AC solid state relay is a kind of contactless on-off electronic switch, in which two small terminals are input control terminals and the other two large terminals are output control terminals. Photoelectric isolation is used between input and output. By adding DC signal to the input, the output can change from off state to on state (blocking state when there is no signal), so as to control large load. The whole device has no movable parts and contacts, which can achieve the same function as the common mechanical electromagnetic relay. It is widely used in electric furnace heating constant temperature system, numerical control machinery, plastic machinery, food machinery, packaging machinery, textile machinery, petrochemical equipment, entertainment facilities and other automation control fields, suitable for all kinds of resistive, inductive and capacitive loads.