The Silicon-Controlled Rectifier (SCR) - All About Circuits wwwboutcircuits››ThyristorsSCR triggering by Complex Circuits. This extra terminal is called the gate, and it is used to trigger the device into conduction (latch it) by the application of a small voltage. To trigger, or fire, an SCR, voltage must be applied between the gate and cathode, positive to the gate and negative to the cathode. SCR Principles And Circuits | Nuts & Volts Magazine Basic operation and practical application circuits for SCRs. An SCR (Silicon Controlled Rectifier) is a controllable medium- to high-power self-latching solid-state DC power switch. This article explains its basic operation and shows some practical ways of using it.