A SCR (Silicon Controlled Rectifier) is a type of electronic device that is used in circuits to allow for the control of AC current flow. It is an electronic switching device, which has three electrodes: a gate, a cathode, and an anode. The SCR is triggered when a voltage is applied to the gate electrode, and current flows through the device once it is “turned on.” Once the current is flowing, the SCR will remain in the “on” state, even when the voltage is removed from the gate. This allows the device to act as an “invisible switch” in AC circuits, allowing for the control of current flow. SCRs are commonly used in a variety of applications, such as motor speed control, light dimming, motor starting, and motor braking.