Thyristor Tutorial

In many ways the Silicon Controlled Rectifier, SCR or just Thyristor as it is more commonly known, is
similar in construction to the transistor.

It is a multi-layer semiconductor device, hence the “silicon” part of its name. It requires a gate signal to turn
it “ON”, the “controlled” part of the name and once “ON” it behaves like a rectifying diode, the “rectifier”
part of the name. In fact the circuit symbol for the thyristor suggests that this device acts like a controlled
rectifying diode.

Thyristor Symbol

However, unlike the junction diode which is a two layer ( P-N ) semiconductor device, or the commonly
used bipolar transistor which is a three layer ( P-N-P, or N-P-N ) switching device, the Thyristor is a four
layer ( P-N-P-N ) semiconductor device that contains three PN junctions in series, and is represented by the
symbol as shown.

Like the diode, the Thyristor is a unidirectional device, that is it will only conduct current in one direction
only, but unlike a diode, the thyristor can be made to operate as either an open-circuit switch or as a
rectifying diode depending upon how the thyristors gate is triggered. In other words, thyristors can operate
only in the switching mode and cannot be used for amplification.

The silicon controlled rectifier SCR, is one of several power semiconductor devices along with Triacs
(Triode AC’s), Diacs (Diode AC’s) and UJT’s (Unijunction Transistor) that are all capable of acting like
very fast solid state AC switches for controlling large AC voltages and currents. So for the Electronics
student this makes these very handy solid state devices for controlling AC motors, lamps and for phase
control.

The thyristor is a three-terminal device labelled: “Anode”, “Cathode” and “Gate” and consisting of three PN
junctions which can be switched “ON” and “OFF” at an extremely fast rate, or it can be switched “ON” for
variable lengths of time during half cycles to deliver a selected amount of power to a load. The operation of
the thyristor can be best explained by assuming it to be made up of two transistors connected back-to-back
as a pair of complementary regenerative switches as shown.

A Thyristors Two Transistor Analogy

 

The two transistor equivalent circuit shows that the collector current of the NPN transistor TR2 feeds directly
into the base of the PNP transistor TR1, while the collector current of TR1 feeds into the base of TR2. These
two inter-connected transistors rely upon each other for conduction as each transistor gets its base-emitter
current from the other’s collector-emitter current. So until one of the transistors is given some base current
nothing can happen even if an Anode-to-Cathode voltage is present.

When the thyristors Anode terminal is negative with respect to the Cathode, the centre N-P junction is
forward biased, but the two outer P-N junctions are reversed biased and it behaves very much like an
ordinary diode. Therefore a thyristor blocks the flow of reverse current until at some high voltage level the
breakdown voltage point of the two outer junctions is exceeded and the thyristor conducts without the
application of a Gate signal.

This is an important negative characteristic of the thyristor, as Thyristors can be unintentionally triggered
into conduction by a reverse over-voltage as well as high temperature or a rapidly rising dv/dt voltage such
as a spike.

If the Anode terminal is made positive with respect to the Cathode, the two outer P-N junctions are now
forward biased but the centre N-P junction is reverse biased. Therefore forward current is also blocked. If a
positive current is injected into the base of the NPN transistor TR2, the resulting collector current flows in
the base of transistor TR1. This in turn causes a collector current to flow in the PNP transistor, TR1 which
increases the base current of TR2 and so on.

Typical Thyristor

Very rapidly the two transistors force each other to conduct to saturation as they are connected in a
regenerative feedback loop that can not stop. Once triggered into conduction, the current flowing through
the device between the Anode and the Cathode is limited only by the resistance of the external circuit as the
forward resistance of the device when conducting can be very low at less than 1Ω so the voltage drop across
it and power loss is also low.
Then we can see that a thyristor blocks current in both directions of an AC supply in its “OFF” state and can
be turned “ON” and made to act like a normal rectifying diode by the application of a positive current to the
base of transistor, TR2 which for a silicon controlled rectifier is called the “Gate” terminal.

The operating voltage-current I-V characteristics curves for the operation of a Silicon Controlled Rectifier
are given as:

Thyristor I-V Characteristics Curves

Once the thyristor has been turned “ON” and is passing current in the forward direction (anode positive), the
gate signal looses all control due to the regenerative latching action of the two internal transistors. The
application of any gate signals or pulses after regeneration is initiated will have no effect at all because the
thyristor is already conducting and fully-ON.

Unlike the transistor, the SCR can not be biased to stay within some active region along a load line between
its blocking and saturation states. The magnitude and duration of the gate “turn-on” pulse has little effect on
the operation of the device since conduction is controlled internally. Then applying a momentary gate pulse
to the device is enough to cause it to conduct and will remain permanently “ON” even if the gate signal is
completely removed.

Therefore the thyristor can also be thought of as a Bistable Latch having two stable states “OFF” or “ON”.
This is because with no gate signal applied, a silicon controlled rectifier blocks current in both directions of
an AC waveform, and once it is triggered into conduction, the regenerative latching action means that it
cannot be turned “OFF” again just by using its Gate.

So how do we turn “OFF” the thyristor?. Once the thyristor has self-latched into its “ON” state and
passing a current, it can only be turned “OFF” again by either removing the supply voltage and therefore the
Anode (IA) current completely, or by reducing its Anode to Cathode current by some external means (the
opening of a switch for example) to below a value commonly called the “minimum holding current”, IH.
The Anode current must therefore be reduced below this minimum holding level long enough for the
thyristors internally latched pn-junctions to recover their blocking state before a forward voltage is again
applied to the device without it automatically self-conducting. Obviously then for a thyristor to conduct in
the first place, its Anode current, which is also its load current, IL must be greater than its holding current
value. That is IL > IH.

Since the thyristor has the ability to turn “OFF” whenever the Anode current is reduced below this minimum
holding value, it follows then that when used on a sinusoidal AC supply the SCR will automatically turn
itself “OFF” at some value near to the cross over point of each half cycle, and as we now know, will remain
“OFF” until the application of the next Gate trigger pulse.

Since an AC sinusoidal voltage continually reverses in polarity from positive to negative on every half-
cycle, this allows the thyristor to turn “OFF” at the 180o zero point of the positive waveform. This effect is
known as “natural commutation” and is a very important characteristic of the silicon controlled rectifier.

Thyristors used in circuits fed from DC supplies, this natural commutation condition cannot occur as the DC
supply voltage is continuous so some other way to turn “OFF” the thyristor must be provided at the
appropriate time because once triggered it will remain conducting.

However in AC sinusoidal circuits natural commutation occurs every half cycle. Then during the positive
half cycle of an AC sinusoidal waveform, the thyristor is forward biased (anode positive) and a can be
triggered “ON” using a Gate signal or pulse. During the negative half cycle, the Anode becomes negative
while the Cathode is positive. The thyristor is reverse biased by this voltage and cannot conduct even if a
Gate signal is present.

So by applying a Gate signal at the appropriate time during the positive half of an AC waveform, the
thyristor can be triggered into conduction until the end of the positive half cycle. Thus phase control (as it is
called) can be used to trigger the thyristor at any point along the positive half of the AC waveform and one
of the many uses of a Silicon Controlled Rectifier is in the power control of AC systems as shown.

Thyristor Phase Control

At the start of each positive half-cycle the SCR is “OFF”. On the application of the gate pulse triggers the
SCR into conduction and remains fully latched “ON” for the duration of the positive cycle. If the thyristor is
triggered at the beginning of the half-cycle ( θ = 0o ), the load (a lamp) will be “ON” for the full positive
cycle of the AC waveform (half-wave rectified AC) at a high average voltage of 0.318 x Vp.

As the application of the gate trigger pulse increases along the half cycle ( θ = 0o to 90o ), the lamp is
illuminated for less time and the average voltage delivered to the lamp will also be proportionally less
reducing its brightness.

Then we can use a silicon controlled rectifier as an AC light dimmer as well as in a variety of other AC
power applications such as: AC motor-speed control, temperature control systems and power regulator
circuits, etc.

Thus far we have seen that a thyristor is essentially a half-wave device that conducts in only the positive half
of the cycle when the Anode is positive and blocks current flow like a diode when the Anode is negative,
irrespective of the Gate signal.

But there are more semiconductor devices available which come under the banner of “Thyristor” that can
conduct in both directions, full-wave devices, or can be turned “OFF” by the Gate signal.

Such devices include “Gate Turn-OFF Thyristors” (GTO), “Static Induction Thyristors” (SITH), “MOS
Controlled Thyristors” (MCT), “Silicon Controlled Switch” (SCS), “Triode Thyristors” (TRIAC) and “Light
Activated Thyristors” (LASCR) to name a few, with all these devices available in a variety of voltage and
current ratings making them attractive for use in applications at very high power levels.

Thyristor Summary
Silicon Controlled Rectifiers known commonly as Thyristors are three-junction PNPN semiconductor
devices which can be regarded as two inter-connected transistors that can be used in the switching of heavy
electrical loads. They can be latched-“ON” by a single pulse of positive current applied to their Gate
terminal and will remain “ON” indefinitely until the Anode to Cathode current falls below their minimum
latching level.

Static Characteristics of a Thyristor

 Thyristors are semiconductor devices that can operate only in the switching mode.
 Thyristor are current operated devices, a small Gate current controls a larger Anode current.
 Conducts current only when forward biased and triggering current applied to the Gate.
 The thyristor acts like a rectifying diode once it is triggered “ON”.
 Anode current must be greater than holding current to maintain conduction.
 Blocks current flow when reverse biased, no matter if Gate current is applied.
 Once triggered “ON”, will be latched “ON” conducting even when a gate current is no longer
applied providing Anode current is above latching current.

Thyristors are high speed switches that can be used to replace electromechanical relays in many circuits as
they have no moving parts, no contact arcing or suffer from corrosion or dirt. But in addition to simply
switching large currents “ON” and “OFF”, thyristors can be made to control the mean value of an AC load
current without dissipating large amounts of power. A good example of thyristor power control is in the
control of electric lighting, heaters and motor speed.

In the next tutorial we will look at some basic Thyristor Circuits and applications using both AC and DC
supplies.

Thyristor Circuit
Thyristors are high-speed solid-state devices which can be used to control motors, heaters and lamps

In the previous tutorial we looked at the basic construction and operation of the Silicon Controlled Rectifier
more commonly known as a Thyristor. This time we will look at how we can use the thyristor and thyristor
switching circuits to control much larger loads such as lamps, motors, or heaters etc.

We said previously that in order to get the Thyristor to turn-“ON” we need to inject a small trigger pulse of
current (not a continuous current) into the Gate, (G) terminal when the thyristor is in its forward direction,
that is the Anode, (A) is positive with respect to the Cathode, (K), for regenerative latching to occur.

Typical Thyristor

Generally, this trigger pulse need only be of a few micro-seconds in duration but the longer the Gate pulse is
applied the faster the internal avalanche breakdown occurs and the faster the turn-“ON” time of the thyristor,
but the maximum Gate current must not be exceeded. Once triggered and fully conducting, the voltage drop
across the thyristor, Anode to Cathode, is reasonably constant at about 1.0V for all values of Anode current
up to its rated value.

But remember though that once a Thyristor starts to conduct it continues to conduct even with no Gate
signal, until the Anode current decreases below the devices holding current, (IH) and below this value it
automatically turns-“OFF”. Then unlike bipolar transistors and FET’s, thyristors cannot be used for
amplification or controlled switching.

Thyristors are semiconductor devices that are specifically designed for use in high-power switching
applications and do not have the ability of an amplifier. Thyristors can operate only in a switching mode,
acting like either an open or closed switch. Once triggered into conduction by its gate terminal, a thyristor
will remain conducting (passing current) always. Therefore in DC circuits and some highly inductive AC
circuits the current has to be artificially reduced by a separate switch or turn off circuit.

DC Thyristor Circuit
When connected to a direct current DC supply, the thyristor can be used as a DC switch to control larger DC
currents and loads. When using the Thyristor as a switch it behaves like an electronic latch because once
activated it remains in the “ON” state until manually reset. Consider the DC thyristor circuit below.

DC Thyristor Switching Circuit

 

This simple “on-off” thyristor firing circuit uses the thyristor as a switch to control a lamp, but it could also
be used as an on-off control circuit for a motor, heater or some other such DC load. The thyristor is forward
biased and is triggered into conduction by briefly closing the normally-open “ON” push button, S1 which
connects the Gate terminal to the DC supply via the Gate resistor, RG thus allowing current to flow into the
Gate. If the value of RG is set too high with respect to the supply voltage, the thyristor may not trigger.

Once the circuit has been turned-“ON”, it self latches and stays “ON” even when the push button is released
providing the load current is more than the thyristors latching current. Additional operations of push button,
S1 will have no effect on the circuits state as once “latched” the Gate looses all control. The thyristor is now
turned fully “ON” (conducting) allowing full load circuit current to flow through the device in the forward
direction and back to the battery supply.

One of the main advantages of using a thyristor as a switch in a DC circuit is that it has a very high current
gain. The thyristor is a current operated device because a small Gate current can control a much larger
Anode current.

The Gate-cathode resistor RGK is generally included to reduce the Gate’s sensitivity and increase its dv/dt
capability thus preventing false triggering of the device.

As the thyristor has self latched into the “ON” state, the circuit can only be reset by interrupting the power
supply and reducing the Anode current to below the thyristors minimum holding current (IH) value.

Opening the normally-closed “OFF” push button, S2 breaks the circuit, reducing the circuit current flowing
through the Thyristor to zero, thus forcing it to turn “OFF” until the application again of another Gate
signal.

However, one of the disadvantages of this DC thyristor circuit design is that the mechanical normally-closed
“OFF” switch S2 needs to be big enough to handle the circuit power flowing through both the thyristor and
the lamp when the contacts are opened. If this is the case we could just replace the thyristor with a large
mechanical switch. One way to overcome this problem and reduce the need for a larger more robust “OFF”
switch is to connect the switch in parallel with the thyristor as shown.

Alternative DC Thyristor Circuit

 

Here the thyristor switch receives the required terminal voltage and Gate pulse signal as before but the larger
normally-closed switch of the previous circuit has be replaced by a smaller normally-open switch in parallel
with the thyristor. Activation of switch S2 momentarily applies a short circuit between the thyristors Anode
and Cathode stopping the device from conducting by reducing the holding current to below its minimum
value.

AC Thyristor Circuit
When connected to an alternating current AC supply, the thyristor behaves differently from the previous DC
connected circuit. This is because AC power reverses polarity periodically and therefore any thyristor used
in an AC circuit will automatically be reverse-biased causing it to turn-“OFF” during one-half of each cycle.
Consider the AC thyristor circuit below.

AC Thyristor Circuit

The above thyristor firing circuit is similar in design to the DC SCR circuit except for the omission of an
additional “OFF” switch and the inclusion of diode D1 which prevents reverse bias being applied to the
Gate. During the positive half-cycle of the sinusoidal waveform, the device is forward biased but with
switch S1 open, zero gate current is applied to the thyristor and it remains “OFF”. On the negative half-cycle,
the device is reverse biased and will remain “OFF” regardless of the condition of switch S1.

If switch S1 is closed, at the beginning of each positive half-cycle the thyristor is fully “OFF” but shortly
after there will be sufficient positive trigger voltage and therefore current present at the Gate to turn the
thyristor and the lamp “ON”.
The thyristor is now latched-“ON” for the duration of the positive half-cycle and will automatically turn
“OFF” again when the positive half-cycle ends and the Anode current falls below the holding current value.

