Silicon Controlled Rectifier (SCR) is a type of high-power electrical component, also known as a thyristor. It has the advantages of small size, high efficiency, and long lifespan. In automatic control systems, it can be used as a high-power driving device to control large-power equipment with small-power control elements. It has been widely applied in AC and DC motor speed control systems, power regulation systems, and servo systems.
There are two types of SCRs: unidirectional SCRs and bidirectional SCRs. Bidirectional SCRs are also called three-terminal bidirectional SCRs, abbreviated as TRIAC. The structure of a bidirectional SCR is equivalent to two unidirectional SCRs connected in reverse. This type of SCR has the function of bidirectional conduction. Its on-off state is determined by the control electrode G. Applying a positive pulse (or negative pulse) to the control electrode G can make it conduct in the forward (or reverse) direction. The advantage of this device is that the control circuit is simple, there is no reverse voltage withstand problem, and therefore it is particularly suitable for use as an AC contactless switch.