Thyristor Tutorial

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

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.
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

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

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

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. The
quadrac 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

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.