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"Toggle button" redirects here. For the GUI widget, see Cycle button. For other uses, see Switch (disambiguation).
Electrical switches. Top, left to right: circuit breaker, mercury switch, wafer switch, DIP switch, surface mount switch, reed switch. Bottom, left to right: wall switch (U.S. style), miniature toggle switch, in‑line switch, push-button switch, rocker switch, microswitch.

In electrical engineering, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another.[1][2] The mechanism of a switch may be operated directly by a human operator to control a circuit (for example, a light switch or a keyboard button), may be operated by a moving object such as a door-operated switch, or may be operated by some sensing element for pressure, temperature or flow. A relay is a switch that is operated by electricity. Switches are made to handle a wide range of voltages and currents; very large switches may be used to isolate high-voltage circuits in electrical substations.




An ideal switch would have no voltage drop when closed, and would have no limits on voltage or current rating. It would have zero rise time and fall time during state changes, and would change state without "bouncing" between on and off positions.



A toggle switch in the "on" position.



Contact terminology[edit]

Triple-pole single-throw (TPST or 3PST) knife switch used to short the windings of a 3‑phase wind turbine for 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 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 separate circuits which are controlled by a single switch. For example, a "2-pole" switch has two separate identical sets of contacts controlled by the same switch. 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.[4]

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") and opened by the switch action. A switch with both types of contact is called a changeover 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 electrical power wiring (i.e., house and building wiring by electricians), names generally involve the suffix "-way"; however, these terms differ between British English and American English (i.e., the terms two way and three  way are used with different meanings).[5]

Electronics specification and abbreviation Expansion
Description Symbol
SPST 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. SPST-Switch.svg
SPDT Single pole, double throw Two-way Three-way A simple changeover switch: C (COM, Common) is connected to L1 or to L2. SPDT-Switch.svg
SPTT, c.o.
Single pole changeover
Single pole, centre off or
Single Pole, Triple Throw
    Similar to SPDT. Some suppliers use SPCO/SPTT for switches with a stable off position in the centre and SPDT for those without.[citation needed]
DPST Double pole, single throw Double pole Double pole Equivalent to two SPST switches controlled by a single mechanism DPST-symbol.svg
DPDT Double pole, double throw Equivalent to two SPDT switches controlled by a single mechanism. DPDT-symbol.svg
DPCO Double pole changeover
or Double pole, centre off
    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.
    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. Crossover-switch-symbol.svg
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 Diagram of 2P6T switch.png

Switches with larger numbers of poles or throws can be described by replacing the "S" or "D" with a number (e.g. 3PST, 4PST, 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[edit]

Snapshot of switch bounce on an oscilloscope. The switch bounces between on and off several times before settling.


The effects of contact bounce can be eliminated by use of mercury-wetted contacts, but these are now infrequently used because of the hazard of mercury release. Alternatively, contact circuits can be low-pass filtered to reduce or eliminate multiple pulses. In digital systems, multiple samples of the contact state can be taken or a time delay can be implemented in order for the contact bounce to settle before the contact input is used to control anything. Bounce in an SPDT switch can be eliminated by an SR latch.[7] All of these methods are referred to as "debouncing" circuits.

By analogy, the term "debounce" has arisen in the software development industry to describe rate-limiting or throttling the frequency of a method's execution.[8]

Arcs and quenching[edit]




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 nonconductive 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.



Power switching[edit]




Inductive loads[edit]


Incandescent loads[edit]

A "T-rated" wall switch (the T is for Tungsten filament)[12] that is suited for incandescent loads.

When turned on, an 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.[12]

Wetting current[edit]

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.[13] The film of oxidation occurs often in areas with high humidity. Providing a sufficient amount of wetting current is a crucial step in designing 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.



Biased switches[edit]

The momentary push-button switch is a type of biased switch. The most common type is a "push-to-make" (or normally-open or NO) switch, which makes contact when the button is pressed and breaks when the button is released. Each key of a computer keyboard, for example, is a normally-open "push-to-make" switch. A "push-to-break" (or normally-closed or NC) switch, on the other hand, breaks contact when the button is pressed and makes contact when it is released. An example of a push-to-break switch is a button used to release a door held closed by an electromagnet. The interior lamp of a household refrigerator is controlled by a switch that is held open when the door is closed.

Rotary switch[edit]

Main article: Rotary switch
A three-deck stacked rotary switch. Any number of switching elements may be stacked in this manner, by using a longer shaft and additional spacing standoffs between each switching element.





Toggle switch[edit]

Large toggle switch, depicted in circuit "open" position, electrical contacts to left; background is 1/4" square graph paper
Bank of toggle switches on a Data General Nova minicomputer front panel.
Toggle switches with the shared cover preventing certain forbidden combinations



The word "toggle" is a reference to a kind of mechanism or joint consisting of two arms, which are almost in line with each other, connected with an elbow-like pivot. However, the phrase "toggle switch" is applied to a switch with a short handle and a positive snap-action, whether it actually contains a toggle mechanism or not. Similarly, a switch where a definitive click is heard, is called a "positive on-off switch"[14] - - the most common use of this type of switch is a typical light switch or electrical outlet switch.

Multiple toggle switches may be mechanically interlocked to prevent forbidden combinations.

Special types[edit]

Opened float switch of a dirty water pump

Switches can be designed to respond to any type of mechanical stimulus: for example, vibration (the trembler switch), tilt, air pressure, fluid level (a float switch), the turning of a key (key switch), linear or rotary movement (a limit switch or microswitch), or presence of a magnetic field (the reed switch). Many switches are operated automatically by changes in some environmental condition or by motion of machinery. A limit switch is used, for example, in machine tools to interlock operation with the proper position of tools. In heating or cooling systems a sail switch ensures that air flow is adequate in a duct. Pressure switches respond to fluid pressure.