During the next negative half-cycle the device is fully “OFF” anyway until the following positive half-cycle
when the process repeats itself and the thyristor conducts again as long as the switch is closed.

Then in this condition the lamp will receive only half of the available power from the AC source as the
thyristor acts like a rectifying diode, and conducts current only during the positive half-cycles when it is
forward biased. The thyristor continues to supply half power to the lamp until the switch is opened.

If it were possible to rapidly turn switch S1 ON and OFF, so that the thyristor received its Gate signal at the
“peak” (90o) point of each positive half-cycle, the device would only conduct for one half of the positive
half-cycle. In other words, conduction would only take place during one-half of one-half of a sine wave and
this condition would cause the lamp to receive “one-fourth” or a quarter of the total power available from
the AC source.

By accurately varying the timing relationship between the Gate pulse and the positive half-cycle, the
Thyristor could be made to supply any percentage of power desired to the load, between 0% and 50%.
Obviously, using this circuit configuration it cannot supply more than 50% power to the lamp, because it
cannot conduct during the negative half-cycles when it is reverse biased. Consider the circuit below.

Half Wave Phase Control

Phase control is the most common form of thyristor AC power control and a basic AC phase-control circuit
can be constructed as shown above. Here the thyristors Gate voltage is derived from the RC charging circuit
via the trigger diode, D1.

During the positive half-cycle when the thyristor is forward biased, capacitor, C charges up via resistor R1
following the AC supply voltage. The Gate is activated only when the voltage at point A has risen enough to
cause the trigger diode D1, to conduct and the capacitor discharges into the Gate of the thyristor turning it
“ON”. The time duration in the positive half of the cycle at which conduction starts is controlled by RC time
constant set by the variable resistor, R1.

Increasing the value of R1 has the effect of delaying the triggering voltage and current supplied to the
thyristors Gate which in turn causes a lag in the devices conduction time. As a result, the fraction of the half-
cycle over which the device conducts can be controlled between 0 and 180o, which means that the average
power dissipated by the lamp can be adjusted. However, the thyristor is a unidirectional device so only a
maximum of 50% power can be supplied during each positive half-cycle.
There are a variety of ways to achieve 100% full-wave AC control using “thyristors”. One way is to include
a single thyristor within a diode bridge rectifier circuit which converts AC to a unidirectional current
through the thyristor while the more common method is to use two thyristors connected in inverse parallel.
A more practical approach is to use a single Triac as this device can be triggered in both directions,
therefore making them suitable for AC switching applications.

Triac Tutorial
A Triac is a high-speed solid-state device that can switch and control AC power in both directions of a
sinusoidal waveform

Being a solid state device, thyristors can be used to control lamps, motors, or heaters etc. However, one of
the problems of using a thyristor for controlling such circuits is that like a diode, the “thyristor” is a
unidirectional device, meaning that it passes current in one direction only, from Anode to Cathode.

For DC switching circuits this “one-way” switching characteristic may be acceptable as once triggered all
the DC power is delivered straight to the load. But in sinusoidal AC switching circuits, this unidirectional
switching may be a problem as it only conducts during one half of the cycle (like a half-wave rectifier) when
the Anode is positive irrespective of whatever the Gate signal is doing. Then for AC operation only half the
power is delivered to the load by a thyristor.

In order to obtain full-wave power control we could connect a single thyristor inside a full-wave bridge
rectifier which triggers on each positive half-wave, or to connect two thyristors together in inverse parallel
(back-to-back) as shown below but this increases both the complexity and number of components used in
the switching circuit.

Thyristor Configurations

There is however, another type of semiconductor device called a “Triode AC Switch” or Triac for short
which is also a member of the thyristor family that be used as a solid state power switching device but more
importantly it is a “bidirectional” device. In other words, a Triac can be triggered into conduction by both
positive and negative voltages applied to its Anode and with both positive and negative trigger pulses
applied to its Gate terminal making it a two-quadrant switching Gate controlled device.
A Triac behaves just like two conventional thyristors connected together in inverse parallel (back-to-back)
with respect to each other and because of this arrangement the two thyristors share a common Gate terminal
all within a single three-terminal package.

Since a triac conducts in both directions of a sinusoidal waveform, the concept of an Anode terminal and a
Cathode terminal used to identify the main power terminals of a thyristor are replaced with identifications
of: MT1, for Main Terminal 1 and MT2 for Main Terminal 2 with the Gate terminal G referenced the same.

In most AC switching applications, the triac gate terminal is associated with the MT1 terminal, similar to the
gate-cathode relationship of the thyristor or the base-emitter relationship of the transistor. The construction,
P-N doping and schematic symbol used to represent a Triac is given below.

Triac Symbol and Construction

We now know that a “triac” is a 4-layer, PNPN in the positive direction and a NPNP in the negative
direction, three-terminal bidirectional device that blocks current in its “OFF” state acting like an open-circuit
switch, but unlike a conventional thyristor, the triac can conduct current in either direction when triggered
by a single gate pulse. Then a triac has four possible triggering modes of operation as follows.

 Ι +  Mode = MT2 current positive (+ve), Gate current positive (+ve)

 Ι –  Mode = MT2 current positive (+ve), Gate current negative (-ve)
 ΙΙΙ +  Mode = MT2 current negative (-ve), Gate current positive (+ve)
 ΙΙΙ –  Mode = MT2 current negative (-ve), Gate current negative (-ve)

And these four modes in which a triac can be operated are shown using the triacs I-V characteristics curves.

Triac I-V Characteristics Curves

 

In Quadrant Ι, the triac is usually triggered into conduction by a positive gate current, labelled above as
mode Ι+. But it can also be triggered by a negative gate current, mode Ι–. Similarly, in Quadrant <ΙΙΙ,
triggering with a negative gate current, –ΙG is also common, mode ΙΙΙ– along with mode ΙΙΙ+. Modes Ι– and
ΙΙΙ+ are, however, less sensitive configurations requiring a greater gate current to cause triggering than the
more common triac triggering modes of Ι+ and ΙΙΙ–.

Also, just like silicon controlled rectifiers (SCR’s), triac’s also require a minimum holding current IH to
maintain conduction at the waveforms cross over point. Then even though the two thyristors are combined
into one single triac device, they still exhibit individual electrical characteristics such as different breakdown
voltages, holding currents and trigger voltage levels exactly the same as we would expect from a single SCR
device.

Triac Applications
The Triac is most commonly used semiconductor device for switching and power control of AC systems as
the triac can be switched “ON” by either a positive or negative Gate pulse, regardless of the polarity of the
AC supply at that time. This makes the triac ideal to control a lamp or AC motor load with a very basic triac
switching circuit given below.

Triac Switching Circuit

 

The circuit above shows a simple DC triggered triac power switching circuit. With switch SW1 open, no
current flows into the Gate of the triac and the lamp is therefore “OFF”. When SW1 is closed, Gate current
is applied to the triac from the battery supply VG via resistor R and the triac is driven into full conduction
acting like a closed switch and full power is drawn by the lamp from the sinusoidal supply.

As the battery supplies a positive Gate current to the triac whenever switch SW1 is closed, the triac is
therefore continually gated in modes Ι+ and ΙΙΙ+ regardless of the polarity of terminal MT2.

Of course, the problem with this simple triac switching circuit is that we would require an additional positive
or negative Gate supply to trigger the triac into conduction. But we can also trigger the triac using the actual
AC supply voltage itself as the gate triggering voltage. Consider the circuit below.

Triac Switching Circuit

The circuit shows a triac used as a simple static AC power switch providing an “ON”-“OFF” function
similar in operation to the previous DC circuit. When switch SW1 is open, the triac acts as an open switch
and the lamp passes zero current. When SW1 is closed the triac is gated “ON” via current limiting resistor R
and self-latches shortly after the start of each half-cycle, thus switching full power to the lamp load.

As the supply is sinusoidal AC, the triac automatically unlatches at the end of each AC half-cycle as the
instantaneous supply voltage and thus the load current briefly falls to zero but re-latches again using the
opposite thyristor half on the next half cycle as long as the switch remains closed. This type of switching
control is generally called full-wave control due to the fact that both halves of the sine wave are being
controlled.
As the triac is effectively two back-to-back connected SCR’s, we can take this triac switching circuit further
by modifying how the gate is triggered as shown below.

Modified Triac Switching Circuit

As above, if switch SW1 is open at position A, there is no gate current and the lamp is “OFF”. If the switch
is moved to position B gate current flows at every half cycle the same as before and full power is drawn by
the lamp as the triac operates in modes Ι+ and ΙΙΙ–.

However this time when the switch is connected to position C, the diode will prevent the triggering of the
gate when MT2 is negative as the diode is reverse biased. Thus the triac only conducts on the positive half-
cycles operating in mode I+ only and the lamp will light at half power. Then depending upon the position of
the switch the load is Off, at Half Power or Fully ON.

Triac Phase Control

Another common type of triac switching circuit uses phase control to vary the amount of voltage, and
therefore power applied to a load, in this case a motor, for both the positive and negative halves of the input
waveform. This type of AC motor speed control gives a fully variable and linear control because the voltage
can be adjusted from zero to the full applied voltage as shown.

Triac Phase Control

 
This basic phase triggering circuit uses the triac in series with the motor across an AC sinusoidal supply.
The variable resistor, VR1 is used to control the amount of phase shift on the gate of the triac which in turn
controls the amount of voltage applied to the motor by turning it ON at different times during the AC cycle.

The triac’s triggering voltage is derived from the VR1 – C1 combination via the Diac (The diac is a
bidirectional semiconductor device that helps provide a sharp trigger current pulse to fully turn-ON the
triac).

At the start of each cycle, C1 charges up via the variable resistor, VR1. This continues until the voltage
across C1 is sufficient to trigger the diac into conduction which in turn allows capacitor, C1 to discharge
into the gate of the triac turning it “ON”.

Once the triac is triggered into conduction and saturates, it effectively shorts out the gate triggering phase
control circuit connected in parallel across it and the triac takes control for the remainder of the half-cycle.

As we have seen above, the triac turns-OFF automatically at the end of the half-cycle and the VR1 – C1
triggering process starts again on the next half cycle.

However, because the triac requires differing amounts of gate current in each switching mode of operation,
for example Ι+ and ΙΙΙ–, a triac is therefore asymmetrical meaning that it may not trigger at the exact same
point for each positive and negative half cycle.

This simple triac speed control circuit is suitable for not only AC motor speed control but for lamp dimmers
and electrical heater control and in fact is very similar to a triac light dimmer used in many homes.
However, a commercial triac dimmer should not be used as a motor speed controller as generally triac light
dimmers are intended to be used with resistive loads only such as incandescent lamps.

Then we can end this Triac Tutorial by summarising its main points as follows:

 A “Triac” is another 4-layer, 3-terminal thyristor device similar to the SCR.

 The Triac can be triggered into conduction in either direction.
 There are four possible triggering modes for a Triac, of which 2 are preferred.

Electrical AC power control using a Triac is extremely effective when used properly to control resistive
type loads such as incandescent lamps, heaters or small universal motors commonly found in portable power
tools and small appliances.

But please remember that these devices can be used and attached directly to the mains AC power source so
circuit testing should be done when the power control device is disconnected from the mains power supply.
Please remember safety first!.

Insulated Gate Bipolar Transistor

The IGBT is a power switching transistor which combines the advantages of MOSFETs and BJTs for use in
power supply and motor control circuits
The Insulated Gate Bipolar Transistor also called an IGBT for short, is something of a cross between a
conventional Bipolar Junction Transistor, (BJT) and a Field Effect Transistor, (MOSFET) making it ideal as
a semiconductor switching device.

The IGBT Transistor takes the best parts of these two types of common transistors, the high input impedance
and high switching speeds of a MOSFET with the low saturation voltage of a bipolar transistor, and
combines them together to produce another type of transistor switching device that is capable of handling
large collector-emitter currents with virtually zero gate current drive.

Typical IGBT

The Insulated Gate Bipolar Transistor, (IGBT) combines the insulated gate (hence the first part of its name)
technology of the MOSFET with the output performance characteristics of a conventional bipolar transistor,
(hence the second part of its name).

The result of this hybrid combination is that the “IGBT Transistor” has the output switching and conduction
characteristics of a bipolar transistor but is voltage-controlled like a MOSFET.

IGBTs are mainly used in power electronics applications, such as inverters, converters and power supplies,
were the demands of the solid state switching device are not fully met by power bipolars and power
MOSFETs. High-current and high-voltage bipolars are available, but their switching speeds are slow, while
power MOSFETs may have higher switching speeds, but high-voltage and high-current devices are
expensive and hard to achieve.

The advantage gained by the insulated gate bipolar transistor device over a BJT or MOSFET is that it offers
greater power gain than the standard bipolar type transistor combined with the higher voltage operation and
lower input losses of the MOSFET. In effect it is an FET integrated with a bipolar transistor in a form of
Darlington type configuration as shown.

Insulated Gate Bipolar Transistor

We can see that the insulated gate bipolar transistor is a three terminal, transconductance device that
combines an insulated gate N-channel MOSFET input with a PNP bipolar transistor output connected in a
type of Darlington configuration.
As a result the terminals are labelled as: Collector, Emitter and Gate. Two of its terminals (C-E) are
associated with the conductance path which passes current, while its third terminal (G) controls the device.

The amount of amplification achieved by the insulated gate bipolar transistor is a ratio between its output
signal and its input signal. For a conventional bipolar junction transistor, (BJT) the amount of gain is
approximately equal to the ratio of the output current to the input current, called Beta.

For a metal oxide semiconductor field effect transistor or MOSFET, there is no input current as the gate is
isolated from the main current carrying channel. Therefore, an FET’s gain is equal to the ratio of output
current change to input voltage change, making it a transconductance device and this is also true of the
IGBT. Then we can treat the IGBT as a power BJT whose base current is provided by a MOSFET.

The Insulated Gate Bipolar Transistor can be used in small signal amplifier circuits in much the same
way as the BJT or MOSFET type transistors. But as the IGBT combines the low conduction loss of a BJT
with the high switching speed of a power MOSFET an optimal solid state switch exists which is ideal for
use in power electronics applications.

Also, the IGBT has a much lower “on-state” resistance, RON than an equivalent MOSFET. This means that
the I2R drop across the bipolar output structure for a given switching current is much lower. The forward
blocking operation of the IGBT transistor is identical to a power MOSFET.

When used as static controlled switch, the insulated gate bipolar transistor has voltage and current ratings
similar to that of the bipolar transistor. However, the presence of an isolated gate in an IGBT makes it a lot
simpler to drive than the BJT as much less drive power is needed.

An insulated gate bipolar transistor is simply turned “ON” or “OFF” by activating and deactivating its Gate
terminal. Applying a positive input voltage signal across the Gate and the Emitter will keep the device in its
“ON” state, while making the input gate signal zero or slightly negative will cause it to turn “OFF” in much
the same way as a bipolar transistor or eMOSFET. Another advantage of the IGBT is that it has a much
lower on-state channel resistance than a standard MOSFET.

IGBT Characteristics

Because the IGBT is a voltage-controlled device, it only requires a small voltage on the Gate to maintain
conduction through the device unlike BJT’s which require that the Base current is continuously supplied in a
sufficient enough quantity to maintain saturation.

