thyristor, semiconductor switch used chiefly in power-control applications. Also called a silicon-controlled rectifier (SCR; see rectifier), it is a variation of the transistor. A thyristor is capable of producing large direct currents by rectification of alternating currents and can be automatically triggered "off" for specified periods of time. Thyristors are used in variable-speed electric motors, power supplies for electrochemical processes, lighting and heating control, and controllers for electric utility power systems.