![]() ![]() ![]() 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. 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. So for the Electronics student this makes these very handy solid state devices for controlling AC motors, lamps and for phase control. 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. ![]() In other words, thyristors can operate only in the switching mode and cannot be used for amplification. 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. However, unlike the junction diode which is a two layer ( P-N ) semiconductor device, or the commonly used bipolar transistor which is a three layer ( P-N-P, or N-P-N ) switching device, the Thyristor 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. ![]()
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