Relay

A relay is an electromechanical switch that uses an electromagnet to open or close one or many sets of contacts.

When a current flows through the induction coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. The movement either makes or breaks a connection with a fixed contact. When the current is switched off, the armature is returned by a spring to its resting position.

Applications

Relays are used:

to control a high-voltage circuit with a low-voltage signal, as in some types of modem,
to control a high-current circuit with a low-current signal, as in the starter solenoid of an automobile,
to isolate the controlling circuit from the controlled circuit when the two are at different potentials, for example when controlling a mains-powered device from a low-voltage switch.

A relay can be made into a buzzer by wiring up the coil in series with the normally-closed contacts. When a current is applied to the relay, it opens the contacts that provide current to the coil, causing the current to stop flowing and the contacts to close again. This cycle repeats continuously, causing the relay to buzz at an audible frequency.

Types of relay

A changeover relay has one moving contact and two fixed contacts. One of these is normally closed when the relay is switched off, and the other is normally open. Switching the relay on causes the normally open contact to close and the normally closed contact to open.

A latching relay is mechanically arranged so that the armature can rest in either of two positions. There are two coils that pull the armature in opposite directions, so the relay can be switched to one position or the other and then left in that state indefinitely. This type of relay has the advantage that it consumes power only for an instant, while it is being switched.

A reed relay has two, usually normally open, contacts inside a glass envelope. This protects the contacts against atmospheric corrosion.