Also the IGBT is a unidirectional device, meaning it can only switch current in the “forward direction”, that
is from Collector to Emitter unlike MOSFET’s which have bi-directional current switching capabilities
(controlled in the forward direction and uncontrolled in the reverse direction).

The principal of operation and Gate drive circuits for the insulated gate bipolar transistor are very similar to
that of the N-channel power MOSFET. The basic difference is that the resistance offered by the main
conducting channel when current flows through the device in its “ON” state is very much smaller in the
IGBT. Because of this, the current ratings are much higher when compared with an equivalent power
MOSFET.

The main advantages of using the Insulated Gate Bipolar Transistor over other types of transistor devices
are its high voltage capability, low ON-resistance, ease of drive, relatively fast switching speeds and
combined with zero gate drive current makes it a good choice for moderate speed, high voltage applications
such as in pulse-width modulated (PWM), variable speed control, switch-mode power supplies or solar
powered DC-AC inverter and frequency converter applications operating in the hundreds of kilohertz range.

A general comparison between BJT’s, MOSFET’s and IGBT’s is given in the following table.

IGBT Comparison Table

Device Power Power

IGBT
Characteristic Bipolar MOSFET

Voltage Rating High <1kV High <1kV Very High >1kV

Current Rating High <500A Low <200A High >500A

Current, hFE Voltage, VGS Voltage, VGE

Input Drive
20-200 3-10V 4-8V

Input Impedance Low High High

Output Impedance Low Medium Low

Switching Speed Slow (uS) Fast (nS) Medium

Cost Low Medium High

We have seen that the Insulated Gate Bipolar Transistor is semiconductor switching device that has the
output characteristics of a bipolar junction transistor, BJT, but is controlled like a metal oxide field effect
transistor, MOSFET.

One of the main advantages of the IGBT transistor is the simplicity by which it can be driven “ON” by
applying a positive gate voltage, or switched “OFF” by making the gate signal zero or slightly negative
allowing it to be used in a variety of switching applications. It can also be driven in its linear active region
for use in power amplifiers.

With its lower on-state resistance and conduction losses as well as its ability to switch high voltages at high
frequencies without damage makes the Insulated Gate Bipolar Transistor ideal for driving inductive loads
such as coil windings, electromagnets and DC motors.

Diac Tutorial
The Diac is a two-junction bidirectional semiconductor device designed to break down when the AC voltage
across it exceeds a certain level passing current in either direction
The DIode AC switch, or Diac for short, is another solid state, three-layer, two-junction semiconductor
device but unlike the transistor the Diac has no base connection making it a two terminal device, labelled A1
and A2.

Diac’s are an electronic component which offer no control or amplification but act much like a bidirectional
switching diode as they can conduct current from either polarity of a suitable AC voltage supply.

In our tutorial about SCR’s and Triacs, we saw that in ON-OFF switching applications, these devices could
be triggered by simple circuits producing steady state gate currents as shown.

When switch, S1 is open no gate current flows and the lamp is “OFF”. When switch S1 is closed, gate
current IG flows and the SCR conducts on the positive half cycles only as it is operating in quadrant Ι.

We remember also that once gated “ON”, the SCR will only switch “OFF” again when its supply voltage
falls to a values such that its Anode current, IA is less than the value of its holding current, IH.

If we wish to control the mean value of the lamp current, rather than just switch it “ON” or “OFF”, we could
apply a short pulse of gate current at a pre-set trigger point to allow conduction of the SCR to occur over
part of the half-cycle only. Then the mean value of the lamp current would be varied by changing the delay
time, T between the start of the cycle and the trigger point. This method is known commonly as “phase
control”.

But to achieve phase control, two things are needed. One is a variable phase shift circuit (usually an RC
passive circuit), and two, some form of trigger circuit or device that can produce the required gate pulse
when the delayed waveform reaches a certain level. One such solid state semiconductor device that is
designed to produce these gate pulses is the Diac.

The diac is constructed like a transistor but has no base connection allowing it to be connected into a circuit
in either polarity. Diacs are primarily used as trigger devices in phase-triggering and variable power control
applications because a diac helps provide a sharper and more instant trigger pulse (as opposed to a steadily
rising ramp voltage) which is used to turn “ON” the main switching device.

The diac symbol and the voltage-current characteristics curves of the diac are given below.

Diac Symbol and I-V Characteristics

 

We can see from the above diac I-V characteristics curves that the diac blocks the flow of current in both
directions until the applied voltage is greater than VBR, at which point breakdown of the device occurs and
the diac conducts heavily in a similar way to the zener diode passing a sudden pulse of voltage. This VBR
point is called the Diacs breakdown voltage or breakover voltage.

In an ordinary zener diode the voltage across it would remain constant as the current increased. However, in
the diac the transistor action causes the voltage to reduce as the current increases. Once in the conducting
state, the resistance of the diac falls to a very low value allowing a relatively large value of current to flow.
For most commonly available diacs such as the ST2 or DB3, their breakdown voltage typically ranges from
about ±25 to 35 volts. Higher breakover voltage ratings are available, for example 40 volts for the DB4 diac.

This action gives the diac the characteristic of a negative resistance as shown above. As the diac is a
symmetrical device, it therefore has the same characteristic for both positive and negative voltages and it is
this negative resistance action that makes the Diac suitable as a triggering device for SCR’s or triacs.

Diac Applications
As stated above, the diac is commonly used as a solid state triggering device for other semiconductor
switching devices, mainly SCR’s and triacs. Triacs are widely used in applications such as lamp dimmers
and motor speed controllers and as such the diac is used in conjunction with the triac to provide full-wave
control of the AC supply as shown.

Diac AC Phase Control

 

As the AC supply voltage increases at the beginning of the cycle, capacitor, C is charged through the series
combination of the fixed resistor, R1 and the potentiometer, VR1 and the voltage across its plates increases.
When the charging voltage reaches the breakover voltage of the diac (about 30 V for the ST2), the diac
breaks down and the capacitor discharges through the diac.

The discharge produces a sudden pulse of current, which fires the triac into conduction. The phase angle at
which the triac is triggered can be varied using VR1, which controls the charging rate of the capacitor.
Resistor, R1 limits the gate current to a safe value when VR1 is at its minimum.

Once the triac has been fired into conduction, it is maintained in its “ON” state by the load current flowing
through it, while the voltage across the resistor–capacitor combination is limited by the “ON” voltage of the
triac and is maintained until the end of the present half-cycle of the AC supply.

At the end of the half cycle the supply voltage falls to zero, reducing the current through the triac below its
holding current, IH turning it “OFF” and the diac stops conduction. The supply voltage then enters its next
half-cycle, the capacitor voltage again begins to rise (this time in the opposite direction) and the cycle of
firing the triac repeats over again.

Triac Conduction Waveform

Then we have seen that the Diac is a very useful device which can be used to trigger triacs and because of its
negative resistance characteristics this allows it to switch “ON” rapidly once a certain applied voltage level
is reached. However, this means that whenever we want to use a triac for AC power control we will need a
separate diac as well. Fortunately for us, some bright spark somewhere replaced the individual diac and triac
with a single switching device called a Quadrac.

The Quadrac
The Quadrac is basically a Diac and Triac fabricated together within a single semiconductor package and as
such are also known as “internally triggered triacs”. This all in one bi-directional device is gate controlled
using either polarity of the main terminal voltage which means it can be used in full-wave phase-control
applications such as heater controls, lamp dimmers, and AC motor speed control, etc.

Like the triac, quadracs are a three-terminal semiconductor switching device labelled MT2 for main terminal
one (usually the anode), MT1 for main terminal two (usually the cathode) and G for the gate terminal.
The quadrac is available in a variety of package types depending upon their voltage and current switching
requirements with the TO-220 package being the most common as it is designed to be an exact replacement
for most triac devices.

Diac Tutorial Summary

In this diac tutorial we have seen that the diac such as the ST2 or DB3 is a two-terminal voltage blocking
device that can conduct in either direction. Diacs posses negative resistance characteristics which allows
them to switch “ON” rapidly once a certain applied voltage level is reached.

Since the diac is a bidirectional device, when paired with the BTAxx-600A or IRT80 series of switching
triacs it makes it useful as a triggering device in phase control and general AC circuits such as light dimmers
and motor speed controls.

Quadracs are simply triacs with an internally connected diac. As with triacs, quadracs are bidirectional AC
switches which are gate controlled for either polarity of main terminal voltage.

Unijunction Transistor
The UJT is a three-terminal, semiconductor device which exhibits negative resistance and switching
characteristics for use as a relaxation oscillator in phase control applications

The Unijunction Transistor or UJT for short, is another solid state three terminal device that can be used
in gate pulse, timing circuits and trigger generator applications to switch and control either thyristors and
triac’s for AC power control type applications.

Like diodes, unijunction transistors are constructed from separate P-type and N-type semiconductor
materials forming a single (hence its name Uni-Junction) PN-junction within the main conducting N-type
channel of the device.

Although the Unijunction Transistor has the name of a transistor, its switching characteristics are very
different from those of a conventional bipolar or field effect transistor as it can not be used to amplify a
signal but instead is used as a ON-OFF switching transistor. UJT’s have unidirectional conductivity and
negative impedance characteristics acting more like a variable voltage divider during breakdown.

Like N-channel FET’s, the UJT consists of a single solid piece of N-type semiconductor material forming
the main current carrying channel with its two outer connections marked as Base 2 ( B2 ) and Base 1 ( B1 ).
The third connection, confusingly marked as the Emitter ( E ) is located along the channel. The emitter
terminal is represented by an arrow pointing from the P-type emitter to the N-type base.

The Emitter rectifying p-n junction of the unijunction transistor is formed by fusing the P-type material into
the N-type silicon channel. However, P-channel UJT’s with an N-type Emitter terminal are also available
but these are little used.
The Emitter junction is positioned along the channel so that it is closer to terminal B2 than B1. An arrow is
used in the UJT symbol which points towards the base indicating that the Emitter terminal is positive and the
silicon bar is negative material. Below shows the symbol, construction, and equivalent circuit of the UJT.

Unijunction Transistor Symbol and Construction

Notice that the symbol for the unijunction transistor looks very similar to that of the junction field effect
transistor or JFET, except that it has a bent arrow representing the Emitter( E ) input. While similar in
respect of their ohmic channels, JFET’s and UJT’s operate very differently and should not be confused.

So how does it work? We can see from the equivalent circuit above, that the N-type channel basically
consists of two resistors RB2 and RB1 in series with an equivalent (ideal) diode, D representing the p-n
junction connected to their center point. This Emitter p-n junction is fixed in position along the ohmic
channel during manufacture and can therefore not be changed.

Resistance RB1 is given between the Emitter, E and terminal B1, while resistance RB2 is given between the
Emitter, E and terminal B2. As the physical position of the p-n junction is closer to terminal B2 than B1 the
resistive value of RB2 will be less than RB1.

The total resistance of the silicon bar (its Ohmic resistance) will be dependent upon the semiconductors
actual doping level as well as the physical dimensions of the N-type silicon channel but can be represented
by RBB. If measured with an ohmmeter, this static resistance would typically measure somewhere between
about 4kΩ and 10kΩ’s for most common UJT’s such as the 2N1671, 2N2646 or the 2N2647.

These two series resistances produce a voltage divider network between the two base terminals of the
unijunction transistor and since this channel stretches from B2 to B1, when a voltage is applied across the
device, the potential at any point along the channel will be in proportion to its position between terminals B2
and B1. The level of the voltage gradient therefore depends upon the amount of supply voltage.

When used in a circuit, terminal B1 is connected to ground and the Emitter serves as the input to the device.
Suppose a voltage VBB is applied across the UJT between B2 and B1 so that B2 is biased positive relative to
B1. With zero Emitter input applied, the voltage developed across RB1 (the lower resistance) of the resistive
voltage divider can be calculated as:

Unijunction Transistor RB1 Voltage

 

For a unijunction transistor, the resistive ratio of RB1 to RBB shown above is called the intrinsic stand-off
ratio and is given the Greek symbol: η (eta). Typical standard values of η range from 0.5 to 0.8 for most
common UJT’s.

If a small positive input voltage which is less than the voltage developed across resistance, RB1 ( ηVBB ) is
now applied to the Emitter input terminal, the diode p-n junction is reverse biased, thus offering a very high
impedance and the device does not conduct. The UJT is switched “OFF” and zero current flows.

However, when the Emitter input voltage is increased and becomes greater than VRB1 (or ηVBB + 0.7V,
where 0.7V equals the p-n junction diode volt drop) the p-n junction becomes forward biased and the
unijunction transistor begins to conduct. The result is that Emitter current, ηIE now flows from the Emitter
into the Base region.

The effect of the additional Emitter current flowing into the Base reduces the resistive portion of the channel
between the Emitter junction and the B1 terminal. This reduction in the value of RB1 resistance to a very low
value means that the Emitter junction becomes even more forward biased resulting in a larger current flow.
The effect of this results in a negative resistance at the Emitter terminal.

Likewise, if the input voltage applied between the Emitter and B1 terminal decreases to a value below
breakdown, the resistive value of RB1 increases to a high value. Then the Unijunction Transistor can be
thought of as a voltage breakdown device.

So we can see that the resistance presented by RB1 is variable and is dependant on the value of Emitter
current, IE. Then forward biasing the Emitter junction with respect to B1 causes more current to flow which
reduces the resistance between the Emitter, E and B1.

In other words, the flow of current into the UJT’s Emitter causes the resistive value of RB1 to decrease and
the voltage drop across it, VRB1 must also decrease, allowing more current to flow producing a negative
resistance condition.

Unijunction Transistor Applications

Now that we know how a unijunction transistor works, what can they be used for. The most common
application of a unijunction transistor is as a triggering device for SCR’s and Triacs but other UJT
applications include sawtoothed generators, simple oscillators, phase control, and timing circuits. The
simplest of all UJT circuits is the Relaxation Oscillator producing non-sinusoidal waveforms.

In a basic and typical UJT relaxation oscillator circuit, the Emitter terminal of the unijunction transistor is
connected to the junction of a series connected resistor and capacitor, RC circuit as shown below.

Unijunction Transistor Relaxation Oscillator

 

When a voltage (Vs) is firstly applied, the unijunction transistor is “OFF” and the capacitor C1 is fully
discharged but begins to charge up exponentially through resistor R3. As the Emitter of the UJT is
connected to the capacitor, when the charging voltage Vc across the capacitor becomes greater than the
diode volt drop value, the p-n junction behaves as a normal diode and becomes forward biased triggering the
UJT into conduction. The unijunction transistor is “ON”. At this point the Emitter to B1 impedance
collapses as the Emitter goes into a low impedance saturated state with the flow of Emitter current through
R1 taking place.

As the ohmic value of resistor R1 is very low, the capacitor discharges rapidly through the UJT and a fast
rising voltage pulse appears across R1. Also, because the capacitor discharges more quickly through the UJT
than it does charging up through resistor R3, the discharging time is a lot less than the charging time as the
capacitor discharges through the low resistance UJT.

When the voltage across the capacitor decreases below the holding point of the p-n junction ( VOFF ), the UJT
turns “OFF” and no current flows into the Emitter junction so once again the capacitor charges up through
resistor R3 and this charging and discharging process between VON and VOFF is constantly repeated while
there is a supply voltage, Vs applied.

