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== History ==
However, an official patent
▲Solely electrical relays got their start as a further improvement to telegraphs, with American scientist [[Joseph Henry]] who is often cited to have invented a relay in 1835 in order to improve his version of the [[electrical telegraph]], developed earlier in 1831.<ref>{{cite book|title=Icons of Invention: The Makers of the Modern World from Gutenberg to Gates|publisher=ABC-CLIO|url=https://books.google.com/books?id=WKuG-VIwID8C&q=Invention+of+the+relay&pg=PA153|page=153|isbn=9780313347436|year=2009}}</ref><ref>{{cite web|title=The electromechanical relay of Joseph Henry|publisher=Georgi Dalakov|url=http://history-computer.com/ModernComputer/Basis/relay.html|access-date=2012-06-21|archive-url=https://web.archive.org/web/20120618121911/http://history-computer.com/ModernComputer/Basis/relay.html|archive-date=2012-06-18|url-status=dead}}</ref><ref>{{cite book|title=Scientific American Inventions and Discoveries: All the Milestones in Ingenuity--From the Discovery of Fire to the Invention of the Microwave Oven|publisher=John Wiley & Sons|url=https://books.google.com/books?id=pDbQVE3IdTcC&q=relay+Joseph+Henry+1835&pg=PA311|page=311|isbn=9780471660248|date=2005-01-28}}</ref><ref>{{cite book| title=Joseph Henry: His Life and Work| url=https://archive.org/details/josephhenryhisli0000coul| url-access=registration| author=Thomas Coulson | publisher =Princeton University Press| location = Princeton| year = 1950}}</ref>
▲However, an official patent wasn't issued until 1840 to [[Samuel Morse]] for his [[Telegraphy|telegraph]], which is now called a relay. The mechanism described acted as a digital amplifier, repeating the telegraph signal, and thus allowing signals to be propagated as far as desired.<ref name="Patent1647">{{cite patent|country=US|number=1647|title=Improvement in the Mode of Communicating Information by Signals by the Application of Electromagnetism|pubdate=June 20, 1840|inventor1-last=Morse|inventor1-first=Samuel E.B.|url=http://www.google.com/patents?id=Xx5AAAAAEBAJ&printsec=abstract&zoom=4&dq=1647}} {{Cite web |url=http://www.google.com/patents?id=Xx5AAAAAEBAJ&printsec=abstract&zoom=4&dq=1647 |title=Patent US1647 - IMPROVEMENT IN THE MODE OF COMMUNICATING INFORMATION BY SIGNALS BY THE - Google Patents |access-date=September 6, 2011 |archive-date=May 24, 2012 |archive-url=https://web.archive.org/web/20120524081816/http://www.google.com/patents?id=Xx5AAAAAEBAJ&printsec=abstract&zoom=4&dq=1647 |url-status=bot: unknown }}</ref>
The word ''relay'' appears in the context of electromagnetic operations from 1860 onwards.<ref>{{cite web|title=Relay|url=http://www.etymonline.com/index.php?allowed_in_frame=0&search=relay&searchmode=none|website=EtymOnline.com}}</ref>
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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>
== Terminology ==
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The National Association of Relay Manufacturers and its successor, the Relay and Switch Industry Association define 23 distinct [[Electrical contact#Contact form|electrical contact forms]] found in relays and switches.<ref>Section 1.6, Engineers' Relay Handbook, 5th ed, Relay and Switch Industry Association, Arlington, VA; 3rd ed, National Association of Relay Manufacturers, Elkhart Ind., 1980; 2nd Ed. Hayden, New York, 1966; large parts of the 5th edition are on line [http://www.esterline.com/powersystems/DesignReference/RelayHandbook.aspx here] {{Webarchive|url=https://web.archive.org/web/20170705143411/http://www.esterline.com/powersystems/DesignReference/RelayHandbook.aspx |date=2017-07-05 }}.</ref> Of these, the following are commonly encountered:
* ''SPST-NO'' (Single-Pole Single-Throw, Normally-Open) relays have a single ''[[
* ''SPST-NC'' (Single-Pole Single-Throw, Normally-Closed) relays have a single ''[[
* ''SPDT'' (Single-Pole Double-Throw) relays have a single set of ''[[
* ''DPST'' – Double-Pole Single-Throw relays are equivalent to a pair of SPST switches or relays actuated by a single coil. Including two for the coil, such a relay has a total of six terminals. The poles may be ''[[
* ''DPDT'' – Double-Pole Double-Throw relays have two sets of ''[[
* [[Form D (switches)|Form D]] – make before break<ref name=":0">{{Cite journal|last=Alexandrovich|first=George|title=The Audio Engineer's Handbook|url=https://www.americanradiohistory.com/Archive-DB-Magazine/60s/DB-1968-09.pdf|journal=Db: The Sound Engineering Magazine|volume=September 1968|pages=10}}</ref>
* [[Form E (switches)|Form E]] – combination of D and B<ref name=":0" />
The ''S'' (''single'') or ''D'' (''double'') designator for the pole count may be replaced with a number, indicating multiple contacts connected to a single [[actuator]]. For example, 4PDT indicates a four-pole double-throw relay that has 12 switching terminals.
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== Types ==
{{
=== Coaxial relay ===
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In one mechanism, two opposing coils with an over-center spring or permanent magnet hold the contacts in position after the coil is de-energized. A pulse to one coil turns the relay on, and a pulse to the opposite coil turns the relay off. This type is widely used where control is from simple switches or single-ended outputs of a control system, and such relays are found in [[avionics]] and numerous industrial applications.
