Solid State Relay(SSR) is industrial control appliances which use power semiconductor devices for main control part, compare with traditional magnetic contact industrial controllers, it is more high-powered, intelligent, digital, systematic and green development. According switching mode SSR has zero-cross and random type. When the input control signal, random type conduction immediately, zero cross type need load voltage turn to zero region just can conduction. Revocation of the control signal, all can cut off when less than maintenance current. zero cross type fast response, suitable for dimmer, thermostats resistive loads and some inductive load applications.