Editing Switch (section)

==Contacts==
[[File:On-Off Switch.jpg|thumb|upright=0.5|A toggle switch in the "on" position]]

In the simplest case, a switch has two conductive pieces, often [[metal]], called ''contacts'', connected to an external circuit, that touch to complete (make) the circuit, and separate to open (break) the circuit. The contact material is chosen for its resistance to corrosion, because most metals form [[Electrical insulation|insulating]] oxides that would prevent the switch from working. Contact materials are also chosen on the basis of [[electrical conductivity]], hardness (resistance to abrasive wear), mechanical strength, low cost and low toxicity. The formation of oxide layers at contact surface, as well as surface roughness and contact pressure, determine the [[contact resistance]], and [[wetting current]] of a mechanical switch. Sometimes the contacts are [[Electroplating|plated]] with [[noble metal]]s, for their excellent conductivity and resistance to corrosion. They may be designed to wipe against each other to clean off any contamination. Nonmetallic conductors, such as conductive plastic, are sometimes used. To prevent the formation of insulating oxides, a minimum wetting current may be specified for a given switch design.

{{anchor|Two-way switch|Three-way switch|Four-way switch|Contact arrangements|Contact terminology}}
{{anchor|contact terminology}}

===Contact terminology===
[[File:Tpst.jpg|upright=1.4|thumb|Triple-pole single-throw (TPST or 3PST) [[knife switch]] used to short the windings of a three‑phase wind turbine for [[Dynamic braking|braking]] purposes. Here the switch is shown in the open position.]]

In electronics, switches are classified according to the arrangement of their contacts. A pair of contacts is said to be "''closed''" when current can flow from one to the other. When the contacts are separated by an [[Insulator (electric)|insulating air gap]], they are said to be "''open''", and no current can flow between them at normal voltages. The terms "''make''" for closure of contacts and "''break''" for opening of contacts are also widely used.

The terms '''pole''' and '''throw''' are also used to describe switch contact variations. The number of "''poles''" is the number of electrically separate switches which are controlled by a single physical actuator. For example, a "''2-pole''" switch has two separate, parallel sets of contacts that open and close in unison via the same mechanism. The number of "''throws''" is the number of separate wiring path choices other than "open" that the switch can adopt for each pole. A single-throw switch has one pair of contacts that can either be closed or open. A double-throw switch has a contact that can be connected to either of two other contacts, a triple-throw has a contact which can be connected to one of three other contacts, etc.<ref>[http://www.herley.com/index.cfm?act=app_notes&notes=switches RF Switch] {{webarchive|url=https://web.archive.org/web/20110423212637/http://www.herley.com/index.cfm?act=app_notes&notes=switches |date=2011-04-23 }} Explanation by [[Herley Industries|Herley&nbsp;– General Microwave]]</ref>

{{anchor|Break-before-make|Make-before-break}}
In a switch where the contacts remain in one state unless actuated, such as a [[push-button]] switch, the contacts can either be '''normally open''' (abbreviated "'''n.o.'''" or "'''no'''") until closed by operation of the switch, or '''normally closed''' ("'''n.c.'''" or "'''nc'''")<ref group="nb" name="NB_NC"/> and opened by the switch action. A switch with both types of contact is called a ''changeover switch'' or ''double-throw switch''. These may be "'''make-before-break'''" ("'''MBB'''" or shorting) which momentarily connects both circuits, or may be "'''break-before-make'''" ("'''BBM'''" or non-shorting) which interrupts one circuit before closing the other.

These terms have given rise to abbreviations for the types of switch which are used in the [[electronics]] industry such as "''single-pole, single-throw''" (SPST) (the simplest type, "on or off") or "''single-pole, double-throw''" (SPDT), connecting either of two terminals to the common terminal. In [[Mains electricity|electrical power]] wiring (i.e.,&nbsp;house and building wiring by [[electrician]]s), names generally involve the suffix ''"-way"''; however, these terms differ between [[British English]] and [[American English]] (i.e., the terms ''two&nbsp;way'' and ''three&nbsp;way'' are used with different meanings).

