Relays are electronic control devices, which have control systems (also called input circuits) and controlled systems (also called the output circuit). They are usually used in automatic control circuits. It is actually with less current to control large current of a kind of "automatic switch", so plays an automatic adjustment, safety protection, conversion circuit. An appliance that turns on or off a controlled output circuit when the input (voltage, current, temperature, etc.) reaches a specified value. Relays are widely used in power protection, automation, motion, remote control, measurement and communication devices.