Silicon controlled rectifiers are three-terminal semiconductor devices (Anode, Cathode and Gate) that conduct current in a single direction (from Anode to Cathode). They are triggered into conduction by applying a positive current into the gate terminal, the device is turned off when the current through it drops below the holding current or when the anode-cathode voltage equals zero while the gate is not biased. They are used in automation applications, lamp dimmers, motor control, and power regulators.