Another latching type has a [[
In another type, a ''ratchet relay'' has a ratchet mechanism that holds the contacts closed after the coil is momentarily energized. A second impulse, in the same or a separate coil, releases the contacts.<ref name="IRS2001
A [[Stepping switch|stepping relay]] is a specialized kind of multi-way latching relay designed for early automatic [[telephone exchange]]s.
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Very [[Mechanical computer#Electro-mechanical computers|early computers]] often stored bits in a magnetically latching relay, such as [[ferreed]] or the later [[remreed]] in the [[1ESS switch]].
{{anchor|Holding circuit}}Some early computers used ordinary relays as a kind of [[latch (electronics)|latch]]—they store bits in ordinary wire-spring relays or reed relays by feeding an output wire back as an input, resulting in a feedback loop or [[sequential circuit]]. Such an electrically latching relay requires continuous power to maintain state, unlike magnetically latching relays or mechanically ratcheting relays. While ''[[self-holding circuit<!-- this circular link with possibilities is already linked to wikidata and other Wikipedias -->|(self-)holding circuit]]s'' are often realized with relays they can also be implemented by other means.
In computer memories, latching relays and other relays were replaced by [[delay-line memory]], which in turn was replaced by a series of ever faster and ever smaller memory technologies.
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A mercury-wetted reed relay is a form of reed relay that employs a [[mercury switch]], in which the contacts are wetted with [[mercury (element)|mercury]]. Mercury reduces the contact resistance and mitigates the associated voltage drop. Surface contamination may result in poor conductivity for low-current signals. For high-speed applications, the mercury eliminates contact bounce, and provides virtually instantaneous circuit closure. Mercury wetted relays are position-sensitive and must be mounted according to the manufacturer's specifications. Because of the toxicity and expense of liquid mercury, these relays have increasingly fallen into disuse.
The high speed of switching action of the mercury-wetted relay is a notable advantage. The mercury globules on each contact [[coalescence (physics)|coalesce]], and the current rise time through the contacts is generally considered to be a few picoseconds.{{citation needed|date=November 2023}} However, in a practical circuit it may be limited by the [[inductance]] of the contacts and wiring. It was quite common, before restrictions on the use of mercury, to use a mercury-wetted relay in the laboratory as a convenient means of generating fast rise time pulses, however although the rise time may be picoseconds, the exact timing of the event is, like all other types of relay, subject to considerable jitter, possibly milliseconds, due to mechanical
The same coalescence process causes another effect, which is a nuisance in some applications. The contact resistance is not stable immediately after contact closure, and drifts, mostly downwards, for several seconds after closure, the change perhaps being 0.5 ohm.{{citation needed|date=November 2023}}
=== Multi-voltage relays ===
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[[File:Solid-state-contactor.jpg|thumb|25 A and 40 A solid state contactors]]
A [[solid-state relay]] (SSR) is a [[solid state (electronics)|solid state]] electronic component that provides a function similar to an [[electromechanical]] relay but does not have any moving components, increasing long-term reliability. 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. An [[optocoupler]] (a [[light-emitting diode]] (LED) coupled with a [[photo transistor]]) can be used to isolate control and controlled circuits.<ref>{{Cite web|url=https://www.electronics-tutorials.ws/blog/optocoupler.html|title=Optocoupler Tutorial|date=16 September 2013 }}</ref>
=== Static relay ===
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[[file:ACRelay.jpg|thumb|upright|A DPDT AC coil relay with "ice cube" packaging]]
Relays are used wherever it is necessary to control a high power or high voltage circuit with a low power circuit, especially when [[galvanic isolation]] is desirable. The first application of relays was in long [[electric telegraph|telegraph]] lines,
Electromechanical switching systems including [[Strowger switch|Strowger]] and [[Crossbar
The use of relays for the logical control of complex switching systems like telephone exchanges was studied by [[Claude Shannon]], who formalized the application of [[Boolean algebra]] to relay circuit design in ''[[A Symbolic Analysis of Relay and Switching Circuits]]''.
Early [[mechanical computer#Electro-mechanical computers|electro-mechanical computers]] such as the [[ARRA (computer)|ARRA]], [[Harvard Mark II]], [[Zuse Z2]], and [[Zuse Z3]] used relays for logic and working registers. However, electronic devices proved faster and easier to use.
=== Protective relays ===
{{Main|
For protection of electrical apparatus and transmission lines, electromechanical relays with accurate operating characteristics were used to detect overload, short-circuits, and other faults. While many such relays remain in use, [[digital protective relay]]s now provide equivalent and more complex protective functions.
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[[Railway signalling]] relays are large considering the mostly small voltages (less than 120 V) and currents (perhaps 100 mA) that they switch. Contacts are widely spaced to prevent flashovers and short circuits over a lifetime that may exceed fifty years.
Since rail signal circuits must be highly reliable, special techniques are used to detect and prevent failures in the relay system. To protect against false feeds, [[double switching]] relay contacts are often used on both the positive and negative side of a circuit, so that two false feeds are needed to cause a false signal.
[[Opto-isolator]]s are also used in some instances with railway signalling, especially where only a single contact is to be switched.
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* Number and type of contacts — normally open, normally closed, (double-throw)
* Contact sequence — "make before break" or "break before make". For example, the old style telephone exchanges required make-before-break so that the connection
* Contact current rating — small relays switch a few amperes, large contactors are rated for up to 3000 amperes, alternating or direct current
* Contact voltage rating — typical control relays rated 300 VAC or 600 VAC, automotive types to 50 VDC, special high-voltage relays to about 15,000 V
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[[Category:German inventions]]
[[Category:American inventions]]
[[Category:Digital electronics]]
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