Relay

relayslatching relayelectric relayelectromechanical relayelectrical relaycontrol relaymercury-wetted relayimpulse relaycontactorsrelay switch
A relay is an electrically operated switch.wikipedia
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Switch

switchestoggle switchcontact bounce
A relay is an electrically operated switch.
A switch that is operated by another electrical circuit is called a relay.

Electromagnet

electromagnetselectro-magnetelectromagnetic
The traditional form of a relay uses an electromagnet to close or open the contacts, but other operating principles have been invented, such as in solid-state relays which use semiconductor properties for control without relying on moving parts.
Electromagnets are widely used as components of other electrical devices, such as motors, generators, electromechanical solenoids, relays, loudspeakers, hard disks, MRI machines, scientific instruments, and magnetic separation equipment.

Protective relay

Distance relaysProtective deviceprotective relays
Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called protective relays.
In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected.

Joseph Henry

Henry, JosephHenryJoseph Henry Papers Project
American scientist Joseph Henry is often claimed to have invented a relay in 1835 in order to improve his version of the electrical telegraph, developed earlier in 1831.
He invented a precursor to the electric doorbell (specifically a bell that could be rung at a distance via an electric wire, 1831) and electric relay (1835).

Solid-state relay

solid state relaySolid-state relaysSolid State Relays
The traditional form of a relay uses an electromagnet to close or open the contacts, but other operating principles have been invented, such as in solid-state relays which use semiconductor properties for control without relying on moving parts. A solid-state relay (SSR) is a solid state electronic component that provides a function similar to an electromechanical relay but does not have any moving components, increasing long-term reliability.
The relay may be designed to switch either AC or DC to the load.

Edward Davy

It is claimed that English inventor Edward Davy "certainly invented the electric relay" in his electric telegraph c.1835.
Edward Davy (16 June 1806 – 26 January 1885) was an English physician, scientist, and inventor who played a prominent role in the development of telegraphy, and invented an electric relay.

Electrical telegraph

electric telegraphtelegraphtelegraph line
American scientist Joseph Henry is often claimed to have invented a relay in 1835 in order to improve his version of the electrical telegraph, developed earlier in 1831. It is claimed that English inventor Edward Davy "certainly invented the electric relay" in his electric telegraph c.1835. 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.
In 1835, Joseph Henry and Edward Davy invented the critical electrical relay.

Contactor

contactorsmagnetic blowoutcontactor relay control
A contactor is a heavy-duty relay with higher current ratings, used for switching electric motors and lighting loads.
Unlike general-purpose relays, contactors are designed to be directly connected to high-current load devices.

Stepping switch

uniselectorstepping relayuniselectors
A stepping relay is a specialized kind of multi-way latching relay designed for early automatic telephone exchanges.
In electrical control engineering, a stepping switch or stepping relay, also known as a uniselector, is an electromechanical device that switches an input signal path to one of several possible output paths, directed by a train of electrical pulses.

Samuel Morse

Samuel F. B. MorseSamuel F.B. MorseMorse
A simple device, which is now called a relay, was included in the original 1840 telegraph patent of Samuel Morse.
With Gale's help, Morse introduced extra circuits or relays at frequent intervals and was soon able to send a message through 10 mi of wire.

Earth leakage circuit breaker

ELCB
An earth leakage circuit breaker includes a specialized latching relay.
An ELCB is a specialised type of latching relay that has a building's incoming mains power connected through its switching contacts so that the ELCB disconnects the power when earth leakage is detected.

Programmable logic controller

PLCPLCsprogrammable logic controllers
Although such relays once were the backbone of automation in such industries as automobile assembly, the programmable logic controller (PLC) mostly displaced the machine tool relay from sequential control applications.
Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop controllers.

Mercury relay

A mercury relay is a relay that uses mercury as the switching element.
A mercury relay (mercury displacement relay, mercury contactor) is a relay that uses mercury as the switching element.

Relay logic

logic switchesrelayrelay circuits
A number of relays could be used together to carry out complex functions (relay logic).
Relay logic is a method of implementing combinational logic in electrical control circuits by using several electrical relays wired in a particular configuration.

Reed relay

ferreedremreedCrossReed
Very early computers often stored bits in a magnetically latching relay, such as ferreed or the later remreed in the 1ESS switch.
A reed relay is a type of relay that uses an electromagnet to control one or more reed switches.

H bridge

H-bridgehalf bridge
A polarity controlled relay needs changeover switches or an H bridge drive circuit to control it.
One way to build an H bridge is to use an array of relays from a relay board.

Telephone exchange

exchangescentral officeexchange
A stepping relay is a specialized kind of multi-way latching relay designed for early automatic telephone exchanges.
Early exchanges were electromechanical systems using motors, shaft drives, rotating switches and relays.

Electromechanics

electromechanicalelectro-mechanicalElectromechanical Engineering
A solid-state relay (SSR) is a solid state electronic component that provides a function similar to an electromechanical relay but does not have any moving components, increasing long-term reliability.
Often involving electromagnetic principles such as in relays, which allow a voltage or current to control another, usually isolated circuit voltage or current by mechanically switching sets of contacts, and solenoids, by which a voltage can actuate a moving linkage as in solenoid valves.

Safety relay

safety relay PNOZ
Safety relays are devices which generally implement protection functions.
Relays and contactors were used to control plant and machinery in the early days of control technology.

Armature (electrical)

armaturearmaturesArmature (electrical engineering)
A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core (a solenoid), an iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts (there are two contacts in the relay pictured).
The pole piece of a permanent magnet or electromagnet and the moving, iron part of a solenoid, especially if the latter acts as a switch or relay, may also be referred to as armatures.

TRIAC

electronic speed controllatching current
A solid-state relay uses a thyristor, TRIAC or other solid-state switching device, activated by the control signal, to switch the controlled load, instead of a solenoid.
A thyristor is analogous to a relay in that a small voltage induced current can control a much larger voltage and current.

Solid-state electronics

solid statesolid-statesolid state electronics
A solid-state relay (SSR) is a solid state electronic component that provides a function similar to an electromechanical relay but does not have any moving components, increasing long-term reliability.
The term is also used for devices in which semiconductor electronics which have no moving parts replace devices with moving parts, such as the solid-state relay in which transistor switches are used in place of a moving-arm electromechanical relay, or the solid-state drive (SSD) a type of semiconductor memory used in computers to replace hard disk drives, which store data on a rotating disk.

Electrical contacts

electrical contactcontactscontact
The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof.
An electrical contact is an electrical circuit component found in electrical switches, relays, connectors and circuit breakers.

Snubber

inductive kickRC snubbersnubbed
In this case a snubber circuit (a capacitor and resistor in series) across the contacts may absorb the surge.
The snubber diode is wired in parallel with an inductive load (such as a relay coil or electric motor).

Diode

diodessemiconductor diodegermanium diode
When the coil is energized with direct current, a diode is often placed across the coil to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to semiconductor circuit components.
For example, diodes are used in (stepper motor and H-bridge) motor controller and relay circuits to de-energize coils rapidly without the damaging voltage spikes that would otherwise occur.