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Relay
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== Terminology == [[File:Relay symbols.svg|thumb|Circuit symbols of relays (C denotes the common terminal in SPDT and DPDT types.)]] Since relays are [[switch]]es, the terminology applied to switches is also applied to relays; a relay switches one or more ''poles'', each of whose [[Electrical contacts|contacts]] can be ''thrown'' by energizing the coil. Normally open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. Normally closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. All of the contact forms involve combinations of NO and NC connections. 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 ''[[Form A (switches)|Form A]]'' or ''make'' contact. These have two terminals which can be connected or disconnected. Including two for the coil, such a relay has four terminals in total. * ''SPST-NC'' (Single-Pole Single-Throw, Normally-Closed) relays have a single ''[[Form B (switches)|Form B]]'' or ''break'' contact. As with an SPST-NO relay, such a relay has four terminals in total. * ''SPDT'' (Single-Pole Double-Throw) relays have a single set of ''[[Form C (switches)|Form C]]'', ''break before make'' or ''transfer'' contacts. That is, a common terminal connects to either of two others, never connecting to both at the same time. Including two for the coil, such a relay has a total of five terminals. * ''DPST'' β 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 ''[[Form A (switches)|Form A]]'' or ''[[Form B (switches)|Form B]]'' (or one of each; the designations ''NO'' and ''NC'' should be used to resolve the ambiguity). * ''DPDT'' β Double-Pole Double-Throw relays have two sets of ''[[Form C (switches)|Form C]]'' contacts. These are equivalent to two SPDT switches or relays actuated by a single coil. Such a relay has eight terminals, including the coil * [[Form D (switches)|Form D]] β 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]] β 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. ''EN 50005'' are among applicable standards for relay terminal numbering; a typical EN 50005-compliant SPDT relay's terminals would be numbered 11, 12, 14, A1 and A2 for the C, NC, NO, and coil connections, respectively.<ref>EN 50005:1976 "Specification for low voltage switchgear and controlgear for industrial use. Terminal marking and distinctive number. General rules." (1976). In the UK published by BSI as BS 5472:1977.</ref> [[DIN 72552]] defines contact numbers in relays for automotive use: * 85 = relay coil - * 86 = relay coil + * 87 = to load (normally open) * 87a = to load (normally closed) * 30 = battery +
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