{{Anchor|Table of switch comparisons|Table of switch types}}
{| class="wikitable"
|-
! Electronics specification and abbreviation
!valign=top| Expansion <br />of<br />abbreviation
! British<br />mains <br />wiring<br />name
! American <br />electrical<br />wiring<br />name
! Description
! Schematic
! Symbol IEC 60617
|-------
| '''SPST'''<br/>(1P1T) || Single pole, single throw || One-way || Two-way || A simple on-off switch: The two terminals are either connected together or disconnected from each other. An example is a [[light switch]]. || rowspan="2" | [[File:SPST-Switch.svg|100px]] || [[File:Symbol circuit breaker (one-pole).svg|50px]]
|-
|'''SPST-NO'''
'''Form A'''<ref name=":0">{{Cite web|url=http://www.esterline.com/powersystems/DesignReference/RelayHandbook.aspx|title=Engineer's Relay Handbook, 5th edition, Chapter 1.6 by RSIA (formerly NARM)|archive-url=https://web.archive.org/web/20170705143411/http://www.esterline.com/powersystems/DesignReference/RelayHandbook.aspx|archive-date=2017-07-05|url-status=dead}}</ref>
|Single pole, single throw, normally open
|
|
|A simple on-off switch. The two terminals are normally disconnected (open) and are closed when the switch is activated. An example is a [[Push switch|pushbutton switch]]. || 
|-
|'''SPST-NC'''<br/>'''Form B'''<ref name=":0" />
|Single pole, single throw, normally closed
|
|
|A simple on-off switch. The two terminals are normally connected together (closed) and are open when the switch is activated. An example is a [[Push switch|pushbutton switch]].
|[[File:SPST-NC-Switch.svg|frameless|100x100px]] || 
|-------
| '''SPDT'''<br/>'''Form C'''<ref name=":0" /> 
| Single pole, double throw || Two-way || Three-way || A simple break-before-make changeover switch: C (COM, Common) is connected either to L1 or to L2. || rowspan="2" | [[File:SPDT-Switch.svg|100px]] || [[File:Symbol change over switch.svg|50px]]
|-------
| '''SPCO<br />SPTT, c.o.''' || Single pole changeover<br />''or''<br />single pole, centre off ''or''<br />single pole, triple throw|| &nbsp; || &nbsp; || Similar to ''SPDT''. Some suppliers use ''SPCO/SPTT'' for switches with a stable off position in the centre and ''SPDT'' for those without. || &nbsp;
|-------
| &nbsp; || &nbsp; || &nbsp; || &nbsp; || Serial switch or two-circuit switch{{citation needed|date=January 2024}} || &nbsp; || [[File:Symbol Series switch.svg|50px]]
|-------
| '''DPST'''<br/>(2P1T) || Double pole, single throw || Double pole ||Double pole || Equivalent to two ''SPST'' switches controlled by a single mechanism. || [[File:DPST-symbol.svg|100px]] || [[File:Symbol circuit breaker (two-pole).svg|50px]]
|-------
| '''DPDT'''<br/>(2P2T) || Double pole, double throw || || || Equivalent to two ''SPDT'' switches controlled by a single mechanism. ||rowspan=2| [[File:DPDT-symbol.svg|100px]] || [[File:Symbol change over switch (two-pole).svg|50px]]
|-------
| '''DPCO''' || Double pole changeover <br /> ''or'' double pole, centre off || &nbsp; || &nbsp; || Schematically equivalent to ''DPDT''. Some suppliers use ''DPCO'' for switches with a stable center position and ''DPDT'' for those without. A DPDT/DPCO switch with a center position can be "off" in the center, not connected to either L1 or L2, or "on", connected to both L1 and L2 at the same time. The positions of such switches are commonly referenced as "on-off-on" and "on-on-on" respectively. || &nbsp;
|-------
| &nbsp; || &nbsp; || Intermediate switch || Four-way switch || ''DPDT'' switch internally wired for polarity-reversal applications: only four rather than six wires are brought outside the switch housing. Also called cross switch, crossover switch or reversing switch.{{citation needed|date=January 2024}}
|| [[File:crossover-switch-symbol.svg|100px]] || [[File:Symbol cross switch.svg|50px]]
|-------
| '''2P6T''' || Two pole, six throw || || || Changeover switch with a COM (Common), which can connect to L1, L2, L3, L4, L5, or L6; with a second switch (2P, two pole) controlled by a single mechanism. || [[File:2P6T-symbol.svg|100px]] || &nbsp;
|}
Switches with larger numbers of poles or throws can be described by replacing the "S" or "D" with a number (e.g. 3PST, SP4T, etc.) or in some cases the letter "T" (for "triple") or "Q" (for "quadruple"). In the rest of this article the terms ''SPST'', ''SPDT'' and ''intermediate'' will be used to avoid the ambiguity.

