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A relay is an electric switching device having one or more contacts which open or close circuits. The switching device is mostly actuated by an electromagnet which closes or opens the contacts by means of a movable armature which it attracts or releases. However, there are also relays which are operated by other than electro-magnetic forces, e.g., electrical attraction forces or mechanical forces such as the flexural force of a bimetallic strip in a thermo-relay.

It was invented by Joseph Henry in 1835. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered to be, in a broad sense, a form of an electrical amplifier. Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits; the link is magnetic and mechanical.

Three electromagnetic relays which are different in the design of the armature are explained in Figs. 1-3. Each relay has a coil of wire with an iron core and an iron yoke which carries the movable armature or may be an integral feature of the latter. The yoke, which serves as an easy path for the magnetic flux, imparts the polarity of the rear end of the core to the armature, which is thus powerfully attracted by the opposite polarity of the front end of the core.

To prevent the armature from remaining sticking to the core by the action of remanent magnetism the residual magnetism which remains in the core even when no current is flowing in the coil, a small separator stud made of a non-magnetic material (brass) maintains an air gap between the armature and electromagnet core.

The current for energizing the coil is supplied through the connections termed as soldering lugs. In the relay in Fig.1 the contacts are normally open: when the relay coil is energized, the core attracts the armatue, which presses the bottom contact up and thus closes the contacts, so that current then flows through the working circuit by way of the connections 1 and 2.

Several sets of contacts can be installed in a relay, as in Figs. 2 and 3 these are simultaneously actuated when the relay is energized. These include normally closed contacts, which open only when the relay is energized and then break the working circuits in which they are installed (connections 2 and 3).

A type of relay which is used only for telegraphy purpose is the polarised relay (Fig.4). The armature, which carries the contacts at its front end, is suspended from a torsion wire and receives the polarity of a magnetic north pole from the suitably mounted permanent magnet.

The rear end of the armature extends into a gap in an iron yoke with magnetic south polarity. Mounted on this yoke is the relay coil which produces the controlling magnetic flux. The superposition of the magnetic fluxes, and therefore of the forces exerted, is shown in Fig.4. A relay of this kind responds differently to energizing currents flowing in different directions.

Following are some of the advantages and disadvantages:

Advantages of relays:

  • Relays can switch AC and DC, transistors can only switch DC.
  • Relays can switch AC and DC, transistors can only switch DC.
  • Relays can switch high voltages, transistors cannot
  • Relays are a better choice for switching large currents (> 5A).
  • Relays can switch many contacts at once.

Disadvantages of relays:

  • Relays are bulkier than transistors for switching small currents.
  • Relays cannot switch rapidly (except reed relays), transistors can switch many times per second.
  • Relays use more power due to the current flowing through their coil.
  • Relays require more current than many ICs can provide, so a low power transistor may be needed to switch the current for the relay's coil.