Mercury tilt switch[edit]

Main article: Mercury switch

The mercury switch consists of a drop of mercury inside a glass bulb with two or more contacts. The two contacts pass through the glass, and are connected by the mercury when the bulb is tilted to make the mercury roll on to them.

This type of switch performs much better than the ball tilt switch, as the liquid metal connection is unaffected by dirt, debris and oxidation, it wets the contacts ensuring a very low resistance bounce-free connection, and movement and vibration do not produce a poor contact. These types can be used for precision works.

It can also be used where arcing is dangerous (such as in the presence of explosive vapour) as the entire unit is sealed.

Knife switch[edit]

Main article: Knife switch
A high-voltage disconnect switch used in an electrical substation. Such switches are used mostly to isolate circuits, and usually cannot break load current. High-voltage switches are available for the highest transmission voltages, up to 1 million volts. This switch is gang-operated so that all three phases are interrupted at the same time.

Knife switches consist of a flat metal blade, hinged at one end, with an insulating handle for operation, and a fixed contact. When the switch is closed, current flows through the hinged pivot and blade and through the fixed contact. Such switches are usually not enclosed. The knife and contacts are typically formed of copper, steel, or brass, depending on the application. Fixed contacts may be backed up with a spring. Several parallel blades can be operated at the same time by one handle. The parts may be mounted on an insulating base with terminals for wiring, or may be directly bolted to an insulated switch board in a large assembly. Since the electrical contacts are exposed, the switch is used only where people cannot accidentally come in contact with the switch or where the voltage is so low as to not present a hazard.

Knife switches are made in many sizes from miniature switches to large devices used to carry thousands of amperes. In electrical transmission and distribution, gang-operated switches are used in circuits up to the highest voltages.

The disadvantages of the knife switch are the slow opening speed and the proximity of the operator to exposed live parts. Metal-enclosed safety disconnect switches are used for isolation of circuits in industrial power distribution. Sometimes spring-loaded auxiliary blades are fitted which momentarily carry the full current during opening, then quickly part to rapidly extinguish the arc.



Reversing switch[edit]

A DPDT switch has six connections, but since polarity reversal is a very common usage of DPDT switches, some variations of the DPDT switch are internally wired specifically for polarity reversal. These crossover switches only have four terminals rather than six. Two of the terminals are inputs and two are outputs. When connected to a battery or other DC source, the 4-way switch selects from either normal or reversed polarity. Such switches can also be used as intermediate switches in a multiway switching system for control of lamps by more than two switches.

Light switches[edit]

Main article: Light switch


Electronic switches[edit]

Three push button switches (Tactile Switches). Major scale is inches.




Many people use metonymy to call a variety of devices "switches" that conceptually connect or disconnect signals and communication paths between electrical devices, analogous to the way mechanical switches connect and disconnect paths for electrons to flow between two conductors. Early telephone systems used an automatically operated Strowger switch to connect telephone callers; telephone exchanges contain one or more crossbar switches today.

Since the advent of digital logic in the 1950s, the term switch has spread to a variety of digital active devices such as transistors and logic gates whose function is to change their output state between two logic levels or connect different signal lines, and even computers, network switches, whose function is to provide connections between different ports in a computer network.[15] The term 'switched' is also applied to telecommunications networks, and signifies a network that is circuit switched, providing dedicated circuits for communication between end nodes, such as the public switched telephone network. The common feature of all these usages is they refer to devices that control a binary state: they are either on or off, closed or open, connected or not connected.

Other switches[edit]

See also[edit]


  1. ^ "Switch". The Free Dictionary. Farlex. 2008. Retrieved 2008-12-27. 
  2. ^ "Switch". The American Heritage Dictionary, College Edition. Houghton Mifflin. 1979. p. 1301. 
  3. ^ "General Electric Contact Materials". Electrical Contact Catalog (Material Catalog). Tanaka Precious Metals. 2005. Retrieved 2007-02-21. 
  4. ^ RF Switch Explanation by Herley – General Microwave
  5. ^ Terminology on "Light wiring" differing US and UK usage of the term 'WAYS' when referring to switches
  6. ^ Walker, PMB, Chambers Science and Technology Dictionary, Edinburgh, 1988, ISBN 1-85296-150-3
  7. ^ Ganssle.com
  8. ^ "Debouncing Javascript Methods"
  9. ^ "Lab Note #105 Contact Life – Unsuppressed vs. Suppressed Arcing" (PDF). Arc Suppression Technologies. April 2011. Retrieved February 5, 2012.  (3.6 Mb)
  10. ^ http://uk.farnell.com/panasonic-ew/abj151261j/microswitch-hinge-lever/dp/2095696?in_merch=New%20Products&in_merch=Featured%20New%20Products&MER=i-9b10-00002068
  11. ^ http://www.nteinc.com/switches/pdf/pg1.pdf
  12. ^ a b Fardo, Stephen; Patrick, Dale (2009-01-01). Electrical Power Systems Technology. The Fairmont Press, Inc. p. 337. Retrieved 2015-01-26. 
  13. ^ Gregory K. McMillan (ed) Process/Industrial Instruments and Controls Handbook (5th Edition) (McGraw Hill, 1999) ISBN 0-07-012582-1 page 7.26
  14. ^ Gladstone, Bernard (1978). The New York times complete manual of home repair. Times Books. p. 399. 
  15. ^ "'Switch'". Telecom definitions. SearchTelecom.com. 2007. Retrieved 2008-12-27. 

External links[edit]