When the relay is working, the electromagnet is electrified, and the armature is sucked down to make D and e contact, and the working circuit is closed. When the electromagnet is powered off, it loses magnetism, and the spring pulls up the holding iron to cut off the working circuit. The common relay is the switch that uses electromagnet to control the working circuit. The advantages of the control circuit realized by the relay are: low voltage control high voltage; remote control; automatic control.