Electromagnetic relay is generally composed of iron core, coil, armature, contact spring, etc. As long as a certain voltage is applied to both ends of the coil, a certain current will flow through the coil, which will produce electromagnetic effect. The armature will overcome the pull of return spring and attract to the iron core under the action of electromagnetic force, thus driving the moving contact and static contact (normally open contact) of armature to pull in. When the coil is cut off, the electromagnetic suction will disappear, and the armature will return to its original position under the reaction force of the spring, so that the moving contact and the original static contact (normally closed contact) are closed. In this way, the circuit can be connected and cut off. For the "normally open and normally closed" contacts of the relay, it can be distinguished as follows: the static contact in the disconnected state when the relay coil is not energized is called "normally open contact"; the static contact in the on state is called "normally closed contact".