A thyristor is like a diode; it can pass electricity in a forward direction but, blocks it in a reverse direction. The difference between a diode and a thyristor is that the thyristor is a three terminal device consisting of an anode, cathode and gate, which will also block current flow in the forward direction until it’s gate is signaled to open or, to use more technical jargon, the thyristor is “fired”. Once turned on, the gate signal is removed and the thyristor remains on until the current through it drops to zero, which conveniently happens during each cycle of the main supply. Thyristors are most commonly known as Silicon Controlled Rectifiers (SCRs), Triacs, Thyristor Surge Protective Devices & SIDACs, and Programmable Unijunction Transistor (PUT). Although it can take many forms, they all have certain things in common. “Normally-off” switches, a small current pulse into the gate electrode can trigger thyristors “on”. Once triggered, the component then stays in the conducting state even when the gate on signal is removed. It only returns to the “off” (blocking) state if the current falls below a certain minimum or if the direction of the current is reversed. The circuit symbol is shown below.

When there is no gate current, the device is in the non-conducting state and will hold off positive and negative bias voltages. The zero gate current characteristic is:

Here are some things that thyristors are used for:

Coilgun – Thyristors are the device of choice when using a basic capacitor discharge coilgun. The thyristor will hold off the capacitor voltage until it is triggered into the forward conduction zone by the gate current signal. The capacitor then discharges through the thyristor into the coil and the current decays to zero. There is no option for turning off the current once the device is conducting. *Coilguns have no industrial application use*

Despike ac lines (snubber circuit) – Alternating current transmission lines are susceptible to spikes. Using thyristors on the primary side of a transformer eliminates spiking. The purpose of using thyristors to despike the voltage (dv/dt) is for enhanced controllability. Historically, replacement of mercury-arc valves by thyristors yielded robust ac/ dc converters, minimized conversion losses, and yielded fast control on transmitted power—so much so that line-to-ground fault clearing became possible without the use of circuit breakers. Instead, by rapidly attaining zero current through the use of current controllers and, in addition, by rapidly recovering the electromagnetic energy stored in the energized line, the faulted dc line could be isolated by low interruption–rating isolators. To wit, if the voltage rate of rise is great enough (high enough), the thyristors will turn on controlling the current without interruption.

Medical devices – Thyristors are used in energy-absorbing circuitry to protect implantable devices under test (DUT).  Shown in the schematic, the thyristors are kept at the edge of conduction, which respond in nanoseconds to limit the spike passed into the instrumentation amplifier inputs to a few volts. As the thyristors conduct, the voltage is dropped across resistors R1-4. The use of these thyristors protects circuits to limit the damaging effects of errant high-energy pulses resulting from either test equipment or ICD malfunction. The solution is a set of energy-absorbing circuits set to shunt voltages of more than 16 V quickly, while minimally loading the circuit. Shunting is performed by thyristors D5-8. To ensure quick action and minimal capacitive loading, they are held at a bias voltage near their trigger point by supply rails at +15 V and -15 V. The back-biased diodes D1-4 isolate the rails from the circuit under normal conditions and prevent leakage current in the thyristors from loading the signal line.

These protection circuits are applied on sensitive circuits to divert unintentional energy (spikes). Without them, the reliability of the interface would be in question after any failure of an ICD under test. A similar but simpler protection circuit is used within the ICD to protect its sensitive input circuits from external defibrillator shocks.

Power switches – When used in cases of a 50 Hz supply, the solid state switches (thyristors) will turn on and off fifty times per second (60 times in the case of a 60 Hz supply). Let’s use a motor as an example. Remember that the torque a motor produces is proportional to the square of the applied voltage. Well, for the principle of the electronic soft start, you ramp the motor voltage up from zero to full volts in a controlled fashion and over a set time period. In doing so, the voltage is varied by allowing only part of the sine wave to pass to the motor. This is achieved using thyristors to turn on and off at predetermined points in each cycle of the mains supply frequency. To achieve full wave (360º), wire two thyristors in parallel and in opposite directions, then with correctly controlled firing, all or part of the ac wave can be passed to the motor. One thyristor will pass the positive part of the cycle and the other will pass the negative.

The most surprising thing about thyristors is that they are becoming obsolete. JFETs and MOSs are replacing thyristors because of its ability to amplify and switch. The gate turn-off (GTO) thyristor semiconductor devices facilitate current turn-on, as well as turnoff by using control signals. This technology has grown very rapidly; consequently, high-power GTOs are now available (100 mm, 6 kV or 150 mm, 9 kV). Full on–off control offered by GTOs has made pulse width–modulated (PWM) inverters easy to realize. Advances in semiconductor technology are yielding new efficient, simple-to-operate devices. The insulated gate bipolar transistor (IGBT), and the metaloxide semiconductor (MOS)–controlled thyristor (MCT), control electric power using low levels of energy from their high-impedance MOS gates, as compared to high-current pulses needed for thyristors or GTOs.

The MOS turn-off (MTO) thyristor combines the advantages of both thyristors and MOS devices by using a current-controlled turn-on (thyristor) and a voltage-controlled turn-off having a high-impedance MOS structure. Hybrid MTOs are being proposed that show substantially low device losses relative to GTOs. Because MTOs use nearly half the parts of GTOs, their application promises significant reliability improvement.

Thyristors are now available in large sizes, eliminating the need for paralleling them for high-current applications. Their voltage ratings have also increased, so that relatively few are required to be connected in series to yield switches or converters for power-transmission applications. Actually, the present trend is to produce high-power electronic building blocks (HPEBBs) to configure high-power switches and converters, thus eliminating the custom-design needs at the device level.

Thanks to Wentworth John-NASA, Coilgun Systems, A.G. Bolt Fluidrive Engineering Co. Ltd. and Mohamed E. El-Hawary, Series Editor, IEEE Press Power Series of Power Engineering