Editing Relay (section)

== Applications ==
[[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, whereas the weak signal received at an intermediate station could control a contact, regenerating the signal for further transmission. High-voltage or high-current devices can be controlled with small, low voltage wiring and pilots switches. Operators can be isolated from the high voltage circuit. Low power devices such as [[microprocessor]]s can drive relays to control electrical loads beyond their direct drive capability. In an automobile, a starter relay allows the high current of the cranking motor to be controlled with small wiring and contacts in the ignition key.

Electromechanical switching systems including [[Strowger switch|Strowger]] and [[Crossbar switch|crossbar]] telephone exchanges made extensive use of relays in ancillary control circuits. The Relay Automatic Telephone Company also manufactured telephone exchanges based solely on relay switching techniques designed by [[:sv:Gotthilf Betulander|Gotthilf Ansgarius Betulander]]. The first public relay based telephone exchange in the  [[UK]] was installed in [[Fleetwood]] on 15 July 1922 and remained in service until 1959.<ref name="Graces Guide">{{cite web | title = Relay Automatic Telephone Company | url = http://www.gracesguide.co.uk/Relay_Automatic_Telephone_Co | access-date = October 6, 2014}}</ref><ref name="British Telecom History">{{cite web | title = British Telecom History 1912-1968 | url = http://www.btplc.com/thegroup/btshistory/1912to1968/1922.htm | access-date = October 8, 2014 | archive-date = October 14, 2014 | archive-url = https://web.archive.org/web/20141014015809/http://www.btplc.com/thegroup/btshistory/1912to1968/1922.htm | url-status = dead }}</ref>

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]]''. Relays can perform the basic operations of Boolean combinatorial logic. For example, the Boolean AND function is realised by connecting normally open relay contacts in series, the OR function by connecting normally open contacts in parallel. Inversion of a logical input can be done with a normally closed contact. Relays were used for control of automated systems for machine tools and production lines. The [[Ladder programming language]] is often used for designing [[relay logic]] networks.

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.

Relays are much more resistant than semiconductors to nuclear radiation, so they are widely used in safety-critical logic, such as the control panels of radioactive waste-handling machinery. Electromechanical [[protective relay]]s are used to detect overload and other faults on electrical lines by opening and closing [[circuit breaker]]s.

=== Protective relays ===
{{Main|Protective relay}}

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.

=== Railway signaling ===
[[File:Relay room.jpg|thumb|Part of a relay [[interlocking]] using UK Q-style miniature plug-in relays]]

[[Railway signalling]] relays are large considering the mostly small voltages (less than 120&nbsp;V) and currents (perhaps 100&nbsp;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. Not all relay circuits can be proved so there is reliance on construction features such as carbon to silver contacts to resist lightning induced contact welding and to provide AC immunity.

[[Opto-isolator]]s are also used in some instances with railway signalling, especially where only a single contact is to be switched.

=== Selection considerations ===
[[file:Uy-multi1-hy.jpg|thumb|Several 30-contact relays in "Connector" circuits in mid-20th century [[Number One Crossbar Switching System|1XB switch]] and [[Number Five Crossbar Switching System|5XB switch]] telephone exchanges; cover removed on one.]]

Selection of an appropriate relay for a particular application requires evaluation of many different factors:

* Number and type of contacts&nbsp;— normally open, normally closed, (double-throw)
* Contact sequence&nbsp;— "make before break" or "break before make". For example, the old style telephone exchanges required make-before-break so that the connection did not get dropped while dialing the number.
* Contact current rating&nbsp;— small relays switch a few amperes, large contactors are rated for up to 3000 amperes, alternating or direct current
* Contact voltage rating&nbsp;— typical control relays rated 300 VAC or 600 VAC, automotive types to 50 VDC, special high-voltage relays to about 15,000&nbsp;V
* Operating lifetime, useful life&nbsp;— the number of times the relay can be expected to operate reliably. There is both a mechanical life and a contact life. The contact life is affected by the type of load switched. Breaking load current causes [[#Undesired arcing|undesired arcing]] between the contacts, eventually leading to contacts that weld shut or contacts that fail due to erosion by the arc.<ref name="electronic-components.com.au">{{cite web | title = Arc Suppression to Protect Relays From Destructive Arc Energy | url = http://www.electronic-components.com.au/products-services/arc-suppression/ | access-date = December 6, 2013}}</ref>
* Coil voltage&nbsp;— machine-tool relays usually 24 VDC, 120 or 250 VAC, relays for switchgear may have 125 V or 250 VDC coils, 
* Coil current&nbsp;— Minimum current required for reliable operation and minimum holding current, as well as effects of power dissipation on coil temperature at various [[duty cycle]]s. "Sensitive" relays operate on a few milliamperes.
* Package/enclosure&nbsp;— open, touch-safe, double-voltage for isolation between circuits, [[Electrical equipment in hazardous areas|explosion proof]], outdoor, oil and splash resistant, washable for [[printed circuit board]] assembly
* Operating environment&nbsp;— minimum and maximum operating temperature and other environmental considerations, such as effects of humidity and salt
* Assembly&nbsp;— Some relays feature a sticker that keeps the enclosure sealed to allow PCB post soldering cleaning, which is removed once assembly is complete.
* Mounting&nbsp;— sockets, plug board, rail mount, panel mount, through-panel mount, enclosure for mounting on walls or equipment
* Switching time&nbsp;— where high speed is required
* "Dry" contacts&nbsp;— when switching very low level signals, special contact materials may be needed such as gold-plated contacts
* Contact protection&nbsp;— suppress arcing in very inductive circuits
* Coil protection&nbsp;— suppress the surge voltage produced when switching the coil current
* Isolation between coil contacts
* Aerospace or radiation-resistant testing, special quality assurance
* Expected mechanical loads due to [[acceleration]]&nbsp;— some relays used in [[aerospace]] applications are designed to function in [[Shock (mechanics)|shock]] loads of 50 [[G-force|''g'']], or more.
* Size&nbsp;— smaller relays often resist mechanical vibration and shock better than larger relays, because of the lower inertia of the moving parts and the higher natural frequencies of smaller parts.<ref name="keller"/> Larger relays often handle higher voltage and current than smaller relays.
* Accessories such as timers, auxiliary contacts, pilot lamps, and test buttons.
* Regulatory approvals.
* Stray magnetic linkage between coils of adjacent relays on a printed circuit board.
There are many considerations involved in the correct selection of a control relay for a particular application, including factors such as speed of operation, sensitivity, and [[hysteresis]]. Although typical control relays operate in the 5 [[millisecond|ms]] to 20 ms range, relays with switching speeds as fast as 100 [[microsecond|μs]] are available. [[Reed relay]]s which are actuated by low currents and switch fast are suitable for controlling small currents.

As with any switch, the contact current (unrelated to the coil current) must not exceed a given value to avoid damage. In high-[[inductance]] circuits such as [[electric motor|motors]], other issues must be addressed. When an inductance is connected to a power source, an [[inrush current|input surge current or electromotor starting current]] larger than the steady-state current exists. When the circuit is broken, the current cannot change instantaneously, which creates a potentially damaging arc across the separating contacts.

Consequently, for relays used to control inductive loads, we must specify the maximum current that may flow through the relay contacts when it actuates, the ''make rating''; the continuous rating; and the ''break rating''. The make rating may be several times larger than the continuous rating, which is larger than the break rating.