Thyristor, DIACs and TRIACs
Objectives by the end of the lesson the learner should be able to:
(i.) Describe the construction of Thyristors, DIACs and TRIACs
(ii.) Distinguish between the THREE semiconductors devices
(iii.)Describe the application of the THREE semiconductor devices
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.
Fig 11.1 Thyristor Symbol
However, unlike the junction diode which is a two layer ( P-N ) semiconductor device,
or the common BJT 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. These very handy solid state devices are used 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
Fig 11.2 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.
Very rapidly the two transistors force each other to conduct to saturation as they are
connected in a regenerative feedback loop that cannot 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
Fig 11.3 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 loses 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 cannot 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 circuit’s 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
11.4 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.
Triac
A Triac is a high-speed solid-state device that can switch and control AC power in both
directions of a sinusoidal waveform
Fig 11.5 TRIAC
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
Fig 11.6 Thyristor Configuration
There is however, another type of semiconductor device called a “Triode AC Switch”
or Triac for short. Triac’s are also a member of the thyristor family, and just like silicon
controlled rectifiers, they can be used as a solid state power switching devices but more
importantly triac’s are “bidirectional” devices. 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.
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
Fig 11.6 Triac I-V Characteristic 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
Fig 11.7 Triac Switch 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.
Diac
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
Fig 11.8 DIAC
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.
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.
Fig 11.9 SCR Switch
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.
https://www.electronics-tutorials.ws/power/thyristor.html
https://www.electronics-tutorials.ws/power/triac.html
https://www.electronics-tutorials.ws/power/diac.html
