Gate Pulse

Thyristors are silicon-controlled rectifiers (SCRs) commonly used as switching elements; they can be driven by electronic control circuits to activate and de-activate high-power devices such as lamps, motors and heating systems.

The thyristor is controlled through its Gate terminal, while the power circuit to the large load is connected across its Anode and Cathode. A small gate or trigger pulse must be applied to the Gate to turn the thyristor on, so that it starts conducting the load current. This trigger pulse needs only to last for a few microseconds to initiate switching, but the longer it is applied, the faster the internal avalanche breakdown and hence thyristor switch-on occurs. However, 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 1.0 V for all values of Anode current up to its rated value.

Note that a thyristor, once triggered, remains on – even with the gate signal removed – until the anode current falls below the device’s holding current, when it turns off. Therefore, thyristors can only be used for switching applications, unlike bipolar transistors or FETs that can be used for amplification functions. This also means that in DC circuits and some AC circuits with highly-inductive loads, the current must be artificially reduced by a separate switch or turn-off circuit.