UJT Oscillator Waveforms

Then we can see that the unijunction oscillator continually switches “ON” and “OFF” without any feedback.
The frequency of operation of the oscillator is directly affected by the value of the charging resistance R3, in
series with the capacitor C1 and the value of η. The output pulse shape generated from the Base1 (B1)
terminal is that of a sawtooth waveform and to regulate the time period, you only have to change the ohmic
value of resistance, R3 since it sets the RC time constant for charging the capacitor.

The time period, T of the sawtoothed waveform will be given as the charging time plus the discharging time
of the capacitor. As the discharge time, τ1 is generally very short in comparison to the larger RC charging
time, τ2 the time period of oscillation is more or less equivalent to T ≅ τ2. The frequency of oscillation is
therefore given by ƒ = 1/T.

UJT Oscillator Example No1

The data sheet for a 2N2646 Unijunction Transistor gives the intrinsic stand-off ratio η as 0.65. If a 100nF
capacitor is used to generate the timing pulses, calculate the timing resistor required to produce an
oscillation frequency of 100Hz.

1. The timing period is given as:

2. The value of the timing resistor, R3 is calculated as:

Then the value of charging resistor required in this simple example is calculated as 95.3kΩ’s to the nearest
preferred value. However, there are certain conditions required for the UJT relaxation oscillator to operate
correctly as the resistive value of R3 can be too large or too small.

For example, if the value of R3 was too large, (Megohms) the capacitor may not charge up sufficiently to
trigger the Unijunction’s Emitter into conduction but must also be large enough to ensure that the UJT
switches “OFF” once the capacitor has discharged to below the lower trigger voltage.

Likewise if the value of R3 was too small, (a few hundred Ohms) once triggered the current flowing into the
Emitter terminal may be sufficiently large to drive the device into its saturation region preventing it from
turning “OFF” completely. Either way the unijunction oscillator circuit would fail to oscillate.

UJT Speed Control Circuit

One typical application of the unijunction transistor circuit above is to generate a series of pulses to fire and
control a thyristor. By using the UJT as a phase control triggering circuit in conjunction with an SCR or
Triac, we can adjust the speed of a universal AC or DC motor as shown.

Unijunction Transistor Speed Control

Using the circuit above, we can control the speed of a universal series motor (or whichever type of load we
want, heaters, lamps, etc) by regulating the current flowing through the SCR. To control the motors speed,
simply change the frequency of the sawtooth pulse, which is achieved by varying the value of the
potentiometer.

Unijunction Transistor Summary

We have seen that a Unijunction Transistor or UJT for short, is an electronic semiconductor device that
has only one p-n junction within a N-type (or P-type) lightly doped ohmic channel. The UJT has three
terminals one labelled Emitter (E) and two Bases (B1 and B2).

Two ohmic contacts B1 and B2 are attached at each ends of the semiconductor channel with the resistance
between B1 and B2, when the emitter is open circuited being called the interbase resistance, RBB. If
measured with an ohmmeter, this static resistance would typically measure somewhere between about 4kΩ
and 10kΩ’s for most common UJT’s.

The ratio of RB1 to RBB is called the intrinsic stand-off ratio, and is given the Greek symbol: η (eta). Typical
standard values of η range from 0.5 to 0.8 for most common UJT’s.

The unijunction transistor is a solid state triggering device that can be used in a variety of circuits and
applications, ranging from the firing of thyristors and triacs, to the use in sawtooth generators for phase
control circuits.The negative resistance characteristic of the UJT also makes it very useful as a simple
relaxation oscillator.

When connected as a relaxation oscillator, it can oscillate independently without a tank circuit or
complicated RC feedback network. When connected this way, the unijunction transistor is capable of
generating a train of pulses of varying duration simply by varying the values of a single capacitor, (C) or
resistor, (R).

Commonly available unijunction transistors include the 2N1671, 2N2646, 2N2647, etc, with the 2N2646
being the most popular UJT for use in pulse and sawtooth generators and time delay circuits. Other types of
unijunction transistor devices available are called Programmable UJTs, which can have their switching
parameters set by external resistors. The most common Programmable Unijunction Transistors are the
2N6027 and the 2N6028.

Switch Mode Power Supply

Linear voltage IC regulators have been the basis of power supply designs for many years as they are very
good at supplying a continuous fixed voltage output.

Linear voltage regulators are generally much more efficient and easier to use than equivalent voltage
regulator circuits made from discrete components such a zener diode and a resistor, or transistors and even
op-amps.

The most popular linear and fixed output voltage regulator types are by far the 78… positive output voltage
series, and the 79… negative output voltage series. These two types of complementary voltage regulators
produce a precise and stable voltage output ranging from about 5 volts up to about 24 volts for use in many
electronic circuits.

There is a wide range of these three-terminal fixed voltage regulators available each with its own built-in
voltage regulation and current limiting circuits. This allows us to create a whole host of different power
supply rails and outputs, either single or dual supply, suitable for most electronic circuits and applications.
There are even variable voltage linear regulators available as well providing an output voltage which is
continually variable from just above zero to a few volts below its maximum voltage output.

Most d.c. power supplies comprise of a large and heavy step-down mains transformer, diode rectification,
either full-wave or half-wave, a filter circuit to remove any ripple content from the rectified d.c. producing a
suitably smooth d.c. voltage, and some form of voltage regulator or stabiliser circuit, either linear or
switching to ensure the correct regulation of the power supplies output voltage under varying load
conditions. Then a typical d.c. power supply would look something like this:

Typical DC Power Supply

These typical power supply designs contain a large mains transformer (which also provides isolation
between the input and output) and a dissipative series regulator circuit. The regulator circuit could consist of
a single zener diode or a three-terminal linear series regulator to produce the required output voltage. The
advantage of a linear regulator is that the power supply circuit only needs an input capacitor, output
capacitor and some feedback resistors to set the output voltage.
Linear voltage regulators produce a regulated DC output by placing a continuously conducting transistor in
series between the input and the output operating it in its linear region (hence the name) of its current-
voltage (i-v) characteristics. Thus the transistor acts more like a variable resistance which continually adjusts
itself to whatever value is needed to maintain the correct output voltage. Consider this simple series pass
transistor regulator circuit below:

Series Transistor Regulator Circuit

Here this simple emitter-follower regulator circuit consists of a single NPN transistor and a DC biasing
voltage to set the required output voltage. As an emitter follower circuit has unity voltage gain, applying a
suitable biasing voltage to the transistors base, a stabilised output is obtained from the emitter terminal.

Since a transistor provides current gain, the output load current will be much higher than the base current
and higher still if a Darlington transistor arrangement is used.

Also, providing that the input voltage is sufficiently high enough to get the desired output voltage, the output
voltage is controlled by the transistors base voltage and in this example is given as 5.7 volts to produce a 5
volt output to the load as approximately 0.7 volts is dropped across the transistor between the base and
emitter terminals. Then depending upon the value of the base voltage, any value of emitter output voltage
can be obtained.

While this simple series regulator circuit will work, the downside to this is that the series transistor is
continually biased in its linear region dissipating power in the form of heat as a result of its V*I product,
since all the load current must pass through the series transistor, resulting in poor efficiency, wasted power
and continuous heat generation.

Also, one of the disadvantages that series voltage regulators have is that, their maximum continuous output
current rating is limited to just a few amperes or so, so are generally used in applications where low power
outputs are required. When higher output voltage or current power supplies are required, the normal practice
is to use a switching regulator commonly known as a switch-mode power supply to convert the mains
voltage into whatever higher power output is required.

Switch Mode Power Supplies, or SMPS, are becoming common place and have replaced in most cases the
traditional linear ac-to-dc power supplies as a way to cut power consumption, reduce heat dissipation, as
well as size and weight. Switch-mode power supplies can now be found in most PC’s, power amplifiers,
TV’s, dc motor drives, etc., and just about anything that requires a highly efficient supply as switch-mode
power supplies are increasingly becoming a much more mature technology.

By definition, a switch mode power supply (SMPS) is a type of power supply that uses semiconductor
switching techniques, rather than standard linear methods to provide the required output voltage. The basic
switching converter consists of a power switching stage and a control circuit. The power switching stage
performs the power conversion from the circuits input voltage, VIN to its output voltage, VOUT which includes
output filtering.

The major advantage of the switch mode power supply is its higher efficiency, compared to standard linear
regulators, and this is achieved by internally switching a transistor (or power MOSFET) between its “ON”
state (saturated) and its “OFF” state (cut-off), both of which produces lower power dissipation. This means
that when the switching transistor is fully “ON” and conducting current, the voltage drop across it is at its
minimal value, and when the transistor is fully “OFF” there is no current flow through it. So the transistor is
acting like an ideal switch.

As a result, unlike linear regulators which only offer step-down voltage regulation, a switch mode power
supply, can offer step-down, step-up and negation of the input voltage using one or more of the three basic
switch mode circuit topologies: Buck, Boost and Buck-Boost. This refers to how the transistor switch,
inductor, and smoothing capacitor are connected within the basic circuit.

Buck Switch Mode Power Supply

The Buck switching regulator is a type of switch mode power supply circuit that is designed to efficiently
reduce DC voltage from a higher voltage to a lower one, that is it subtracts or “Bucks” the supply voltage,
thereby reducing the voltage available at the output terminals without changing the polarity. In other words,
the buck switching regulator is a step-down regulator circuit, so for example a buck converter can convert
say, +12 volts to +5 volts.

The buck switching regulator is a DC-to-DC converter and one of the simplest and most popular type of
switching regulator. When used within a switch mode power supply configuration, the buck switching
regulator uses a series transistor or power MOSFET (ideally an insulated gate bipolar transistor, or IGBT) as
its main switching device as shown below.

The Buck Switching Regulator

We can see that the basic circuit configuration for a buck converter is a series transistor switch, TR1 with an
associated drive circuit that keeps the output voltage as close to the desired level as possible, a diode, D1, an
inductor, L1 and a smoothing capacitor, C1. The buck converter has two operating modes, depending on if
the switching transistor TR1 is turned “ON” or “OFF”.

When the transistor is biased “ON” (switch closed), diode D1 becomes reverse biased and the input voltage,
VIN causes a current to flow through the inductor to the connected load at the output, charging up the
capacitor, C1. As a changing current flows through the inductor coil, it produces a back-emf which opposes
the flow of current, according to Faraday’s law, until it reaches a steady state creating a magnetic field
around the inductor, L1. This situation continues indefinitely as long as TR1 is closed.

When transistor TR1 is turned “OFF” (switch open) by the controlling circuitry, the input voltage is instantly
disconnected from the emitter circuit causing the magnetic field around the inductor to collapse inducing a
reverse voltage across the inductor. This reverse voltage causes the diode to become forward biased, so the
stored energy in the inductors magnetic field forces current to continue to flow through the load in the same
direction, and return back through diode.

Then the inductor, L1 returns its stored energy back to the load acting like a source and supplying current
until all the inductor’s energy is returned to the circuit or until the transistor switch closes again, whichever
comes first. At the same time the capacitor also discharges supplying current to the load. The combination of
the inductor and capacitor forms an LC filter smoothing out any ripple created by the switching action of the
transistor.

Therefore, when the transistor solid state switch is closed, current is supplied from the supply, and when the
transistor switch is open, current is supplied by the inductor. Note that the current flowing through the
inductor is always in the same direction, either directly from the supply or via the diode but obviously at
different times within the switching cycle.

As the transistor switch is being continuously closed and opened, the average output voltage value will
therefore be related to the duty cycle, D which is defined as the conduction time of the transistor switch
during one full switching cycle. If VIN is the supply voltage, and the “ON” and “OFF” times for the
transistor switch are defined as: tON and tOFF, then the output voltage VOUT is given as:

Buck Converter Duty Cycle

The buck converters duty cycle can also be defined as:

So the larger the duty cycle, the higher the average DC output voltage from the switch mode power supply.
From this we can also see that the output voltage will always be lower than the input voltage since the duty
cycle, D can never reach one (unity) resulting in a step-down voltage regulator. Voltage regulation is
obtained by varying the duty cycle and with high switching speeds, up to 200kHz, smaller components can
be used thereby greatly reducing a switch mode power supply’s size and weight.

Another advantage of the buck converter is that the inductor-capacitor (LC) arrangement provides very good
filtering of the inductor current. Ideally the buck converter should be operated in a continuous switching
mode so that the inductor current never falls to zero. With ideal components, that is zero voltage drop and
switching losses in the “ON” state, the ideal buck converter could have efficiencies as high as 100%.

As well as the step-down buck switching regulator for the basic design of a switch mode power supply, there
is another operation of the fundamental switching regulator that acts as a step-up voltage regulator called the
Boost Converter.
Boost Switch Mode Power Supply
The Boost switching regulator is another type of switch mode power supply circuit. It has the same types
of components as the previous buck converter, but this time in different positions. The boost converter is
designed to increase a DC voltage from a lower voltage to a higher one, that is it adds too or “Boosts” the
supply voltage, thereby increasing the available voltage at the output terminals without changing the
polarity. In other words, the boost switching regulator is a step-up regulator circuit, so for example a boost
converter can convert say, +5 volts to +12 volts.

We saw previously that the buck switching regulator uses a series switching transistor within its basic
design. The difference with the design of the boost switching regulator is that it uses a parallel connected
switching transistor to control the output voltage from the switch mode power supply. As the transistor
switch is effectively connected in parallel with the output, electrical energy only passes through the inductor
to the load when the transistor is biased “OFF” (switch open) as shown.

The Boost Switching Regulator

In the Boost Converter circuit, when the transistor switch is fully-on, electrical energy from the supply, VIN
passes through the inductor and transistor switch and back to the supply. As a result, none of it passes to the
output as the saturated transistor switch effectively creates a short-circuit to the output. This increases the
current flowing through the inductor as it has a shorter inner path to travel back to the supply. Meanwhile,
diode D1 becomes reverse biased as its anode is connected to ground via the transistor switch with the
voltage level on the output remaining fairly constant as the capacitor starts to discharge through the load.

When the transistor is switched fully-off, the input supply is now connected to the output via the series
connected inductor and diode. As the inductor field decreases the induced energy stored in the inductor is
pushed to the output by VIN, through the now forward biased diode. The result of all this is that the induced
voltage across the inductor L1 reverses and adds to the voltage of the input supply increasing the total output
voltage as it now becomes, VIN + VL.

Current from the smoothing capacitor, C1 which was used to supply the load when the transistor switch was
closed, is now returned to the capacitor by the input supply via the diode. Then the current supplied to the
capacitor is the diode current, which will always be ON or OFF as the diode is continually switched between
forward and reverse status by the switching actions of transistor. Then the smoothing capacitor must be
sufficiently large enough to produce a smooth steady output.

As the induced voltage across the inductor L1 is negative, it adds to the source voltage, VIN forcing the
inductor current into the load. The boost converters steady state output voltage is given by:
 

As with the previous buck converter, the output voltage from the boost converter depends upon the input
voltage and duty cycle. Therefore, by controlling the duty cycle, output regulation is achieved. Not also that
this equation is independent of the value of the inductor, the load current, and the output capacitor.

We have seen above that the basic operation of a non-isolated switch mode power supply circuit can use
either a buck converter or boost converter configuration depending upon whether we require a step-down
(buck) or step-up (boost) output voltage. While buck converters may be the more common SMPS switching
configuration, boost converters are commonly used in capacitive circuit applications such as battery
chargers, photo-flashes, strobe flashes, etc, because the capacitor supplies all of the load current while the
switch is closed.