===Contact bounce===
<!-- This section is linked from [[Elektronika BK]] -->

==== Bounce ====
[[File:Bouncy Switch.png|thumb|[[Oscilloscope]] snapshot showing voltage fluctuations caused by a switch bouncing between on and off (even swinging above V{{Subscript|high}} and below V{{Subscript|low}}) many times over 2.6 [[Millisecond|ms]] before settling|260x260px]]
Contact bounce (also called ''chatter'') is a common problem with mechanical switches, [[relay]]s and [[battery contact]]s, which arises as the result of electrical [[contact resistance]] (ECR) phenomena at interfaces. Switch and relay contacts are usually made of springy metals. When the contacts strike together, their momentum and elasticity act together to cause them to bounce apart one or more times before making steady contact. The result is a rapidly pulsed electric current instead of a clean transition from zero to full current. The effect is usually unimportant in power circuits, but causes problems in some [[analogue electronics|analogue]] and [[logic circuit]]s that respond fast enough to misinterpret the on‑off pulses as a data stream.<ref name="Walker1998">Walker, PMB, ''Chambers Science and Technology Dictionary'', Edinburgh, 1988, {{ISBN|1-85296-150-3}}</ref> In the design of micro-contacts, controlling surface structure ([[surface roughness]]) and minimizing the formation of [[Passivation (chemistry)|passivated layers]] on metallic surfaces are instrumental in inhibiting chatter.

In the [[Hammond organ]], multiple wires are pressed together under the piano keys of the manuals. Their bouncing and non-synchronous closing of the switches is known as ''Hammond Click'' and compositions exist that use and emphasize this feature. Some [[electronic organ]]s have a switchable replica of this sound effect.<ref>Features of Technics E-33</ref>

==== Debouncing ====
[[File:TS contact bounce filter.svg|thumb|right|SPST switch bounce filter using a [[Schmitt trigger]] and capacitor]]
The effects of contact bounce can be eliminated by:
* Use of [[Mercury-wetted relay|mercury-wetted contacts]], but these are now infrequently used because of the hazards of mercury.
* Alternatively, contact circuit voltages can be [[low-pass filter]]ed to reduce or eliminate multiple pulses from appearing.
* In digital systems, multiple samples of the contact state can be taken at a low rate and examined for a steady sequence, so that contacts can settle before the contact level is considered reliable and acted upon. See {{Section link|Keyboard technology#Debouncing}}.
* Bounce in [[#Contact terminology|SPDT]] ("single-pole, double-throw") switch contacts signals can be filtered out using an [[Multivibrator#Bistable|SR flip-flop]] (latch) or [[Schmitt trigger]].
All of these methods are referred to as 'debouncing'.

===Arcs and quenching===
When the power being switched is sufficiently large, the electron flow across opening switch contacts is sufficient to [[ionize]] the air molecules across the tiny gap between the contacts as the switch is opened, forming a [[gas plasma]], also known as an [[electric arc]]. The plasma is of low resistance and is able to sustain power flow, even with the separation distance between the switch contacts steadily increasing. The plasma is also very hot and is capable of eroding the metal surfaces of the switch contacts (the same true for vacuum switches). Electric current arcing causes significant [[Contact protection#Contact wear|degradation of the contacts]] and also significant [[electromagnetic interference]] (EMI), requiring the use of [[arc suppression]] methods.<ref>{{cite web | title = Lab Note #105 ''Contact Life&nbsp;– Unsuppressed vs. Suppressed Arcing'' | publisher = Arc Suppression Technologies | date = April 2011 | url = http://www.arcsuppressiontechnologies.com/arc-suppression-facts/lab-app-notes/ | access-date = February 5, 2012 | format = [[PDF]] | url-status = live | archive-url = https://web.archive.org/web/20131203004750/http://www.arcsuppressiontechnologies.com/arc-suppression-facts/lab-app-notes/ | archive-date = December 3, 2013 }} (3.6 Mb)</ref>

Where the voltage is sufficiently high, an arc can also form as the switch is closed and the contacts approach. If the voltage potential is sufficient to exceed the [[breakdown voltage]] of the air separating the contacts, an arc forms which is sustained until the switch closes completely and the switch surfaces make contact.

In either case, the standard method for minimizing arc formation and preventing contact damage is to use a fast-moving switch mechanism, typically using a spring-operated [[Tipping point (physics)|tipping-point mechanism]] to assure quick motion of switch contacts, regardless of the speed at which the switch control is operated by the user. Movement of the switch control lever applies tension to a spring until a tipping point is reached, and the contacts suddenly snap open or closed as the spring tension is released.

