🛠A relay is an electrically operated switch.
🛠It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals.
🛠The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof.
🛠Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts.
🛠Relays are used where it is necessary to control a circuit by an independent low-power signal, or where several circuits must be controlled by one signal.
🛠Relays were first used in long-distance telegraph circuits as signal repeaters: they refresh the signal coming in from one circuit by transmitting it on another circuit.
🛠Relays were used extensively in telephone exchanges and early computers to perform logical operations.
🛠The traditional electromechanical form of a relay uses an electromagnet to close or open the contacts, but relays using other operating principles have also been invented, such as in solid-state relays which use semiconductor properties for control without relying on moving parts.
🛠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 or safety relays.