But we can also combine these two basic switching topologies into a single non-isolating switching
regulator circuit called unsurprisingly, a Buck-Boost Converter.

Buck-Boost Switching Regulator

The Buck-Boost switching regulator is a combination of the buck converter and the boost converter that
produces an inverted (negative) output voltage which can be greater or less than the input voltage based on
the duty cycle. The buck-boost converter is a variation of the boost converter circuit in which the inverting
converter only delivers the energy stored by the inductor, L1, into the load. The basic buck-boost switch
mode power supply circuit is given below.

The Buck-Boost Switching Regulator

When the transistor switch, TR1, is switched fully-on (closed), the voltage across the inductor is equal to the
supply voltage so the inductor stores energy from the input supply. No current is delivered to the connected
load at the output because diode, D1, is reverse biased. When the transistor switch is fully-off (open), the
diode becomes forward biased and the energy previously stored in the inductor is transferred to the load.

In other words, when the switch is “ON”, energy is delivered into the inductor by the DC supply (via the
switch), and none to the output, and when the switch is “OFF”, the voltage across the inductor reverses as
the inductor now becomes a source of energy so the energy stored previously in the inductor is switched to
the output (through the diode), and none comes directly from the input DC source. So the voltage dropped
across the load when the switching transistor is “OFF” is equal to the inductor voltage.

The result is that the magnitude of the inverted output voltage can be greater or smaller (or equal to) the
magnitude of the input voltage based on the duty cycle. For example, a positive-to-negative buck-boost
converter can convert 5 volts to 12 volts (step-up) or 12 volts to 5 volts (step-down).

The buck-boost switching regulators steady state output voltage, VOUT is given as:

Then the buck-boost regulator gets its name from producing an output voltage that can be higher (like a
boost power stage) or lower (like a buck power stage) in magnitude than the input voltage. However, the
output voltage is opposite in polarity from the input voltage.

Switch Mode Power Supply Summary

The modern switch mode power supply, or SMPS, uses solid-state switches to convert an unregulated DC
input voltage to a regulated and smooth DC output voltage at different voltage levels. The input supply can
be a true DC voltage from a battery or solar panel, or a rectified DC voltage from an AC supply using a
diode bridge along with some additional capacitive filtering.

In many power control applications, the power transistor, MOSFET or IGFET, is operated in its switching
mode were it is repeatedly turned “ON” and “OFF” at high speed. The main advantage of this is that the
power efficiency of the regulator can be quite high because the transistor is either fully-on and conducting
(saturated) or full-off (cut-off).

There are several types of DC-to-DC converter (as opposed to a DC-to-AC converter which is an inverter)
configurations available, with the three basic switching power supply topologies looked at here being the
Buck, Boost, and the Buck-Boost switching regulators. All three of these topologies are non-isolated, that is
their input and output voltages share a common ground line.

Each switching regulator design has its own unique properties with regards to the steady-state duty cycles,
relationship between the input and output current, and the output voltage ripple produced by the solid-state
switch action. Another important property of these switch mode power supply topologies is the frequency
response of the switching action to the output voltage.

Regulation of the output voltage is achieved by the percentage control of the time that the switching
transistor is in the “ON” state compared to the total ON/OFF time. This ratio is called the duty cycle and by
varying the duty cycle, (D the magnitude of the output voltage, VOUT can be controlled.

The use of a single inductor and diode as well as fast switching solid-state switches capable of operating at
switching frequencies in the kilohertz range, within the switch mode power supply design, allows for the
size and weight of the power supply to be greatly reduced. This is because there would be no large and
heavy step-down (or step-up) voltage mains transformers within their design. However, if isolation is
required between the input and output terminals, a transformer must be included before the converter.

The two most popular non-isolated switching configurations are the buck (subtractive) and the boost
(additive) converters.
The buck converter is a type of switch-mode power supply that is designed to convert electrical energy from
one voltage to a lower one. The buck converter operates with a series connected switching transistor. As the
duty cycle, D < 1, the output voltage of the buck is always smaller than the input voltage, VIN.

The boost converter is a type of switch-mode power supply that is designed to convert electrical energy from
one voltage to a higher one. The boost converter operates with a parallel connected switching transistor
which results in a direct current path between VIN and VOUT via the inductor, L1 and diode, D1. This means
there is no protection against short-circuits on the output.

By varying the duty cycle, (D) of a boost converter, the output voltage can be controlled and with D < 1, the
DC output from the boost converter is greater than input voltage VIN as a consequence of the inductors self-
induced voltage.

Also, the output smoothing capacitors in Switch-mode Power Supplies is assumed to be very large, which
results in a constant output voltage from the switch mode supply during the transistors switching action.

Transient Suppression Devices

Transient suppression devices can significantly reduce the amount of energy released as a result of over
voltage spikes and surges.

We would like to think that the AC or DC power supplies we use to power our circuits are both clean and
well-regulated supplies. However, the switching of AC inductive loads or the switching of DC relay contacts
and DC motors as part of a micro-controller project all combine to produce a quality of power supply that is
difficult to maintain.

These inductive switching transients occur when some form of inductive or reactive load, such as a motor, a
solenoid coil or a relay coil, is suddenly switched off. The rapidly collapsing of its magnetic field induces a
transient voltage which becomes superimposed onto the steady-state supply. These inductive switching
voltage transients can reach the 1,000’s of volts.

Transients are very steep voltage steps that occur in electrical circuits due to the sudden release of a
previously stored energy, either inductive or capacitive, which results in a high voltage transient, or surge
being created. This sudden release of energy back into the circuit due to some switching action creates a
transient voltage spike in the form of a steep impulse of energy which can in theory be of any infinite value.
This high dv/dt transient switching spike can exist either for a very short period of time (milli-seconds or
micro-seconds), or they can occur every so often over short periods of time, for example randomly two or
three times a day.

We must also realise that voltage transients do not always start at zero volts or at the beginning of a cycle,
but can be superimposed onto another voltage level. Either way, transients are bad as they can damage
electronic equipment and therefore need to be suppressed and controlled.

Transient suppression devices can take on many forms from arc contacts, to filters, to solid state
semiconductor devices. Discrete semiconductor transient suppression devices such as the Metal-oxide
Varistor, or MOV, are by far the most common as they are available in a variety of energy absorbing and
voltage ratings making it possible to exercise tight control over unwanted and potentially destructive
transients or over voltage spikes.

Transient suppression devices can be used in series with the load to either attenuate or reduce the energy
value of a transient preventing its propagation through a circuit, or they can be used in parallel with the load
to divert the transient away, usually to ground, and so limit or clamp the residual voltage.

Attenuation of a voltage transient is usually accomplished using low-pass filters connected in series with the
load circuit. When a voltage transient occurs it is usually a fast moving, high frequency spike so the filter
attenuates or blocks this high frequency transient while still allowing the low frequency power or signal
component to continue undisturbed. A good example of transient attenuators are mains filtered extension
cords.

Diverting a transient is usually accomplished using a voltage-clamping type device or by using what are
commonly called a crowbar type device. These parallel connected devices exhibit a nonlinear impedance
characteristic as the current flowing through them is not linear to the voltage across their terminals as given
by Ohms Law.

A voltage-clamping device such as an MOV, has a variable impedance depending on the current flowing
through the device or on the voltage across its terminal. Under normal steady-state operating conditions, the
device offers a high impedance and has therefore no effect on the connected circuit.

However, when a voltage transient occurs, the impedance of the device changes increasing the current
drawn through the device as the voltage across it rises. The result is an apparent clamping of the transient
voltage. The volt-ampere characteristic of a clamping devices is generally time-dependent as the large
increase in current results in the device dissipating a lot of energy.

Crowbar devices are another type of transient suppression device which diverts over voltage spikes away
from a circuit as a result of a switching type turn-on action. Crowbar devices are similar in operation to a
zener diode in that under normal steady-state conditions they have no effect on the circuit. When a transient
is detected, they rapidly switch “ON” offering a very low impedance path which diverts the transient away
from the parallel-connected load.
Then discrete transient suppression devices can be divided into three basic categories depending on their
type of connection and operation.

 Series (blocking) connected Low Pass Filters.

 Parallel (shunting) connected Voltage Clampers and Voltage Clippers.
 Parallel (shunting) connected Crowbar devices.

and this can be shown as:

Transient Suppression Devices

Series Transient Suppression Filters

Transients on an AC power line can range from a few volts to over several kilo-volts above the normal
mains voltage. Suppression devices which attenuate or block these transient use filter circuits to effectively
eliminate these mains born transients by inserting a 100Hz filter in series with the connected load.

The frequency component of a fast switching voltage transient can be much higher than the slow moving
fundamental frequency of the AC source. Thus an obvious choice to attenuate and control these unwanted
transients is to use a low-pass filter section between the source and the load.

Low pass filters, such as an LC filter, can be used to attenuate any high frequency transients and allow the
low-frequency power or signal to pass through undisturbed. The simplest form of transient suppression filter
is that of a resistor-capacitor RC filter placed directly across the power line to attenuate any high frequencies
transients.

Filters intended for AC power applications generally comprise of inductances and capacitors to form
multistage LC filters whose degree of attenuation depends on the number of LC stages in the filter. A typical
series connected AC mains transient suppression filter is shown below.

Typical Transient Suppression Filter

This basic two-stage low-pass AC filter provides a high insertion loss between line-to-line and line-to-
ground throughout the frequency range offering effective transient voltage protection by stopping any high-
frequency transient and noise from reaching the connected load equipment. Also as well as reducing voltage
spikes and transients, these mains power filters can help eliminate any radio-frequency interference or
emissions given off by the power supply.

Voltage Clamping Transient Suppressors

Voltage clampers are used to limit the amplitude of a transient across a circuit. A voltage clamping device
begins conducting when a preset threshold voltage is exceeded, then returns back to a non-conducting mode
when the overvoltage drops below its threshold level. Thus overvoltage spikes are clipped off to a safe level
by the clamping device.

Voltage clamping devices are generally placed across the supply and in parallel with the load to protect it
against any unwanted high dv/dt voltage transients. A voltage clamper can be something as simple as a zener
diode across a DC supply, but for bi-directional AC supplies we need to use metal oxide varistor (MOV),
suppression diodes or voltage dependent resistor (VDR) for over-voltage protection.

Note that voltage clamping devices divert surge currents, they do not absorb them as with a filter, so care
must be taken to ensure that the path used to divert the transient does not produce or create its own problems
for the circuit.

Zener Diode Transient Suppressors

Zener diodes are used for protection on DC supplies (unidirectional) as they behave like normal diodes in
their forward biased direction, but break down and conduct in their reverse biased direction. Thus a zener
diode’s reverse breakdown voltage, VZ can be used as the reference or clamping voltage level.

In the reverse direction and below the their zener breakdown voltage, VZ zener diodes exhibit high
impedance to the supply and conducts very little leakage current. However, when the voltage across the
zener is greater than its zener voltage, it starts to breakdown with its conduction increasing gradually as the
voltage across it increases exhibiting a very low impedance path to the over voltage transient.

Zener Transient Suppression

When connected across a supply or across the components being protected, the zener diode is effectively
“invisible” until a transient voltage appears as it has a high impedance below its reverse breakdown voltage
and a low impedance above its reverse breakdown voltage.

When the zener is in the breakdown mode of operation, that is when suppressing a transient, the diode
clamps the over voltage instantly to limit the spike to a safe level and then returns back to normal once the
transient voltage is below the zener voltage, VZ. Then the clamping voltage, VC is therefore equal to the
zener’s reverse breakdown voltage. Because of these clamping characteristics, the zener diode is used to
suppress transients as it clamps potentially damaging currents away from the protected load.

The surge current and power capability of the zener diode is approximately proportional to its junction area.
Most zener diodes are designed to operate at low power and voltage levels. Zener diodes designed to operate
at higher voltage levels and absorb higher surge currents without damage are known as Avalanche Diodes.
We said earlier that a single zener diode can only be used for transient suppression on steady state DC
supplies due to their forward biased diode characteristics. But by connecting two zener diodes “back-to-
back” we can use their clamping characteristics across a bidirectional AC supply.

Zener Transient Suppression

By connecting two zener diodes back-to-back we can now protect both the positive half cycle from
overvoltage transients with one zener diode and the negative half cycle with the other.

If both the zener diodes are of the same reverse breakdown voltage, then a transient voltage of either polarity
will be clamped at the same zener voltage level as one zener diode will be effectively in its reverse bias
mode while the other will be in its forward bias mode.

While two back-to-back zener diodes can be used for transient suppression of an AC supply, transient
voltage suppressor (TVS) devices are available with opposing junctions built into a single device making
them ideal for AC power applications. Bidirectional avalanche diodes are available in a range of voltage and
power levels.

MOV Transient Suppressors

While zener diodes and fast recovery avalanche diodes are fast acting and effective at clamping
overvoltages, the most common overvoltage suppression clamping technique is the use of metal oxide
varistors, or MOV’s. As well as their high voltage ratings, metal oxide varistors are capable of handling
much larger surge currents, be it at a slower rate, and can be used in both DC and AC power lines to protect
from voltage extremes such as overvoltage transients.

The MOV is a semiconductor voltage-dependent variable resistor which is placed in parallel (shunt) with the
load, or component to be protected. MOV’s have a high resistance at low voltage and low resistance at high
voltage and their non-linear voltage-current characteristics make them useful in guarding against power-line
surges and overvoltage transients.

MOV’s behave in a similar manner to back-to-back zener diodes as they can be used for bidirectional
voltage clamping with the conduction of the transient increasing as the voltage across it increases. These
small disk-shaped metal-oxide type of varistors offer high breakdown voltages in both directions and can
absorb higher amounts of energy, they are often rated in joules rather than watts.

MOV Transient Suppression

Being a voltage clamping device, metal oxide varistors offer very high resistances when the voltage across
its terminals is below its pre-determined breakdown value acting more like a voltage dependent resistor
(VDR). When exposed to high transient voltage of either polarity, the electrical characteristics of the device
changes and its resistance becomes very small clamping the voltage to a safe level.

Then the main purpose of the metal oxide varistor when used as a transient suppression device is to clamp
the voltage appearing across it to a safe level as in most applications, the device is placed in parallel with the
circuit or device to be protected.

Crowbar Transient Suppressors

Another type of parallel (shunt) connected transient suppression device is known as crowbar protection
Electronic crowbar devices conduct when a preset threshold voltage is exceeded by triggering to a
conductive on-state resulting in a voltage drop of only a few volts, hence the name crowbar.

Crowbar devices and circuits effectively create a short circuit when a trigger voltage is reached and are
commonly found in stabilised power supplies which have been designed to produce a fixed output voltage,
for example a constant 12 volts or 5 volts, but can also be used to protect a circuit or load from transient
over voltages.

Semiconductor-based active crowbar circuits are placed in parallel (shunt) with the load and are capable of
attenuating very large surge currents. Thyristors are generally used in crowbar circuits as they have a low
“on-state” voltage and can keep voltage levels well below damaging levels. Once fired they can divert a
substantial amount of transient energy to ground through themselves as they act as very low impedance type
switch.

The disadvantage here is that this short circuit may cause circuit fuses or circuit breakers to operate if
additional commutation circuitry is not provided to turn “OFF” the crowbar clamp once switched “ON”
especially in a DC system because the power supply is shorted by the crowbar device and the output voltage
will therefore be zero. Consider the simple crowbar clamping circuit below.

