Silicon controlled rectifier

Silicon controlled rectifier (SCR) is a type of thyristor. It is a four-layer solid-state current-controlling device.
SCRs are unidirectional devices: they conduct current only in one direction. Normally, SCRs can be triggered only by currents going into the gate.
A rectifier is an electrical device that converts alternating current (AC) to direct current (DC). SCRs are used for rectification of high-power AC in high-voltage direct-current power transmission. They are also used in the control of welding machines.
SCRs are mainly used in devices for the control of high power, perhaps with high voltage. They are suitable for use in medium- to high-voltage AC power control applications, such as lamp dimming, power regulators and motor control.