See J. C. McDonald, Fundamentals of Digital Switching (1990).
A switch is a mechanical device used to connect and disconnect an electric circuit at will. Switches cover a wide range of types, from subminiature up to industrial plant switching megawatts of power on high voltage supply and distribution lines.
In applications where multiple switching options are required (e.g., a telephone service), mechanical switches have long been replaced by electronic switching devices which can be automated and intelligently controlled.
The switch is referred to as a "gate" when abstracted to mathematical form. In the philosophy of logic, operational arguments are represented as logic gates. The use of electronic gates to function as a system of logical gates is the fundamental basis for the computer—i.e. a computer is a system of electronic switches which function as logical gates.
A railroad switch is not electrical, but a mechanical device to divert a train from one track to another.
In the simplest case, a switch has two pieces of metal called contacts that touch to make a circuit, and separate to 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.
A pair of contacts is said to be 'closed' when there is no space between them, allowing electricity to flow from one to the other. When the contacts are separated by an insulating air gap, an air space, they are said to be 'open', and no electricity can flow at typical voltages.
Switches can be and are classified according to the arrangement of their contacts in electronics fields— but electricians in the electrical wiring service business and their electrical supplier industries use different nomenclature, such as "one-way", "two-way", "three-way" and "four-way" switches—which have different meanings in North American and British cultural regions as is delineated in the table below.
Some contacts are normally open (Abbreviated "n.o." or "no") until closed by operation of the switch, while others are normally closed ("n.c. or "nc") and opened by the switch action, where the abbreviations given are commonly used on electronics diagrams for clarity of operation in assembly, analysis or troubleshooting. The serve to synchronize meaning with possible mistakes in wiring assembly, where wiring part of switch one way and part another (usually opposite) way will pretty much guarantee things won't work as designed.
A switch with both types of contact is called a changeover switch or "make-before-break" switch contact, whereas most switches have a spring loaded action which momentarily disconnect the load and so are "break-before-make" types by contrast— which type is used could be important, if for example, the switch selects two different power sources instead of switching circuit loads, or the circuit load will not and cannot tolerate any interruption in applied power.
The terms pole and throw are also used to describe switch contact variations. A pole is a set of contacts, the switch's electrical terminals that are connected to and belong to a single circuit, usually a load. A throw is one of two or more positions (the nomenclature is also applied to rotary switches, which can have many 'throw' positions) that the switch can adopt, which normally, but not always correspond to the number positions the switch handle or rotor can take when connecting between the common lead of the switch and a pole or poles. A throw position which connects no terminals (poles), has a mis-match between positions and positions which connect terminals, but are quite useful to turn things "Off" or for example, alternatively select between two scaled modes of operation. (e.g. Bright illumination, moderate illumination, no illumination.)
These terms give 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 involving the suffixed word "-way" are used; however, these terms differ between British and American English and the terms two way and three way are used in both with different meanings.
|Electronics specification and abbreviation|| Expansion |
| British |
| American |
|SPST||Single pole, single throw||One way||Two way||A simple on-off switch: The two terminals are either connected together or not connected to anything. An example is a light switch.||-------||SPDT||Single pole, double throw||Two way||Three way||A simple changeover switch: C (COM, Common) is connected to L1 or to L2.|
| 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.|
|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: A is connected to B and D to E, or A is connected to C and D to F.|||-------||DPCO|| Double pole changeover |
or Double pole, centre off
|Equivalent to DPDT. Some suppliers use DPCO for switches with a stable off position in the centre and DPDT for those without.|
|Intermediate switch||4-way switch||DPDT switch internally wired for polarity-reversal applications: only four rather than six wires are brought outside the switch housing; with the above, B is connected to F and C to E; hence A is connected to B and D to C, or A is connected to C and D to B.|
The momentary push-button switch is a type of biased switch. The most common type is a push-to-make switch, which makes contact when the button is pressed and breaks when the button is released. A push-to-break 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 open by an electromagnet. Changeover push button switches do exist but are even less common.
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.
The "knife", a flat metal swinging arm, is moved by the user between two or more contact areas. The knife and contacts are typically formed of copper, steel, or brass, depending on the application.
The primary advantage of a knife switch is the extremely high current capability inherent to the design. The amount of surface area on the "knife" that shorts the contacts is also extremely high, allowing a wide range of high voltage or high amperage applications with no circuit degradation, choke, or arcing during the switch throw. Thicker components need only be accompanied by wider contacts to conduct higher currents, which allows the design to scale extremely well with size.
Although knife switches are inferior to traditional switches in applications where user safety is paramount, knife switches are still commonly employed in everyday high-voltage applications such as building transformers, large power relays, and air-conditioning units.
