A relay is an electrical control device. When the change of the input quantity (excitation quantity) reaches the specified requirements, the relay makes a predetermined step change of the controlled quantity in the electrical output circuit. It has an interactive relationship between the control system (also known as the input loop) and the controlled system (also known as the output loop). 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 adjustment, safety protection, and conversion circuit in the circuit. Relays are widely used in industry. Therefore, today we will introduce the structure, function and classification of relays. We hope that after reading this article, you can have a general idea of relays.