A relay is an electrical control device, which is an electrical appliance that makes a predetermined step change of the controlled quantity in the electrical output circuit when the change of the input quantity meets the specified requirements. It has the interactive relationship between the control system and the controlled system. Usually used in automatic control circuits, a relay is actually an automatic switch that uses a small current to control the operation of a large current. Therefore, it plays the role of automatic adjustment, safety protection, and conversion circuit in the circuit.