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) TEST YOUR UNDERSTANDING: Silicon Controlled Rectifiers is another example of using AC Power in electronics circuits. Follow the topics through to SCR Phase control (the same principle applies to the Triac & Diac topic) adjust the load value above to below the default value to see that as you minimise or maximise the load requirements it only uses a part of the AC mains cycle thereby avoiding the wasted energy that might otherwise occur if a purely restive circuit were used.