As the power being switched increases, other methods are used to minimize or prevent arc formation. A plasma is hot and will rise due to [[convection]] air currents. The arc can be quenched with a series of non-conductive blades spanning the distance between switch contacts, and as the arc rises, its length increases as it forms ridges rising into the spaces between the blades, until the arc is too long to stay sustained and is extinguished. A ''puffer'' may be used to blow a sudden high velocity burst of gas across the switch contacts, which rapidly extends the length of the arc to extinguish it quickly.

Extremely large switches often have switch contacts surrounded by something other than air to more rapidly extinguish the arc. For example, the switch contacts may operate in a vacuum, immersed in [[mineral oil]], or in [[Sulfur hexafluoride#Dielectric medium|sulfur hexafluoride]].

In AC power service, the current periodically passes through zero; this effect makes it harder to sustain an arc on opening. Manufacturers may rate switches with lower voltage or current rating when used in DC&nbsp;circuits.

===Power switching===
{{See also|Power management integrated circuit|Solid-state electronics|Solid-state relay}}

When a switch is designed to switch significant power, the transitional state of the switch as well as the ability to withstand continuous operating currents must be considered. When a switch is in the on state, its resistance is near zero and very little power is dropped in the contacts; when a switch is in the off state, its resistance is extremely high and even less power is dropped in the contacts. However, when the switch is flicked, the resistance must pass through a state where a quarter of the load's rated power{{Citation needed|date=October 2013}} (or worse if the load is not purely resistive) is briefly dropped in the switch.

For this reason, power switches intended to interrupt a load current have spring mechanisms to make sure the transition between on and off is as short as possible regardless of the speed at which the user moves the rocker.

Power switches usually come in two types. A momentary on‑off switch (such as on a [[laser pointer]]) usually takes the form of a button and only closes the circuit when the button is depressed. A regular on‑off switch (such as on a [[flashlight]]) has a constant on-off feature. Dual-action switches incorporate both of these features.

===Inductive loads===
When a strongly [[inductance|inductive]] load such as an [[electric motor]] is switched off, the current cannot drop instantaneously to zero; a [[Electric arc|spark]] will jump across the opening contacts. Switches for [[inductance|inductive]] loads must be rated to handle these cases. The spark will cause [[electromagnetic interference]] if not suppressed; a [[snubber]] network of a [[resistor]] and [[capacitor]] in series will quell the spark.<ref>{{cite web |url=https://www.illinoiscapacitor.com/pdf/Papers/RC_snubber.pdf |title=Cornell Dubilier Capacitors – CDE (En-US) |access-date=2017-10-05 |url-status=live |archive-url=https://web.archive.org/web/20170215074956/http://www.illinoiscapacitor.com/pdf/papers/rc_snubber.pdf |archive-date=2017-02-15 }}</ref>

===Incandescent loads===
[[File:General-Electric-silent-T-rated-light-switch.jpg|thumb|upright=0.5|A "T-rated" wall switch (the T is for ''Tungsten'' filament)<ref name="Electrical_Power_Systems_Technology_pg337">{{cite book|title=Electrical Power Systems Technology|last1=Fardo|first1=Stephen|last2=Patrick|first2=Dale|date=2009-01-01|publisher=The Fairmont Press, Inc.|page=337|isbn=9780881735864|url=https://books.google.com/books?id=bohwbWi3bUMC&q=switch%20T-rated&pg=PA337|access-date=2015-01-26|url-status=live|archive-url=https://web.archive.org/web/20171224053707/https://books.google.com/books?id=bohwbWi3bUMC&lpg=PA337&dq=switch%20T-rated&pg=PA337#v=onepage&q=switch%20T-rated&f=false|archive-date=2017-12-24 }}</ref> that is suited for incandescent loads]]

When turned on, an [[Incandescent light bulb|incandescent lamp]] draws a large [[inrush current]] of about ten times the steady-state current; as the filament heats up, its resistance rises and the current decreases to a steady-state value. A switch designed for an incandescent lamp load can withstand this inrush current.<ref name="Electrical_Power_Systems_Technology_pg337"/>

===Wetting current===
{{Main|Wetting current}}

''Wetting current'' is the minimum current needing to flow through a mechanical switch while it is operated to break through any film of [[oxidation]] that may have been deposited on the switch contacts.<ref>Gregory K. McMillan (ed) ''Process/Industrial Instruments and Controls Handbook (5th Edition)'' (McGraw Hill, 1999) {{ISBN|0-07-012582-1}} page 7.26</ref> The film of oxidation occurs often in areas with high [[humidity]]. Providing a sufficient amount of wetting current is a crucial step in designing [[Control engineering|systems]] that use delicate switches with small contact pressure as sensor inputs. Failing to do this might result in switches remaining electrically "open" due to contact oxidation.