Switching a load on or off from two locations (for instance, turning a light on or off from either end of a flight of stairs) requires two SPDT switches. There are two basic methods of wiring to achieve this, and another not recommended.
In the first method, mains is fed into the common terminal of one of the switches; the switches are then connected through the L1 and L2 terminals (swapping the L1 and L2 terminals will just make the switches work the other way round), and finally a feed to the light is taken from the common of the second switch. A connects to B or C, D connects to B or C; the light is on if A connects to D, i.e. if A and D both connect to B or both connect to C.
The second method is to join the three terminals of one switch to the corresponding terminals on the other switch and take the incoming supply and the wire out to the light to the L1 and L2 terminals. Through one switch A connects to B or C, through the other also to B or C; the light is on if B connects to C, i.e. if A connects to B with one switch and to C with the other.
Wiring needed in addition to the mains network (not including protective earths):
If the mains and the load are connected to the system of switches at one of them, then in both methods we need three wires between the two switches. In the first method one of the three wires just has to pass through the switch, which tends to be less convenient than being connected. When multiple wires come to a terminal they can often all be put directly in the terminal. When wires need to be joined without going to a terminal a crimped joint, piece of terminal block, wirenut or similar device must be used and the bulk of this may require use of a deeper backbox.
Using the first method, there are four possible combinations of switch positions: two with the light on and two with the light off. N.B. This diagram uses the American electrical wiring name from the table above.
If there is a hot (a unique phase) and a neutral wire in both switches and just one wire between them where the light is connected (as in the picture), you can then solve the two way switch problem easily: just plug the hot in the top from switch, the neutral in the bottom from switch and the wire that goes to the light in the middle from the switch. This in both switches. Now you have a fully functional two way switch.
This works like the first method above: there are four possibilities and just in two of them there is a hot and a neutral connected in the poles of the light. In the other ones, both poles are neutral or hot and then no current flows because the potential difference is zero.
The advantage of this method is that it uses just one wire to the light, having a hot and neutral in both switches.
The reason why this is not recommended is that the light socket pins may still be hot even with the light off, which poses a risk when changing a bulb. Another problem with this method is that in both switches there will be hot and neutral wires entering a single switch, which can lead to a short circuit in the event of switch failure, unlike the other methods.
This method is in defiance of the National Electrical Code (USA) and the Canadian Electrical Code. In nearly any and all applications, neutral conductors should never be switched. Not only is this a shock hazard due to mistakenly believing that a hot conductor is switched off; it is also a fire hazard and can destroy sensitive equipment due to excessive and unbalanced current flowing on hot conductors that would otherwise flow back to ground on the neutral conductor.
For more than two locations, the two cores connecting the L1 and L2 of the switches must be passed through an intermediate switch (as explained above) wired to swap them over. Any number of intermediate switches can be inserted, allowing for any number of locations.
Wiring needed in addition to the mains network (not including protective earths):
Using the first method, there are eight possible combinations of switch positions: four with the light on and four with the light off. N.B. This diagram also uses the American electrical wiring name from the table above.
As mentioned above, the above circuit can be extended by using multiple 4-way switches between the 3-way switches to extend switching ability to any number of locations.
For this reason, most power switches (most light switches and almost all larger switches) have spring mechanisms in them 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.
The reason for the difference remains a bit of a mystery. A few hypotheses are often put forward, (for example in the USA if the switch spring fails it cannot cause the switch to accidentally turn on, in other words it will fail safe), but none have been validated. Since there is no significant technical reason for either preference, the standards likely developed due to chance and some degree of cultural isolation.
In countries prone to earthquakes, such as Japan, most switches rock sideways to prevent the switch from inadvertently being turned on or off by falling objects.
Sequential digital logic circuits are particularly vulnerable to contact bounce. The voltage waveform produced by switch bounce usually violates the amplitude and timing specifications of the logic circuit. The result is that the circuit may fail, due to problems such as metastability, race conditions, runt pulses and glitches.
There are a number of techniques for debouncing (mitigating the effects of switch bounce). They can be split into wet contacts, timing based techniques and Hysteresis based techniques.
Mercury wetted switches are not a popular option today, primarily due to mercury's toxicity.
If an on/off switch is used with a pull up (or pull down) resistor and a single capacitor is placed over the switch (or across the resistor, but this can cause nasty spikes of current on the power supply lines) then when the switch is closed (generally pressed) the capacitor will almost instantly discharge through the switch. But when the switch is opened (generally released) the capacitor takes some time to recharge. Therefore contact bounce will have negligible effect on the output. The slow edges can be cleaned up with a Schmitt trigger if necessary. This method has the advantage of fast response to the initial press but the current surges through the switch may be undesirable. Other RC based systems are also possible with various responses and such systems are probably the easiest method when constructing with simple logic gates and discrete components.