A relay controls the operation of large currents through small currents, and plays the role of automatic adjustment, safety protection, and circuit conversion.

The working principle of the relay is based on the electromagnetic principle. When a certain voltage or current is applied to both ends of the coil of the relay, an electromagnetic effect will be generated in the coil, attracting the armature to move toward the iron core. This movement drives the moving contact and the static contact of the contact system to attract, thereby achieving the purpose of conduction or disconnection in the circuit. When the coil is de-energized, the electromagnetic attraction disappears, and the armature returns to its original position under the reaction force of the spring, so that the moving contact and the original static contact are released, completing a complete switching operation.

The "normally open" and "normally closed" contacts of the relay are distinguished according to the state of the relay coil when it is not energized. When the relay coil is not energized, the static contact in the disconnected state is called the "normally open contact", and the static contact in the connected state is called the "normally closed contact".

In addition, relays can also be used to achieve more complex functions, such as time delay, current measurement and signal amplification, etc. They play an important role in modern electrical and electronic technology, such as in home, automobile, communication, industry and other fields.