A relay is an electromechanical switch that uses an electromagnet to open or close one or many sets of contacts.
It was invented by Joseph Henry in 1835.
When a current flows through the 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.
Relays are used:
1. To control a high-voltage circuit with a low-voltage signal, as in some types of modem.
2. To control a high-current circuit with a low-current signal, as in the starter solenoid of an automobile.
3. To detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers.
4. 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. The latter is often applied to control office lighting as the low voltage wires are easily installed in partitions, which may be often moved as needs change. They may also be controlled by room occupancy detectors in an effort to conserve energy.
5. To perform logic functions. For example, the Boolean AND function is realised by connecting relay contacts in series, the OR function by connecting contacts in parallel. Due to the failure modes of a relay compared with a semiconductor, they are widely used in safety critical logic, such as the control panels of radioactive waste handling machinery.
6. To generate sound. 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. This can be further improved into an electric bell by extending the moving contact so it can repeatedly strike a bell.
Types of relay
A changeover relay has one moving contact (common or C) and two fixed contacts. One of these is Normally Closed (NC) when the relay is switched off, and the other is Normally Open (NO). Switching the relay on causes the normally open contact to close and the normally closed contact to open.
Relay as typically used in a car (cover removed)
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, and it retains its last setting across a power outage. (Some common relays may be wired to electrically latch, which offers no power saving but does ensure that the relay returns to a known state during and after a power outage.)
A reed relay has two, usually normally open, contacts inside a vacuum or inert gas filled glass tube. This protects the contacts against atmospheric corrosion. The two contacts are closed by magnetism from a coil around the glass tube, or a permanent magnet moved towards it.