Editing Relay (section)

== Basic design and operation ==
[[file:Relay Parts.jpg|thumb|left|Simple electromechanical relay]]
[[file:Relay animation without flyback diode .gif|thumb|Operation without [[flyback diode]], arcing causes degradation of the switch contacts]]
[[file:Relay animation with flyback diode.gif|thumb|Operation with flyback diode, arcing in the control circuit is avoided]]

A simple [[Electromagnetism|electromagnetic]] relay consists of a coil of wire wrapped around a [[Magnetic core|soft iron core]] (a solenoid), an iron yoke which provides a low [[Magnetic reluctance|reluctance]] path for magnetic flux, a movable iron [[Armature (electrical engineering)|armature]], and one or more sets of contacts (there are two contacts in the relay pictured). The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. The armature is held in place by a [[Spring (device)|spring]] so that when the relay is de-energized there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the [[printed circuit board]] (PCB) via the [[yoke]], which is soldered to the PCB.

When an [[electric current]] is passed through the coil it generates a [[magnetic field]] that activates the armature, and the consequent movement of the movable contact(s) either makes or breaks (depending upon construction) a connection with a fixed contact. If the set of contacts was closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low-voltage application this reduces noise; in a high voltage or current application it reduces [[#Undesired arcing|arcing]].

[[file:Relais-Finder-12A.webm|thumb|left|Operation of a 12 A relay]]

When the coil is energized with [[direct current]], a [[flyback diode]] or [[snubber]] [[resistor]] is often placed across the coil to dissipate the energy from the collapsing magnetic field ([[back EMF]]) at deactivation, which would otherwise generate a [[voltage spike]] dangerous to [[semiconductor]] circuit components. Such diodes were not widely used before the application of [[transistor]]s as relay drivers, but soon became ubiquitous as early [[Bipolar junction transistor#Germanium transistors|germanium transistors]] were easily destroyed by this surge. Some automotive relays include a diode inside the relay case. Resistors, while more durable than diodes, are less efficient at eliminating voltage spikes generated by relays<ref>{{cite web |title=Understanding Relays & Wiring Diagrams |url=https://www.swe-check.com.au/editorials/understanding_relays.php |website=Swe-Check |access-date=16 December 2020}}</ref> and therefore not as commonly used.

[[file:Relay2.jpg|thumb|A small cradle relay often used in electronics. The "cradle" term refers to the shape of the relay's armature]]

If the relay is driving a large, or especially a [[Electrical reactance|reactive]] load, there may be a similar problem of surge currents around the relay output contacts. In this case a snubber circuit (a capacitor and resistor in series) across the contacts may absorb the surge. Suitably rated capacitors and the associated resistor are sold as a single packaged component for this commonplace use.

If the coil is designed to be energized with [[alternating current]] (AC), some method is used to split the flux into two out-of-phase components which add together, increasing the minimum pull on the armature during the AC cycle. Typically this is done with a small copper "shading ring" crimped around a portion of the core that creates the delayed, out-of-phase component,<ref>{{cite web |title = Art & Science of Protective Relaying, Chapter 2, GE Consumer & Electrical |last= Mason |first= C. R. |url = http://www.gedigitalenergy.com/multilin/notes/artsci/ |access-date = October 9, 2011}}</ref> which holds the contacts during the zero crossings of the control voltage.<ref>{{cite conference |conference=Electrical Contacts |title=Design of Shading Coils for Minimizing the Contact Bouncing of AC Contactors |url=https://www.researchgate.net/publication/224355001 |first1=J.R. |last1=Riba |first2=A.G. |last2=Espinosa |first3=J. |last3=Cusidó |first4=J.A. |last4=Ortega |first5=L. |last5=Romeral |date=November 2008 |access-date=2018-01-07 |page=130}}</ref>

Contact materials for relays vary by application. Materials with low contact resistance may be oxidized by the air, or may tend to "stick" instead of cleanly parting when opening. Contact material may be optimized for low electrical resistance, high strength to withstand repeated operations, or high capacity to withstand the heat of an arc. Where very low resistance is required, or low thermally-induced voltages are desired, gold-plated contacts may be used, along with palladium and other non-oxidizing, semi-precious metals. Silver or silver-plated contacts are used for signal switching. Mercury-wetted relays make and break circuits using a thin, self-renewing film of liquid mercury. For higher-power relays switching many amperes, such as motor circuit contactors, contacts are made with a mixtures of silver and cadmium oxide, providing low contact resistance and high resistance to the heat of arcing. Contacts used in circuits carrying scores or hundreds of amperes may include additional structures for heat dissipation and management of the arc produced when interrupting the circuit.<ref>Ian Sinclair, ''Passive Components for Circuit Design'', Elsevier, 2000 {{ISBN|008051359X}}, pp. 161–164</ref> Some relays have field-replaceable contacts, such as certain machine tool relays; these may be replaced when worn out, or changed between normally open and normally closed state, to allow for changes in the controlled circuit.<ref>{{cite book |first=Joseph E. |last=Fleckenstein |title=Three-Phase Electrical Power |publisher=CRC Press |date=2017 |isbn=978-1498737784 |page=321}}</ref>