Solid state relays are contactless switches composed of microelectronic circuit, discrete electronic device and power electronic power device. The input of solid state relays uses small control signals to drive large current loads directly. Solid state relays can be used directly in control systems because they have short circuit protection, overload protection and overheat protection. Solid state relays have been widely used in computer peripheral interface equipment, constant temperature system, temperature control, electric furnace heating control, motor control, numerical control machinery, remote control system, industrial automation devices.