Relay

タイトルまたは説明

A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching
mechanism mechanically, but other operating principles 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. The first relays were used in
long distance telegraph circuits, repeating the signal coming in from one circuit and re-transmitting it to
another. Relays were used extensively in telephone exchanges and early computers to perform logical
operations.
A type of relay that can handle the high power required to directly control an electric motor is called a
contactor. Solid-state relays control power circuits with no moving, 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"