In electrical engineering, a switch is an electrical component that
can "make" or "break" an electrical circuit, interrupting the current
or diverting it from one conductor to another. The mechanism of
a switch removes or restores the conducting path in a circuit when it
is operated. It may be operated manually, for example, a light switch
or a keyboard button, may be operated by a moving object such as a
door, or may be operated by some sensing element for pressure,
temperature or flow. A switch will have one or more sets of contacts,
which may operate simultaneously, sequentially, or alternately.
Switches in high-powered circuits must operate rapidly to prevent
destructive arcing, and may include special features to assist in
rapidly interrupting a heavy current. Multiple forms of actuators are
used for operation by hand or to sense position, level, temperature or
1 Description 2 Contacts
2.1 Contact terminology 2.2 Contact bounce 2.3 Arcs and quenching 2.4 Power switching 2.5 Inductive loads 2.6 Incandescent loads 2.7 Wetting current
3.1 Biased switches 3.2 Rotary switch 3.3 Toggle switch
4.1 Mercury tilt switch 4.2 Knife switch 4.3 Footswitch 4.4 Reversing switch
5 Light switches 6 Electronic switches 7 Other switches 8 See also 9 References 10 External links
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.
The most familiar form of switch is a manually operated electromechanical device with one or more sets of electrical contacts, which are connected to external circuits. Each set of contacts can be in one of two states: either "closed" meaning the contacts are touching and electricity can flow between them, or "open", meaning the contacts are separated and the switch is nonconducting. The mechanism actuating the transition between these two states (open or closed) are usually (there are other types of actions) either an "alternate action" (flip the switch for continuous "on" or "off") or "momentary" (push for "on" and release for "off") type. A switch may be directly manipulated by a human as a control signal to a system, such as a computer keyboard button, or to control power flow in a circuit, such as a light switch. Automatically operated switches can be used to control the motions of machines, for example, to indicate that a garage door has reached its full open position or that a machine tool is in a position to accept another workpiece. Switches may be operated by process variables such as pressure, temperature, flow, current, voltage, and force, acting as sensors in a process and used to automatically control a system. For example, a thermostat is a temperature-operated switch used to control a heating process. A switch that is operated by another electrical circuit is called a relay. Large switches may be remotely operated by a motor drive mechanism. Some switches are used to isolate electric power from a system, providing a visible point of isolation that can be padlocked if necessary to prevent accidental operation of a machine during maintenance, or to prevent electric shock. 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. Practical switches fall short of this ideal; they have resistance, limits on the current and voltage they can handle, finite switching time, etc. The ideal switch is often used in circuit analysis as it greatly simplifies the system of equations to be solved, but this can lead to a less accurate solution. Theoretical treatment of the effects of non-ideal properties is required in the design of large networks of switches, as for example used in telephone exchanges. Contacts
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 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. Sometimes the contacts are plated with noble metals. 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.
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
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.
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 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 electrical power wiring (i.e., house and building
wiring by electricians), names generally involve the suffix "-way";
however, these terms differ between
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-NO Form A
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 pushbutton switch.
SPST-NC Form B
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 pushbutton switch.
SPDT Form C
Single pole, double throw Two-way Three-way A simple break-before-make changeover switch: C (COM, Common) is connected to L1 (NC, Normally Closed) or to L2 (NO, Normally Open).
SPCO SPTT, c.o. Single pole changeover or 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.
DPST Double pole, single throw Double pole Double pole Equivalent to two SPST switches controlled by a single mechanism.
DPDT Double pole, double throw
Equivalent to two SPDT switches controlled by a single mechanism.
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.
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.
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
Snapshot of switch bounce on an oscilloscope. The switch bounces between on and off several times before settling.
Contact bounce (also called chatter) is a common problem with
mechanical switches and relays.
A "T-rated" wall switch (the T is for Tungsten filament) 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. 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. 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. Actuator The moving part that applies the operating force to the contacts is called the actuator, and may be a toggle or dolly, a rocker, a push-button or any type of mechanical linkage (see photo). Biased switches A switch normally maintains its set position once operated. A biased switch contains a mechanism that springs it into another position when released by an operator. 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 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.
