A relay is an electro-mechanical switching device which allows one or more control circuits to switch a load circuit which may have differing voltage or current requirements.
Relays were first used in long-distance telegraph circuits as signal repeaters: they were used to refresh the signal coming in from one circuit by transmitting it on another circuit, each relay and circuit having it's own local power supply.
Prior to the invention of the transistor, relays were used extensively in telephone exchanges and early computers to perform logical operations.
One example of circuits with differing current requirements would be the headlights on a car. These are often operated via a relay, with the switch on the dashboard or steering column operating a low current control circuit that - via the relay - switches on the high current headlight circuit. This means that items such as the headlight indication light on the dashboard does not have to carry the full load of the headlight and taillight bulbs through it's filament and cabling, and reduces contact wear on the light switch.
Relays are used extensively in automotive and industrial control applications.
The traditional form of relay uses an electromagnet to close or open the circuit contacts and these are still very much in widespread use, but other types have been developed, such as solid-state relays which use semiconductor properties for control without relying on moving parts.
Latching relays require only a single pulse of control power to operate the switch persistently. Another pulse applied to a second set of control terminals, or a pulse with opposite polarity, resets the switch, while repeated pulses of the same kind have no effect. Magnetic latching relays are useful in applications when interrupted power should not affect the circuits that the relay is controlling.
Relays
A relay is essentially an electrically operated switch. Relays are used where it is necessary to control a circuit by a separate signal, where several circuits must be controlled by one signal, or where the controlling equipment and the controlled equipment must remain electrically separate.
In the Fire and Security industry, relays are typically used to integrate otherwise incompatible systems, or incorporate third party equipment.
Relays can be purchased as momentary, latching or timed types. The number of switches (poles) that are available within the relay unit can vary from one to six. The switches (poles) within a relay all ‘change state’ at the same time.
A relay switch ‘changes state’ when the electromagnet coil is energised or de-energised by the control circuit. For this reason the switches are usually supplied with three contact terminals. These are usually labelled:
C – Switch ‘common’ connection
N/O or NO – Switch ‘normally open’ connection (open circuit when the control coil is de-energised)
N/C or NC – Switch ‘normally closed’ connection (closed circuit when the control coil is de-energised)
‘Changing state’ involves the changeover of the N/O and N/C settings - normally open to closed and normally closed to open.
It is worthwhile to know in advance before specifying a relay for a task the following information:
Coil voltage: the voltage of the control circuit and if the voltage is AC or DC.
Switched voltage: the voltage of the relay controlled circuit
Switched current: the maximum current draw through the relay switch contacts.
Momentary, timed or latching operation
Number of switches (poles) required and type:
Single throw (N/O OR N/C terminals) or double throw (N/O AND N/C terminals).