Relay is an automatic switching element with isolation function, which can make a predetermined step change of the controlled power in the output circuit when the change of the excitation in the input circuit reaches a specified value. It has a sensing mechanism to respond to external excitation (electrical or non-electrical), an actuator to control the controlled circuit "on" and "off", and an intermediate comparison mechanism to compare, judge and convert the magnitude of the excitation. Relays are widely used in remote control, telemetry, communication, automatic control, mechatronics, and aerospace technology to control, protect, regulate, and transmit information.

Relays generally have an induction mechanism (input part) that reflects certain input variables (e.g., current, voltage, power, impedance, frequency, temperature, pressure, speed, light, etc.); an actuator (output part) that provides "on" and "off" control of the controlled circuit; and an intermediate mechanism (drive part) that couples and isolates the input quantity, processes the function, and drives the output part between the input and output parts of the relay. Between the input and output parts of the relay, there is an intermediate mechanism (drive part) that couples and isolates the input, processes the function and drives the output.

As a control element, the relay has several roles.

(1) Expanding the control range: For example, a multi-contact relay control signal up to a certain value can be switched, opened and turned on multiple circuits at the same time according to the different forms of contact groups.

(2) Amplification: For example, sensitive relays, intermediate relays, etc., with a very small amount of control, you can control a very high-power circuit.

(3) Integrated signals: For example, when multiple control signals are fed into a multi-winding relay in a prescribed form, they are compared and integrated to achieve a predetermined control effect.

(4) Automatic, remote control, monitoring: For example, relays on automatic devices, together with other electrical appliances, can form programmed control lines, thus enabling automated operation.