What is Relay?

relay is an electricity operated switch. It consists of a set of input terminals for a single or multiple control signals. And a set of operating contact terminals. 

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. Moreover, 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 use extensively in telephone exchanges and early computers to perform logical operations.

Electrical relay
Electrical relay


American scientist Joseph Henry is claim to have invent a relay in 1835.

A simple device, which is now a relay. it was include in the original 1840 telegraph patent of Samuel Morse.Moreover, The word relay appears in the context of electromagnetic operations from 1860.

Design and Operation

A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core. Which provides a low reluctance path for magnetic flux, a movable iron armature.Therefore, The armature is hing to the yoke and mechanically link to one or more sets of moving contacts.

Therefore, The armature is place by a spring so that when the relay is de-energize. Thus, There is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay picture is close, and the other set is open.


Relays are use wherever it is necessary to control a high power or high voltage circuit with a low power circuit.

first application of relays was in long telegraph lines.In an automobile, a starter relay allows the high current of the cranking motor to be control with small wiring and contacts in the ignition key.

Secondly, Electro mechanical switching systems including Stronger and Crossbar telephone exchanges made extensive use of relays in ancillary control circuits. 

3 Comments to “What is Relay?”

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