Basic Crowbar Clamping Circuit

Here a thyristor or SCR is placed across the supply and the load with the voltage divider circuit set up by
resistors R1 and R2 set to bias the thyristors gate at a level low enough for it not to be triggered “ON” during
normal operation. Then the SCR is cut-off and non-conducting.

However when an over voltage transient occurs and rises above a predetermined level, the voltage drop
across resistor R2 also increases and becomes sufficient to trigger the gate of the SCR into conduction which
in turn clamps the voltage transient protecting the load. The problem here is that while the load is protected
from the over voltage, it does not protect the power supply thereby blowing the fuse of the power supply.
Then the protection of the load from the transient created by short-circuiting the power supply may be
greater than the event that triggered it.

As well as using thyristors, for over voltage protection of AC power supplies, triacs can be used as a
crowbar device and triggered into conduction in a similar way. The advantage of using thyristors or triacs
for crowbar protection of AC supplies is that they will automatically turn-off at every half-cycle.

So if a short-duration transient of a fraction of a millisecond triggers the crowbar device, the shunting action
would only short-circuit the AC power line to which it is connected for at least one half-cycle which may be
too fast for the fuse-link to blow.

Zener Crowbar Transient Suppressors

We can improve the transient sensing and performance of the basic crowbar circuit above by using a zener
diode to detect an over voltage condition. Here the resistive voltage divider circuit has been replaced by a
zener diode as shown.

Zener Crowbar Clamping Circuit

The DC supply voltage, VS is monitored by the zener diode which is acting like a transient detection
component, and whose zener voltage, VZ rating determines the voltage level at which the SCR turns on.
When the DC supply voltage is lower than the reverse bias rating of the zener diode, the zener diode does
not conduct so no voltage or current is applied to the gate of the SCR so remains turned “OFF”, non-
conducting.

If the supply voltage increases above the zener voltage rating as in the case of an overvoltage transient, the
zener diode starts to conduct allowing gate current to flow into the SCR turning it “ON” and shorting out the
load supply voltage and blowing the fuse. Then the load is protected from transient voltages above the zener
voltage, VZ as the zener diode only carries the gate current for SCR to turn “ON”, as the SCR itself will
carry the bulk of the shunt current.

While this zener crowbar circuit is an improvement on the basic voltage divider network, it suffers from a
soft turn-on action because the knee at zener breakdown voltage is curved rather than sharp rise. The basic
crowbar circuit that can be modified and improved further by adding some voltage gain to the detection and
triggering circuit in the form of a single amplifier circuit or op-amp circuit.

To that end, thyristors with an over voltage trigger built in have been designed to crowbar unidirectional or
bidirectional transients and voltage surges. Such as the The RCA SK9345 series of IC crowbars which are
designed to protect power supplies of 15 volts, the SK9346 which protects 112 volts, and the SK9347
protects which 115 volt supplies.

All use an integrated circuit with a built-in zener diode, transistors, and an SCR. The MC3423 over voltage
crowbar sensing circuit is a single IC designed to be used with an external crowbar SCR.

Transient Suppression Devices Summary

As we use more electronic devices in our daily lives, we are becoming more dependent on over voltage
protective devices for their role in protecting our equipment from voltage pikes and surges. Transient over
voltages are usually caused by inductive or capacitive switching circuits which release sudden, high-voltage
spikes.

These voltage spikes and surges can consist of high energy for a short period of time, or intermittently for
short periods of time and are superimposed on top of a stead-state value such as an AC mains waveform.

Over voltage protection circuits can take many forms from series connected filters which are designed to
pass power-line frequency voltages and currents while rejecting unwanted high frequency harmonics and
noise, to parallel connected clamping and crowbar circuits which dissipates the over voltage to ground.

The simplest type of AC power-line filter is a capacitor placed across the voltage source. The impedance of
the capacitor changes resulting in attenuation of high-frequency transients. In most applications, the
transient suppression device is placed in parallel with the protected load, or in parallel with some component
to be protected.

The main purpose of a voltage suppression circuit is to clamp the voltage to a safe level. The most common
form of voltage clamping devices are metal oxide varistors, MOV’s and Zener Diodes. MOV’s are best
suited for protection on bidirectional AC power supplies, while zener diodes are best suited for smaller low
energy DC supplies.

Solid-state crowbar circuits which use an SCR or triac as a “crowbar” rapidly shorts the voltage transient
across the power supply to blow the fuse for over-voltage protection. Hybrid transient/surge protectors
combine a crowbar with a clamp, or a clamp/crowbar with a filter, in one module and there are many
different combinations are possible.

Solid State Relay

Solid State Relays are semiconductor equivalents of the electromechanical relay and can be used to control
electrical loads without the use of moving parts
Unlike electro-mechanical relays (EMR) which use coils, magnetic fields, springs and mechanical contacts
to operate and switch a supply, the solid state relay, or SSR, has no moving parts but instead uses the
electrical and optical properties of solid state semiconductors to perform its input to output isolation and
switching functions.

Just like a normal electro-mechanical relay, SSR’s provide complete electrical isolation between their input
and output contacts with its output acting like a conventional electrical switch in that it has very high, almost
infinite resistance when nonconducting (open), and a very low resistance when conducting (closed). Solid
state relays can be designed to switch both AC or DC currents by using an SCR, TRIAC, or switching
transistor output instead of the usual mechanical normally-open (NO) contacts.

While the solid state relay and electro-mechanical relay are fundamentally similar in that their low voltage
input is electrically isolated from the output that switches and controls a load, electro-mechanical relays
have a limited contact life cycle, can take up a lot of room and have slower switch speeds, especially large
power relays and contactors. Solid state relays have no such limitations.

Thus the main advantages solid state relays have over conventional electro-mechanical relays is that they
have no moving parts to wear out, and therefore no contact bounce issues, are able to switch both “ON” and
“OFF” much faster than a mechanical relays armature can move, as well as zero voltage turn-on and zero
current turn-off eliminating electrical noise and transients.

Solid state relays can be bought in standard off-the-shelf packages ranging from just a few volts or amperes
to many hundreds of volts and amperes of output switching capability. However, solid state relays with very
high current ratings (150A plus) are still too expensive to buy due to their power semiconductor and heat
sinking requirements, and as such, cheaper electro-mechanical contactors are still used.

Similar to an electro-mechanical relay, a small input voltage, typically 3 to 32 volts DC, can be used to
control a much large output voltage, or current. For example 240V, 10Amps. This makes them ideal for
microcontroller, PIC and Arduino interfacing as a low-current, 5-volt signal from say a micro-controller or
logic gate can be used to control a particular circuit load, and this is achieved with the use of opto-isolators.

Solid State Relay Input

One of the main components of a solid state relay (SSR) is an opto-isolator (also called an optocoupler)
which contains one (or more) infra-red light-emitting diode, or LED light source, and a photo sensitive
device within a single case. The opto-isolator isolates the input from the output.

The LED light source is connected to the SSR’s input drive section and provides optical coupling through a
gap to an adjacent photo sensitive transistor, darlington pair or triac. When a current passes through the
LED, it illuminates and its light is focused across the gap to a photo-transistor/photo-triac.
Thus the output of an opto-coupled SSR is turned “ON” by energising this LED, usually with low-voltage
signal. As the only connection between the input and output is a beam of light, high voltage isolation
(usually several thousand volts) is achieved by means of this internal opto-isolation.

Not only does the opto-isolator provide a higher degree of input/output isolation, it can also transmit dc and
low-frequency signals. Also, the LED and photo-sensitive device could be totally separate from each other
and optically coupled by means of an optical fibre.

The input circuitry of an SSR may consist of just a single current limiting resistor in series with the LED of
the opto-isolator, or of a more complex circuit with rectification, current regulation, reverse polarity
protection, filtering, etc.

To activate or turn “ON” a sold state relay into conduction, a voltage greater than its minimum value
(usually 3 volts DC) must be applied to its input terminals (equivalent to the electro-mechanical relay coil).
This DC signal may be derived from a mechanical switch, a logic gate or micro-controller, as shown.

Solid State Relay DC Input Circuit

When using mechanical contacts, switches, push-buttons, other relay contacts, etc, as the activating signal,
the supply voltage used can be equal to the SSR’s minimum input voltage value, whereas when using solid
state devices such as transistors, gates and micro-controllers, the minimum supply voltage needs to be one or
two volts above the SSR’s turn-on voltage to account for the switching devices internal voltage drop.

But as well as using a DC voltage, either sinking or sourcing, to switch the solid state relay into conduction,
we can also use a sinusoidal waveform as well by adding a bridge rectifier for full-wave rectification and a
filter circuit to the DC input as shown.

Solid State Relay AC Input Circuit

 

Bridge rectifiers convert a sinusoidal voltage into full-wave rectified pulses at twice the input frequency.
The problem here is that these voltage pulses start and end from zero volts which means that they will fall
below the minimum turn-on voltage requirements of the SSR’s input threshold causing the output to turn
“on” and “off” every half cycle.

To overcome this erratic firing of the output, we can smooth out the rectified ripples by using a smoothing
capacitor, (C1) on the output of the bridge rectifier. The charging and discharging effect of the capacitor will
raise the the DC component of the rectified signal above the maximum turn-on voltage value of the solid
state relays input. Then even though a constantly changing sinusoidal voltage waveform is used, the input of
the SSR see’s a constant DC voltage.

The values of the voltage dropping resistor, R1 and the smoothing capacitor, C1 are chosen to suit the supply
voltage, 120 volts AC or 240 volts AC as well as the input impedance of the solid state relay. But something
around 40kΩ and 10uF would do.

Then with this bridge rectifier and smoothing capacitor circuit added, a standard DC solid state relay can be
controlled using either an AC or non-polarised DC supply. Of course, manufacturers produce and sell AC
input solid state relays (usually 90 to 280 volts AC) already.

Solid State Relay Output

The output switching capabilities of a solid state relay can be either AC or DC similar to its input voltage
requirements. The output circuit of most standard solid state relays are configured to perform only one type
of switching action giving the equivalent of a normally-open, single-pole, single-throw (SPST-NO)
operation of an electro-mechanical relay.

For most DC SSR’s the solid state switching device commonly used are power transistors, Darlington’s and
MOSFETs, whereas for an AC SSR, the switching device is either a triac or back-to-back thyristors.
Thyristors are preferred due to their high voltage and current capabilities. A single thyristor can also be used
within a bridge rectifier circuit as shown.

Solid State Relay Output Circuit

 

The most common application of solid state relays is in the switching of an AC load, whether that is to
control the AC power for ON/OFF switching, light dimming, motor speed control or other such applications
where power control is needed, these AC loads can be easily controlled with a low current DC voltage using
a solid state relay providing long life and high switching speeds.

One of the biggest advantages of solid state relays over an electromechanical relay is its ability to switch
“OFF” AC loads at the point of zero load current, thereby completely eliminating the arcing, electrical noise
and contact bounce associated with conventional mechanical relays and inductive loads.

This is because AC switching solid state relays use SCR’s and TRIAC’s as their output switching device
which continues conducting, once the input signal is removed, until the AC current flowing through the
device falls below its threshold or holding current value. Then the output of an SSR can never switch OFF in
the middle of a sine wave peak.

Zero current turn-off is a major advantage for using a solid state relay as it reduces electrical noise and the
back-emf associated with the switching of inductive loads as seen as arcing by the contacts of an electro-
mechanical relay. Consider the output waveform diagram below of a typical AC solid state relay.

Solid State Relay Output Waveform

With no input signal applied, no load current flows through the SSR as it is effectively OFF (open-circuited)
and the output terminals see the full AC supply voltage. With the application of a DC input signal, no matter
which part of the sinusoidal waveform, either positive or negative the cycle is going through, due to zero-
voltage switching characteristics of the SSR, the output only turns-on when the waveform crosses over the
zero point.

As the supply voltage increases in either a positive or negative direction, it reaches the minimum value
required to turn the output thyristors or triac fully ON (usually less than about 15 volts). The voltage drop
across the SSR’s output terminals is that of the switching devices on-state voltage drop, VT (usually less than
2 volts). Thus any high inrush currents associated with reactive or lamp loads are greatly reduced.

When the DC input voltage signal is removed, the output does not suddenly turn-off as once triggered into
conduction, the thyristor or triac used as the switching device stays ON for the remainder of the half cycle
until the load currents drops below the devices holding current, at which point it switches OFF. Thus the
high dv/dt back emf’s associated with switching inductive loads in the middle of a sine wave is greatly
reduced.

Then the main advantages of the AC solid state relay over the electro-mechanical relay are its zero crossing
function which turns ON the SSR when the AC load voltage is close to zero volts, thus suppressing any high
inrush currents as the load current will always start from a point close to 0V, and the inherent zero current
turn-off characteristic of the thyristor or triac. Therefore there is a maximum possible turn-off delay
(between the removal of the input signal and the removal of load current) of one half cycle.

Phase Dimming Solid State Relay

While solid state relays can perform straight forward zero-crossing switching of a load, they can also
perform much more complicated functions by means of digital logic circuits, microprocessors and
memories. Another excellent application of a solid state relay is in lamp dimming applications, whether in
the home or for a show or concert.

Non-zero (instant-on) switching solid state relays turn-on immediately after the application of the input
control signal as opposed to the zero crossing SSR above which waits until the next zero-crossing point of
the AC sine-wave. This random-fire switching is used in resistive applications such as lamp dimming and
applications that require the load only to be energised for a small portion of the AC cycle.

Random Switching Output Waveform

While this allows for the phase control of the load waveform, the main problem random turn-on SSR’s is
that the initial load surge current at the instant the relay turns-on, may be high due to the SSR switching
power when the supply voltage is close to its peak value (90o). When the input signal is removed, it stops
conducting when the load current falls below the thyristors or triacs holding current as shown. Obviously for
a DC SSR, the ON-OFF switching action is instant.
The solid state relay is ideal for a wide range of ON/OFF switching applications as they have no moving
parts or contacts unlike an electro-mechanical relay (EMR). There are many different commercial types to
choose from for both AC and DC input control signals as well as AC and DC output switching as they
employ semiconductor switching elements, such as thyristors, triacs and transistors.

But by using a combination of a good opto-isolator and a triac, we can make our own inexpensive and
simple solid state relay to control an AC load such as a heater, lamp or solenoid. As an opto-isolator only
needs a small amount of input/control power to operate, the control signal could be from a PIC, Arduino,
Raspberry PI, or any other such micro-controller.

Solid State Relay Example No1

Lets assume we want a micro-controller with a digital output port signal of only +5 volts to control a 120V
AC, 600 watt heating element. For this we could use the MOC 3020 opto-triac isolator, but the internal triac
can only pass a maximum current (ITSM) of 1 Amps peak at the peak of a 120V AC supply so an additional
switching triac must also be used.

First lets consider the input characteristics of the MOC 3020 opto-isolator (other opto-triacs are available).
The opto-isolators datasheet tells us that the forward voltage, (VF) drop of the input light emitting diode is
1.2 volts and the maximum forward current, (IF) is 50mA.

The LED needs about 10mA to shine reasonably brightly up to its maximum value of 50mA. However the
digital output port of the micro-controller can only supply a maximum of 30mA. Then the value of current
required lies somewhere between 10 and 30 milli-amperes. Therefore:

Thus a series current limiting resistor with a value between 126 and 380Ω’s can be used. As the digital
output port always switches +5 volts and to reduce the power dissipation through the opto-couplers LED, we
will choose a preferred resistive value of 240Ω’s. This gives an LED forward current of less than 16mA. In
this example, any preferred resistor value between 150Ω and 330Ω’s would do.

