A relay is an electronic control device. It has a control system (also called an input circuit) and a controlled system. It is usually used in an automatic control circuit. It actually uses a small current to control a large current. An "automatic switch." The relay is composed of two parts of the coil and the contact group, so the graphical symbol of the relay in the circuit diagram also includes two parts: a long box represents the coil; a group of contact symbols represents the contact group. If the relay has two coils, draw two parallel long boxes. At the same time, the relay's text symbol “J” is marked in the long box or beside the long box. In the circuit plays an automatic adjustment, security protection, conversion circuit and other functions.