Silicon Controlled Rectifier (SCR for short) is a high-power electrical component, also known as thyristor. It has the advantages of small volume, high efficiency and long service life. In the automatic control system, it can be used as a high-power driving device to control high-power equipment with low-power controls. It has been widely used in AC/DC motor speed regulation system, power regulation system and servo system. Silicon controlled rectifier is divided into unidirectional silicon controlled rectifier and bidirectional silicon controlled rectifier. The TRIAC is also called triac. The bidirectional thyristor is structurally equivalent to the reverse connection of two unidirectional thyristors, which has the function of bidirectional conduction. Its on-off state is determined by the control pole g. Adding a positive pulse (or negative pulse) to the control electrode G can make it turn on in the forward (or reverse) direction. The advantage of this device is that the control circuit is simple and there is no reverse withstand voltage problem, so it is especially suitable for AC contactless switch.