The heating element load is 600 watts resistive. Using a 120V AC supply would give us a load current of 5
amperes (I = P/V). As we want to control this load current in both half cycles (all 4 quadrants) of the AC
waveform, we would require a mains switching triac.

The BTA06 is a 6 amps (IT(RMS)) 600 volt triac suitable for general purpose ON/OFF switching of AC loads,
but any similar 6 to 8 amp rated triac would do. Also this switching triac requires only 50mA of gate drive to
start conduction which is far less than the 1 amp maximum rating of the MOC 3020 opto-isolator.

Consider that the output triac of the opto-isolator has switched ON at the peak value (90o) of the 120VRMS
AC supply voltage. This peak voltage has a value of: 120 x 1.414 = 170Vpk. If the opto-triacs maximum
current (ITSM) is 1 ampere peak, then the minimum value of series resistance require is 170/1 = 170Ω’s, or
180Ω’s to the nearest preferred value. This value of 180Ω’s will protect the opto-coupler output triac, as
well as the gate of the BTA06 triac on a 120VAC supply.

If the triac of the opto-isolator switches ON at the zero crossover value (0o) of the 120VRMS AC supply
voltage, then the minimum voltage required to supply the required 50mA gate drive current forcing the
switching triac into conduction will be: 180Ω x 50mA = 9.0 volts. Then the triac fires into conduction when
the sinusoidal Gate-to-MT1 voltage is greater than 9 volts.

Thus the minimum voltage required after the zero crossover point of the AC waveform would be 9 volts
peak with the power dissipation in this series gate resistor being very small so a 180Ω, 0.5 watt rated resistor
could safely be used. Consider the circuit below.

AC Solid State Relay Circuit

This type of optocoupler configuration forms the basis of a very simple solid state relay application which
can be used to control any AC mains powered load such as lamps and motors. Here we have used the MOC
3020 which is a random switching isolator. The MOC 3041 opto-triac isolator has the same characteristics
but with built-in zero-crossing detection allowing the load to receive full power without the heavy inrush
currents when switching inductive loads.

Diode D1 prevents damage due to reverse connection of the input voltage, while the 56 ohm resistor (R3)
shunts any di/dt currents when the triac is OFF eliminating false triggering. It also ties the gate terminal to
MT1 ensuring the triac turns-off fully.

If used with a pulse width modulated, PWM input signal, the ON/OFF switching frequency should be set to
less than 10Hz maximum for an AC load otherwise the output switching of this solid state relay circuit may
not be able to keep up.

Single Phase Rectification

Rectification is the process of linking an AC power supply to a connected DC load by means of solid state
semiconductor devices

Rectification converts an oscillating sinusoidal AC voltage source into a constant current DC voltage supply
by means of diodes, thyristors, transistors, or converters. This rectifying process can take on many forms
with half-wave, full-wave, uncontrolled and fully-controlled rectifiers transforming a single-phase or three-
phase supply into a constant DC level. In this tutorial we will look at single-phase rectification and all its
forms.

Rectifiers are one of the basic building blocks of AC power conversion with half-wave or full-wave
rectification generally performed by semiconductor diodes. Diodes allow alternating currents to flow
through them in the forward direction while blocking current flow in the reverse direction creating a fixed
DC voltage level making them ideal for rectification.

However, direct current which has been rectified by diodes is not as pure as that obtained from say, a battery
source, but has voltage changes in the form of ripples superimposed on it as a result of the alternating
supply.

But for single phase rectification to take place, we need an AC sinusoidal waveform of a fixed voltage and
frequency as shown.

AC Sinusoidal Waveform

AC waveforms generally have two numbers associated with them. The first number expresses the degree of
rotation of the waveform along the x-axis by which the alternator has rotated from 0-to-360o. This value is
known as the period (T) which is defined as the interval taken to complete one full cycle of the waveform.
Periods are measured in units of degrees, time, or radians. The relationship between a sine waves periods
and frequency is defined as: T = 1/ƒ.

The second number indicates the amplitude of the value, either current or voltage, along the y-axis. This
number gives the instantaneous value from zero to some peak or maximum value ( AMAX, VMAX or IMAX )
indicating the sine waves greatest amplitude before returning back to zero again. For a sinusoidal waveform
there are two maximum or peak values, one for the positive and one for the negative half-cycles.

But as well as these two values, there are two more which are of interest to us for rectification purposes. One
is the waveforms Average Value and the other is its RMS Value. The average value of a waveform is
obtained by adding the instantaneous values of voltage (or current) over one half-cycle and is found as:
0.6365*VP. Note that the average value over one complete cycle of a symmetrical sine wave is zero.

The RMS, root mean squared or effective value of a sinusoid (a sinusoid is another name for a sine wave)
delivers the same amount of energy to a resistance as does a DC supply of the same value. The root mean
square (rms) value of a sinusoidal voltage (or current) is defined as: 0.7071*VP.
Single Phase Rectifier
All single phase rectifiers use solid state devices as their primary AC-to-DC converting device. Single phase
uncontrolled half-wave rectifiers are the simplest and possibly the most widely used rectification circuit for
small power levels as their output is heavily affected by the reactance of the connected load.

For uncontrolled rectifier circuits, semiconductor diodes are the most commonly used device and are so
arranged to create either a half-wave or a full-wave rectifier circuit. The advantage of using diodes as the
rectification device is that by design they are unidirectional devices having an inbuilt one-way pn-junction.

This pn-junction converts the bi-directional alternating supply into a one-way unidirectional current by
eliminating one-half of the supply. Depending upon the connection of the diode, it could for example pass
the positive half of the AC waveform when forward-biased, while eliminating the negative half-cycle when
the diode becomes reverse-biased.

The reverse is also true by eliminate the positive half or the waveform and passing the negative half. Either
way, the output from a single diode rectifier consists of only one half of the 360o waveform as shown.

Half-wave Rectification

The single-phase half-wave rectifier configuration above passes the positive half of the AC supply
waveform with the negative half being eliminated. By reversing the direction of the diode we can pass
negative halves and eliminate the positive halves of the AC waveform. Therefore the output will be a series
of positive or negative pulses.

Thus there is no voltage or current applied to the connected load, RL for half of each cycle. In other words,
the voltage across the load resistance, RL consists of only half waveforms, either positive or negative, as it
operates during only one-half of the input cycle, hence the name of half-wave rectifier.

Hopefully we can see that the diode allows current to flow in one direction only producing an output which
consists of half-cycles. This pulsating output waveform not only varies ON and OFF every cycle, but is only
present 50% of the time and with a purely resistive load, this high voltage and current ripple content is at its
maximum.

This pulsating DC means that the equivalent DC value dropped across the load resistor, RL is therefore only
one half of the sinusoidal waveforms average value. Since the maximum value of the waveforms sine
function is 1 ( sin(90o) ), the Average or Mean DC value taken over one-half of a sinusoid is defined as:
0.637 x maximum amplitude value.

So during the positive half-cycle, AAVE equals 0.637*AMAX. However as the negative half-cycles are removed
due to rectification by the diode, the average value during this period will be zero as shown.
Sinusoids Average Value

So for a half-wave rectifier, 50% of the time there is an average value of 0.637*AMAX and 50% of the time
there is zero. If the maximum amplitude is 1, the average or DC value equivalent seen across the load
resistance, RL will be:

Thus the corresponding expressions for the average value of voltage or current for a half-wave rectifier is
given as:

VAVE = 0.318*VMAX

IAVE = 0.318*IMAX

Note that the maximum value, AMAX is that of the input waveform, but we could also use its RMS, or root
mean squared value to find the equivalent DC output value of a single phase half-wave rectifier. To
determine the average voltage for a half-wave rectifier, we multiply the RMS value by 0.9 (form factor) and
divide the product by 2, that is multiplying it by 0.45 giving:

VAVE = 0.45*VRMS

IAVE = 0.45*IRMS

Then we can see that a half-wave rectifier circuit converts either the positive or negative halves of an AC
waveform into a pulsed DC output that has a value of 0.318*AMAX or 0.45*ARMS as shown.

Half-wave Rectifier Average Voltage

Rectification Example No1

A single phase half-wave rectifier is connected to a 50V RMS 50Hz AC supply. If the rectifier is used to
supply a resistive load of 150 Ohms. Calculate the equivalent DC voltage developed across the load, the
load current and power dissipated by the load. Assume ideal diode characteristics.
First we need to convert the 50 volts RMS to its peak or maximum voltage equivalent.

a) Maximum Voltage Amplitude, VM

VM = 1.414*VRMS = 1.414*50 = 70.7 volts

b) Equivalent DC Voltage, VDC

VDC = 0.318*VM = 0.318*70.7 = 22.5 volts

c) Load Current, IL

IL = VDC ÷ RL = 22.5/150 = 0.15A or 150mA

d) Power Dissipated by the Load, PL

PL = V*I or I2*RL = 22.5*0.15 = 3.375W ≅  3.4W

In practice, VDC would be slightly less due to the forward biased 0.7 volt voltage drop across the rectifying
diode.

One of the main disadvantages of a single-phase half-wave rectifier is that there is no output during half of
the available input sinusoidal waveform resulting in a low average value as we have seen. One way to
overcome this is to use more diodes to produce a full-wave rectifier.

Full-wave Rectification
Unlike the previous half-wave rectifier, the full-wave rectifier utilises both halves of the input sinusoidal
waveform to provide a unidirectional output. This is because the full-wave rectifier basically consists of two
half-wave rectifiers connected together to feed the load.

The single phase full-wave rectifier does this by using four diodes arranged in a bridge arrangement passing
the positive half of the waveform as before but inverting the negative half of the sine wave to create a
pulsating DC output. Even though the the voltage and current output from the rectifier is pulsating, it does
not reverse direction using the full 100% of the input waveform and thus providing full-wave rectification.

Single Phase Full-wave Bridge Rectifier

This bridge configuration of diodes provides full-wave rectification because at any time two of the four
diodes are forward biased while the other two are reverse biased. Thus there are two diodes in the
conduction path instead of the single one for the half-wave rectifier. Therefore there will be a difference in
voltage amplitude between VIN and VOUT due to the two forward voltage drops of the serially connected
diodes. Here as before, for simplicity of the maths we will assume ideal diodes.

So how does the single phase full-wave rectifier work. During the positive half cycle of VIN, diodes D1 and
D4 are forward biased while diodes D2 and D3 are reverse biased. Then for the positive half cycle of the input
waveform, current flows along the path of: D1 – A – RL – B – D4 and back to the supply.

During the negative half cycle of VIN, diodes D3 and D2 are forward biased while diodes D4 and D1 are
reverse biased. Then for the negative half cycle of the input waveform, current flows along the path of: D3 –
A – RL – B – D2 and back to the supply.

In both cases the positive and negative half-cycles of the input waveform produce positive output peaks
regardless of polarity of input waveform and as such the load current, i always flows in the SAME direction
through the load, RL between points or nodes A and B. Thus the negative half-cycle of the source becomes a
positive half-cycle at load.

So whichever set of diodes are conducting, node A is always more positive than node B. Therefore the load
current and voltage are unidirectional or DC giving us the following output waveform.

Full-wave Rectifier Output Waveform

Although this pulsating output waveform uses 100% of the input waveform, its average DC voltage (or
current) is not at the same value. We remember from above that the average or mean DC value taken over
one-half of a sinusoid is defined as: 0.637 x maximum amplitude value. However unlike half-wave
rectification above, full-wave rectifiers have two positive half-cycles per input waveform giving us a
different average value as shown.

Full-wave Rectifier Average Value

Here we can see that for a full-wave rectifier, for each positive peak there is an average value of 0.637*AMAX
and as there are two peaks per input waveform, this means there are two lots of average value summed
together. Thus the DC output voltage of a full-wave rectifier is twice that of the previous half-wave rectifier.
If the maximum amplitude is 1, the average or DC value equivalent seen across the load resistance, RL will
be:

Thus the corresponding expressions for the average value of voltage or current for a full-wave rectifier is
given as:

VAVE = 0.637*VMAX

IAVE = 0.637*IMAX

As before, the maximum value, AMAX is that of the input waveform, but we could also use its RMS, or root
mean squared value to find the equivalent DC output value of a single phase full-wave rectifier. To
determine the average voltage for a full-wave rectifier, we multiply the RMS value by 0.9 giving:

VAVE = 0.9*VRMS

IAVE = 0.9*IRMS

Then we can see that a full-wave rectifier circuit converts BOTH the positive or negative halves of an AC
waveform into a pulsed DC output that has a value of 0.637*AMAX or 0.9*ARMS as shown.

Full-wave Rectifier Average Voltage

Rectification Example No2

Four diodes are used to construct a single phase full-wave bridge rectifier circuit which is required to supply
a purely resistive load of 1kΩ at 220 volts DC. Calculate the RMS value of the input voltage required, the
total load current drawn from the supply, the load current passed by each diode and the total power
dissipated by the load. Assume ideal diode characteristics.

a) Rectifier Supply Voltage, VRMS

VDC = 0.9*VRMS  therefore: VRMS = VDC ÷ 0.9 = 220/0.9 = 244.4 VRMS

b) Load Current, IL

IL = VDC ÷ RL = 220/1000 = 0.22A or 220mA
c) Load Current Passed by Each Diode, ID

The load current is supplied by two diodes per cycle, thus:

ID = IL ÷ 2 = 0.22/2 = 0.11A or 110mA

d) Power Dissipated by the Load, PL

PL = V*I or I2*RL = 220*0.22 = 48.4W

Full-wave Half-controlled Bridge Rectifier

Full-wave rectification has many advantages over the simpler half-wave rectifier, such as the output voltage
is more consistent, has a higher average output voltage, the input frequency is doubled by the process of
rectification, and requires a smaller capacitance value smoothing capacitor if one is required. But we can
improve on the design of the bridge rectifier by using thyristors instead of diodes in its design.

By replacing the diodes within a single phase bridge rectifier with thyristors, we can create a phase-
controlled AC-to-DC rectifier for converting the constant AC supply voltage into a controlled DC output
voltage. Phase controlled rectifiers either half-controlled or fully controlled, have many applications in
variable voltage power supplies and motor control.

The single phase bridge rectifier is what is termed an “uncontrolled rectifier” in that the applied input
voltage is passed directly to the output terminals providing a fixed average DC equivalent value. To convert
an uncontrolled bridge rectifier into a single phase half-controlled rectifier circuit we just need to replace
two of the diodes with thyristors (SCR’s) as shown.

Half-controlled Bridge Rectifier

In the half-controlled rectifier configuration, the average DC load voltage is controlled using two thyristors
and two diodes. As we learnt in our tutorial about Thyristors, a thyristor will only conduct (“ON” state)
when its Anode, (A) is more positive than its Cathode, (K) and a firing pulse is applied to its Gate, (G)
terminal. Otherwise it remains inactive.

We also learnt that once “ON”, a thyristor is only turned “OFF” again when its gate signal is removed and
the anode current has fallen below the thyristors holding current, IH as the AC supply voltage reverse biases
it. So by delaying the firing pulse applied to the thyristors gate terminal for a controlled period of time, or
angle (α), after the AC supply voltage has passed the zero-voltage crossing of the anode-to-cathode voltage,
we can control when the thyristor starts to conduct current and hence control the average output voltage.

