Electronics Principles V11

Clive W. Humphris


The SCR Silicon Controlled Rectifier is also known as a Thyristor. This device is a diode that has to be triggered into conduction, once holding current flows the gate pulse can be removed and it will continue to conduct.

The only way to switch the SCR off is by removing the anode voltage, thereby stopping the holding current. SCRs are able to handle very large currents making them ideal for power supply applications.

The operation of the SCR can best be explained by connecting together a PNP and an NPN transistor as shown. When TR2 is made to conduct by placing a positive trigger voltage on its Gate the collector current flows through TR1 base which is forward biased, causing TR1 to conduct heavily.

This in turn provides the base drive for TR2 which continues to flow even when the gate trigger voltage is removed. The only way to stop this regenerative conduction is to either remove the Anode potential or make the Cathode voltage higher than that on the Anode (reverse biasing).

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