A rotary switch operates with a twisting motion of the operating handle with at least two positions. One or more positions of the switch may be momentary (biased with a spring), requiring the operator to hold the switch in the position. Other positions may have a detent to hold the position when released. A rotary switch may have multiple levels or "decks" in order to allow it to control multiple circuits. One form of rotary switch consists of a spindle or "rotor" that has a contact arm or "spoke" which projects from its surface like a cam. It has an array of terminals, arranged in a circle around the rotor, each of which serves as a contact for the "spoke" through which any one of a number of different electrical circuits can be connected to the rotor. The switch is layered to allow the use of multiple poles, each layer is equivalent to one pole. Usually such a switch has a detent mechanism so it "clicks" from one active position to another rather than stalls in an intermediate position. Thus a rotary switch provides greater pole and throw capabilities than simpler switches do. Other types use a cam mechanism to operate multiple independent sets of contacts. Rotary switches were used as channel selectors on television receivers until the early 1970s, as range selectors on electrical metering equipment, as band selectors on multi-band radios and other similar purposes. In industry, rotary switches are used for control of measuring instruments, switchgear, or in control circuits. For example, a radio controlled overhead crane may have a large multi-circuit rotary switch to transfer hard-wired control signals from the local manual controls in the cab to the outputs of the remote control receiver. Toggle switch
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
Toggle switches with the shared cover preventing certain forbidden combinations
A toggle switch is a class of electrical switches that are manually
actuated by a mechanical lever, handle, or rocking mechanism.
Toggle switches are available in many different styles and sizes, and
are used in numerous applications. Many are designed to provide the
simultaneous actuation of multiple sets of electrical contacts, or the
control of large amounts of electric current or mains voltages.
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". A very common use of this type of switch
is to switch lights or other electrical equipment on or off. Multiple
toggle switches may be mechanically interlocked to prevent forbidden
In some contexts, particularly computing, a toggle switch, or the
action of toggling, is understood in the different sense of a
mechanical or software switch that alternates between two states each
time it is activated, regardless of mechanical construction. For
example, the caps lock key on a computer causes all letters to be
generated in capitals after it is pressed once; pressing it again
reverts to lower-case letters.
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 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 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. Footswitch A footswitch is a rugged switch which is operated by foot pressure. An example of use is in the control of a machine tool, allowing the operator to have both hands free to manipulate the workpiece. The foot control of an electric guitar is also a footswitch. Reversing switch 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 Main article: Light switch In building wiring, light switches are installed at convenient locations to control lighting and occasionally other circuits. By use of multiple-pole switches, multiway switching control of a lamp can be obtained from two or more places, such as the ends of a corridor or stairwell. A wireless light switch allows remote control of lamps for convenience; some lamps include a touch switch which electronically controls the lamp if touched anywhere. In public buildings several types of vandal resistant switches are used to prevent unauthorized use. Electronic switches
Three push button switches (Tactile Switches). Major scale is inches.
See also: Electronic switch
A relay is an electrically operated switch. Many relays use an
electromagnet to operate a switching mechanism mechanically, but other
operating principles are also used.
Centrifugal switch Company switch Dead man's switch Fireman's switch Hall-effect switch Inertial switch Isolator switch Kill switch Latching switch Light switch Load control switch Membrane switch MEMS switch Optical switch Photonic switch Piezo switch Pull switch Push switch Sense switch Slotted optical switch Stepping switch Thermal switch Time switch Touch switch Transfer switch Zero speed switch
^ "Switch". The Free Dictionary. Farlex. 2008. Retrieved
^ "Switch". The American Heritage Dictionary, College Edition.
Houghton Mifflin. 1979. p. 1301.
Media related to Electric switches at Wikimedia Commons
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Analogue switch Banyan switch Battery isolator Cam switch Centrifugal switch Company switch Contact protection Crossbar switch Crossover switch Cryotron DIP switch Dry contact Electric switchboard Float switch Half-moon switch Humidistat Infinite switch Inertial switch Kill switch Key switch Knife switch Limit switch Latching switch Light switch Lightning switch Magnetic proximity fuze Magnetic starter Magnetic switch Mercury switch Miniature snap-action switch Motion-triggered contact insufficiency Optical switch Photoswitch Piezo switch Placebo button Pull switch Push switch Push-button Reed switch Rotary switch Sail switch Sea switch Silicone rubber keypad Softswitch Spark gap Staircase timer Stepping switch Time switch Touch switch Transfer switch Vacuum switch Vandal-resistant switch Wireless light switch Zero