S NAGA MALLESWARA REDDY

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. Diacs have 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.

Diac Symbol and I-V Characteristics

Diac Applications

Diac AC Phase Control

Triac Conduction Waveform

The Quadrac

Diac Tutorial Summary

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 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

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.

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.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 main advantages of using the Insulated Gate Bipolar Transistorover 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

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.

In the previous tutorial we looked at the construction and operation of the Silicon Controlled Rectifier more commonly known as a Thyristor. 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 Anodeto 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 Circuitsthis 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.

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".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).

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!.

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 switching circuits to control 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. Thyristors can operate only in the switching mode, where they act like either an open or closed switch and once triggered it will remain conducting. 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, RGthus 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.

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.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".

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, theThyristor 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 Ahas 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.

Thyristor Basics

In many ways the Silicon Controlled Rectifier, or the Thyristor as it is more commonly known, is similar 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 diode which is a two layer ( P-N ) semiconductor device, or the transistor which is a three layer ( P-N-P, or N-P-N ) device, theThyristor 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 TR2feeds 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 risingdv/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.