A relay is an electrical component that controls a circuit by opening and closing it using an electromagnet. It is also used to allow a low-power electrical signal, such as one from a switch or a sensor, to control a high-power electrical circuit, such as a car’s headlights or starter motor. They are used in a broad range of automotive systems, such as the lighting system, the starter system, the fuel pump, and the air conditioning system.
Many relays use an electromagnet to operate a switch mechanically, but other operating principles, such as solid-state relays, are also used. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal.
Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching.
Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems, these functions are performed by digital instruments, still called “protective relays.”