In the electrical world relays are electromechanical devices that use electromagnets to operate other switches and circuits. The relay was invented by Joseph Henry in 1835. Since relays can control output circuits that are of higher power than the input circuits, they are often considered, in a broad sense, to be a type of electrical amplifier. These contacts can be normally-open, normally-closed, or change-over contacts. Normally-open contacts connect the circuit when the relay is activated and disconnected when the relay is inactive. Normally-closed contacts disconnect the circuit when the relay is activated and connect it the relay is inactive. Change-over contacts control one normally-open contact and one normally-closed contact.
A relay has a coil and a spring-loaded armature in the circuit. When an electrical current is passed, a magnetic field is induced and it attracts the armature that touches the contact. When the current is stopped, the spring-loaded armature goes back to its original position. There are some latching relays that require a second coil to reset the contact position. By analogy with the functions of the original electromagnetic device, a solid-state relay operates a thyristor or other type of solid-state switching device with a transformer or light-emitting diode to set it off.
There are several types of relays including: change over, with one moving and a pair of fixed contacts; latching relay, where the armature can move in opposite directions to make contact; reed relay, which has two contacts in an tube of inert gas; mercury wetted relay, which the contacts are wetted with mercury and machine tool relay, used in machine tools; a contactor is a heavy-duty relay used for switching electric motors and lighting loads; A Buchholz relay is a safety device that can sense the amount of gas in large oil-filled transformers; A Solid State Relay (SSR) is a solid state electronic device that is similar to an electromechanical relay but doesn’t have any moving components.
While selecting relays you need to specify the types and number of contacts, contact ratings; coil voltages; mounting details, arcing suppressions, voltage surge protections, etc. Relays are used in modems, starter switches of automobiles, as protection relays to detect faults in transmission lines, in logic functions and so on.