Relay (English name: relay) is an electrical control device that makes a predetermined step change in the controlled quantity in the electrical output circuit when the change in the input quantity (excitation quantity) meets the specified requirements. It has an interactive relationship between the control system (also called input circuit) and the controlled system (also called output circuit). Usually used in automatic control circuits, it 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 regulation, safety protection, and conversion circuit in the circuit.