Half-controlled Bridge Rectifier

During the positive half cycle of the input waveform, current flows along the path of: SCR1 and D2, and back
to the supply. During the negative half cycle of VIN, conduction is through SCR2 and D1 and back to the
supply.

It is clear then that one thyristor from the top group (SCR1 or SCR2) and its corresponding diode from the
bottom group (D2 or D1) must conduct together for for any load current to flow.

Thus the average output voltage, VAVE is dependant on the firing angle α for the two thyristors included in
the half-controlled rectifier as the two diodes are uncontrolled and pass current whenever forward biased. So
for any gate firing angle, α, the average output voltage is given by:

Half-controlled Rectifier Average Output Voltage

Note that the maximum average output voltage occurs when α = 1 but is still only 0.637*VMAX the same as
for the single phase uncontrolled bridge rectifier.

We can take this idea of controlling the average output voltage of the bridge one step further by replacing all
four diodes with thyristors giving us a Fully-controlled Bridge Rectifier circuit.

Fully-controlled Bridge Rectifier

Single phase fully-controlled bridge rectifiers are known more commonly as AC-to-DC converters. Fully-
controlled bridge converters are widely used in the speed control of DC machines and is easily obtained by
replacing all four diodes of a bridge rectifier with thyristors as shown.

Fully-controlled Bridge Rectifier

In the fully-controlled rectifier configuration, the average DC load voltage is controlled using two thyristors
per half-cycle. Thyristors SCR1 and SCR4 are fired together as a pair during the positive half-cycle, while
thyristors SCR3 and SCR4 are also fired together as a pair during the negative half-cycle. That is 180o after
SCR1 and SCR4.

Then during continuous conduction mode of operation the four thyristors are constantly being switched as
alternate pairs to maintain the average or equivalent DC output voltage. As with the half-controlled rectifier,
the output voltage can be fully controlled by varying the thyristors firing delay angle (α).

Thus the expression for the average DC voltage from a single phase fully-controlled rectifier in its
continuous conduction mode is given as:

Fully-controlled Rectifier Average Output Voltage

with the average output voltage varying from VMAX/π to -VMAX/π by varying the firing angle, α from π to 0
respectively. So when α < 90o the average DC voltage is positive and when α > 90o the average DC voltage
is negative. That is power flows from the DC load to the AC supply.

Then we have seen here in this tutorial about single phase rectification that single phase rectifiers can take
on many forms to convert AC voltage to DC voltage from uncontrolled single diode half-wave rectifiers to
fully-controlled full-wave bridge rectifiers using four thyristors.

The advantages of the half-wave rectifier are its simplicity and low cost as it requires only one diode.
However, it is not very efficient as only half of the input signal is used producing a low average output
voltage.

The full-wave rectifier is more efficient than the half-wave rectifier as it uses both half-cycles of the input
sine wave producing a higher average or equivalent DC output voltage. A disadvantage of the full-wave
bridge circuit is that is that it requires four diodes.
Phase controlled rectification uses combinations of diodes and thyristors (SCR’s) to convert the AC input
voltage into a controlled DC output voltage. Fully-controlled rectifiers use four thyristors in their
configuration, whereas half-controlled rectifiers use a combination of both thyristors and diodes.

Then no matter how we do it, the conversion of a sinusoidal AC waveform to a steady state DC supply is
called Rectification.

Three Phase Rectification

3-phase rectification is the process of converting a balanced 3-phase power supply into a fixed DC supply
using solid state diodes or thyristors

We saw in the previous tutorial that the process of converting an AC input supply into a fixed DC supply is
called Rectification with the most popular circuits used to perform this rectification process is one that is
based on solid-state semiconductor diodes. In fact, rectification of alternating voltages is one of the most
popular applications of diodes, as diodes are inexpensive, small and robust allowing us to create numerous
types of rectifier circuits using either individually connected diodes or with just a single integrated bridge
rectifier module.

Single phase supplies such as those in houses and offices are generally 120 Vrms or 240 Vrms phase-to-
neutral, also called line-to-neutral (L-N), and nominally of a fixed voltage and frequency producing an
alternating voltage or current in the form of a sinusoidal waveform being given the abbreviation of “AC”.

Three-phase rectification, also known as poly-phase rectification circuits are similar to the previous single-
phase rectifiers, the difference this time is that we are using three, single-phase supplies connected together
that have been produced by one single three-phase generator.

The advantage here is that 3-phase rectification circuits can be used to power many industrial applications
such as motor control or battery charging which require higher power requirements than a single-phase
rectifier circuit is able to supply.

3-phase supplies take this idea one step further by combining together three AC voltages of identical
frequency and amplitude with each AC voltage being called a “phase”. These three phases are 120 electrical
degrees out-of-phase from each other producing a phase sequence, or phase rotation of: 360o ÷ 3 = 120o as
shown.

Three-phase Waveform
The advantage here is that a three-phase alternating current (AC) supply can be used to provide electrical
power directly to balanced loads and rectifiers. Since a 3-phase supply has a fixed voltage and frequency it
can be used by a rectification circuit to produce a fixed voltage DC power which can then be filtered
resulting in an output DC voltage with less ripple compared to a single-phase rectifying circuit.

Three-phase Rectification
Having seen that a 3-phase supply is just simply three single-phases combined together, we can use this
multi-phase property to create 3-phase rectifier circuits.

As with single-phase rectification, three-phase rectification uses diodes, thyristors, transistors, or converters
to create half-wave, full-wave, uncontrolled and fully-controlled rectifier circuits transforming a given three-
phase supply into a constant DC output level. In most applications a three-phase rectifier is supplied directly
from the mains utility power grid or from a three-phase transformer if different DC output level is required
by the connected load.

As with the previous single-phase rectifier, the most basic three-phase rectifier circuit is that of an
uncontrolled half-wave rectifier circuit which uses three semiconductor diodes, one diode per phase as
shown.

Half-wave Three-phase Rectification

So how does this three-phase half-wave rectifier circuit work. The anode of each diode is connected to one
phase of the voltage supply with the cathodes of all three diodes connected together to the same positive
point, effectively creating a diode-“OR” type arrangement. This common point becomes the positive (+)
terminal for the load while the negative (-) terminal of the load is connected to the neutral (N) of the supply.
Assuming a phase rotation of Red-Yellow-Blue (VA – VB – VC) and the red phase (VA) starts at 0o. The first
diode to conduct will be diode 1 (D1) as it will have a more positive voltage at its anode than diodes D2 or
D3. Thus diode D1 conducts for the positive half-cycle of VA while D2 and D3 are in their reverse-biased
state. The neutral wire provides a return path for the load current back to the supply.

120 electrical degrees later, diode 2 (D2) starts to conduct for the positive half-cycle of VB (yellow phase).
Now its anode becomes more positive than diodes D1 and D3 which are both “OFF” because they are
reversed-biased. Similarly, 120o later VC (blue phase) starts to increase turning “ON” diode 3 (D3) as its
anode becomes more positive, thus turning “OFF” diodes D1 and D2.

Then we can see that for three-phase rectification, whichever diode has a more positive voltage at its anode
compared to the other two diodes it will automatically start to conduct, thereby giving a conduction pattern
of: D1 D2 D3 as shown.

Half-wave Three-phase Rectifier Conduction Waveform

From the above waveforms for a resistive load, we can see that for a half-wave rectifier each diode passes
current for one third of each cycle, with the output waveform being three times the input frequency of the
AC supply. Therefore there are three voltage peaks in a given cycle, so by increasing the number of phases
from a single-phase to a three-phase supply, the rectification of the supply is improved, that is the output DC
voltage is smoother.

For a three-phase half-wave rectifier, the supply voltages VA VB and VC are balanced but with a phase
difference of 120o giving:

VA = VP*sin(ωt – 0o)

VB = VP*sin(ωt – 120o)

VC = VP*sin(ωt – 240o)

Thus the average DC value of the output voltage waveform from a 3-phase half-wave rectifier is given as:
As the voltage supplies peak voltage, VP is equal to VRMS*1.414, it follows that VP is equal to VP/1.414
giving 0.707*VP, so the average DC output voltage of the rectifier can be expressed in terms of the rms
(root-mean-squared) phase voltage giving:

3-phase Rectification Example No1

A half-wave 3-phase rectifier is constructed using three individual diodes and a 120VAC 3-phase star
connected transformer. If it is required to power a connected load with an impedance of 50Ω, Calculate, a)
the average DC voltage output to the load. b) the load current, c) the average current per diode. Assume
ideal diodes.

a). The average DC load voltage:

VDC = 1.17*Vrms = 1.17*120 = 140.4 volts

Note that if we were given the peak voltage (Vp) value, then:

VDC would equal 0.827*Vp or 0.827*169.68 = 140.4V.

b). The DC load current:

IL = VDC/RL = 140.4/50 = 2.81 amperes

c). The average current per diode:

ID = IL/3 = 2.81/3 = 0.94 amperes

One of the disadvantages of half-wave 3-phase rectification is that it requires a 4-wire supply, that is three
phases plus a neutral (N) connection. Also the average DC output voltage is low at a value represented by
0.827*VP as we have seen. This is because the output ripple content is three times the input frequency. But
we can improve on these disadvantages by adding three more diodes to the basic rectifier circuit creating a
three-phase full-wave uncontrolled bridge rectifier.

Full-wave Three-phase Rectification

The full-wave three-phase uncontrolled bridge rectifier circuit uses six diodes, two per phase in a similar
fashion to the single-phase bridge rectifier. A 3-phase full-wave rectifier is obtained by using two half-wave
rectifier circuits. The advantage here is that the circuit produces a lower ripple output than the previous half-
wave 3-phase rectifier as it has a frequency of six times the input AC waveform.
Also, the full-wave rectifier can be fed from a balanced 3–phase 3-wire delta connected supply as no fourth
neutral (N) wire is required. Consider the full-wave 3-phase rectifier circuit below.

Full-wave Three-phase Rectification

As before, assuming a phase rotation of Red-Yellow-Blue (VA – VB – VC) and the red phase (VA) starts at 0o.
Each phase connects between a pair of diodes as shown. One diode of the conducting pair powers the
positive (+) side of load, while the other diode powers the negative (-) side of load.

Diodes D1 D3 D2 and D4 form a bridge rectifier network between phases A and B, similarly diodes D3 D5 D4
and D6 between phases B and C and D5 D1 D6 and D2 between phases C and A.

Thus diodes D1 D3 and D5 feed the positive rail and depending on which one has a more positive voltage at
its anode terminal conducts. Likewise, diodes D2 D4 and D6 feed the negative rail and whichever diode has a
more negative voltage at its cathode terminal conducts.

Then we can see that for three-phase rectification, the diodes conduct in matching pairs giving a conduction
pattern for the load current of: D1-2 D1-6 D3-6 D3-6 D3-4 D5-4 D5-2 and D1-2 as shown.

Full-wave Three-phase Rectifier Conduction Waveform

In 3-phase power rectifiers, conduction always occurs in the most positive diode and the corresponding most
negative diode. Thus as the three phases rotate across the rectifier terminals, conduction is passed from
diode to diode. Then each diode conducts for 120o (one-third) in each supply cycle but as it takes two diodes
to conduct in pairs, each pair of diodes will conduct for only 60o (one-sixth) of a cycle at any one time as
shown above.

Therefore we can correctly say that for a 3-phase rectifier being fed by “3” transformer secondaries, each
phase will be separated by 360o/3 thus requiring 2*3 diodes. Note also that unlike the previous half-wave
rectifier, there is no common connection between the rectifiers input and output terminals. Therefore it can
be fed by a star connected or a delta connected transformer supply.

So the average DC value of the output voltage waveform from a 3-phase full-wave rectifier is given as:

Where: VS is equal to (VL(PEAK) ÷ √3) and where VL(PEAK) is the maximum line-to-line voltage (VL*1.414).

3-phase Rectification Example No2

A 3-phase full-wave bridge rectifier is required to fed a 150Ω resistive load from a 3-phase 127 volt, 60Hz
delta connected supply. Ignoring the voltage drops across the diodes, calculate: 1. the DC output voltage of
the rectifier and 2. the load current.

1. the DC output voltage:

The RMS (Root Mean Squared) line voltage is 127 volts. Therefore the line-to-line peak voltage (VL-L(PEAK))
will be:

As the supply is 3-phase, the phase to neutral voltage (VP-N) of any phase will be:

Note that this is basically the same as saying:

Thus the average DC output voltage from the 3-phase full-wave rectifier is given as:

Again, we can reduce the maths a bit by correctly saying that for a given line-to-line RMS voltage value, in
our example 127 volts, the average DC output voltage is:
2. the rectifiers load current.

The output from the rectifier is feeding a 150Ω resistive load. Then using Ohms law the load current will be:

Uncontrolled 3-phase rectification uses diodes to provide an average output voltage of a fixed value relative
to the value of the input AC voltages. But to vary the output voltage of the rectifier we need to replace the
uncontrolled diodes, either some or all of them, with thyristors to create what are called half-controlled or
fully-controlled bridge rectifiers.

Thyristors are three terminal semiconductor devices and when a suitable trigger pulse is applied to the the
thyristors gate terminal when its Anode–to-Cathode terminal voltage is positive, the device will conduct and
pass a load current. So by delaying the timing of the trigger pulse, (firing angle) we can delay the instant in
time at which the thyristor would naturally switch “ON” if it were a normal diode and the moment it starts to
conduct when the trigger pulse is applied.

Thus with a controlled 3-phase rectification which uses thyristors instead of diodes, we can control the value
of the average DC output voltage by controlling the firing angle of the thyristor pairs and so the rectified
output voltage becomes a function of the firing angle, α.

Therefore the only difference to the formula used above for the average output voltage of a 3-phase bridge
rectifier is in the cosine angle, cos(α) of the firing or triggering pulse. So if the firing angle is zero, (cos(0) =
1), the controlled rectifier performs similar to the previous 3-phase uncontrolled diode rectifier with the
average output voltages being the same.

An example of a fully-controlled 3-phase bridge rectifier is given below:

Fully-controlled 3-phase Bridge Rectifier

Three-phase Rectification Summary

We have seen in this tutorial that three-phase rectification is the process of converting a 3-phase AC supply
into a pulsating DC voltage as rectification converts the input power supply of a sinusoidal voltage and
frequency into a fixed voltage DC power. Thus power rectification changes an alternating supply into a
unidirectional supply.

But we have also seen that 3-phase half-wave uncontrolled rectifiers, which use one diode per phase, require
a star connected supply as a fourth neutral (N) wire to close the circuit from load to source. The 3-phase full-
wave bridge rectifier which use two diodes per phase requires just three mains lines, without neutral, such as
that provided by a delta connected supply.

Another advantage of a full-wave bridge rectifier is that the load current is well balanced across the bridge
improving efficiency (the ratio of output DC power to input power supplied) and reducing the ripple content,
both in amplitude and frequency, as compared to the half-wave configuration.

By increasing the number of phases and diodes within the bridge configuration it is possible to obtain a
higher average DC output voltage with less ripple amplitude as for example, in 6-phase rectification each
diode would conduct for only one-sixth of a cycle. Also, multi-phase rectifiers produce a higher ripple
frequency means less capacitive filtering and a much smoother output voltage. Thus 6, 12, 15 and even 24-
phase uncontrolled rectifiers can be designed to improve the ripple factor for various applications.