A switch is a mechanical device used to connect and disconnect a circuit at will. Switches cover a wide range of types, from subminiature up to industrial plant switching megawatts of power on high voltage 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applications where multiple switching options are required (e. g. , a telephone service), mechanical switches have long been replaced by electronic switching devices which can be automated and intelligently controlled. Basic principle A traditional landline telephone system also known as "plain old telephone service" (POTS, commonly handles both signaling and audio information
The prototypical model is perhaps a mechanical device (for example a railroad switch) which can be disconnected from one course and connected to another. This article primarily uses North American terminology British and Commonwealth terms are given in parentheses
The switch is referred to as a "gate" when abstracted to mathematical form. A logic gate performs a logical operation on one or more logic inputs and produces a single logic output Mathematics is the body of Knowledge and Academic discipline that studies such concepts as Quantity, Structure, Space and In the philosophy of logic, operational arguments are represented as logic gates. Logic is the study of the principles of valid demonstration and Inference. In Logic, an argument is a Set of one or more Declarative sentences (or "propositions") known as the Premises along A logic gate performs a logical operation on one or more logic inputs and produces a single logic output The use of electronic gates to function as a system of logical gates is the fundamental basis for the computer—i. A computer is a Machine that manipulates data according to a list of instructions. e. a computer is a system of electronic switches which function as logical gates.
A simple semiconductor switch is a transistor. A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that In Electronics, a transistor is a Semiconductor device commonly used to amplify or switch electronic signals
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 M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across The contact material is chosen for its resistance to corrosion, because most metals form insulating oxides that would prevent the switch from working. Corrosion means the breaking down of essential properties in a material due to Chemical reactions with its surroundings An insulator, also called a Dielectric, is a material that resists the flow of Electric current. An oxide is a Chemical compound containing at least one Oxygen atom as well as at least one other element Contact materials are also chosen on the basis of electrical conductivity, hardness (resistance to abrasive wear), mechanical strength, low cost and low toxicity. Electrical conductivity or specific conductivity is a measure of a material's ability to conduct an Electric current. Hardness refers to various properties of Matter in the Solid phase that give it high resistance to various kinds of shape change when Force In Materials science, wear is the erosion of material from a Solid Surface by the action of another substance In Materials science, the strength of a material refers to the material's ability to resist an applied force Toxicity is the degree to which a substance is able to damage an exposed organism
Sometimes the contacts are plated with noble metals. Electroplating is the process of using electrical current to reduce Cations of a desired material from a solution and coat a conductive object Noble metals are Metals that are resistant to Corrosion or Oxidation, unlike most Base metals They tend to be Precious metals often They may be designed to wipe against each other to clean off any contamination. Design is used both as a Noun and a Verb. The term is often tied to the various Applied arts and Engineering (See design disciplines Nonmetallic conductors, such as conductive plastic, are sometimes used. In Science and engineering, a conductor is a material which contains movable Electric charges. Plastic is the general common term for a wide range of synthetic or semisynthetic organic solid materials suitable for the manufacture of industrial products
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). An actuator is a mechanical device for moving or controlling a mechanism or system
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 a space, they are said to be 'open', and no electricity can flow.
Switches can be classified according to the arrangement of their contacts. Some contacts are normally open until closed by operation of the switch, while others are normally closed and opened by the switch action. A switch with both types of contact is called a changeover switch. A changeover switch, more commonly called an SPDT switch is a type of Switch that can be positioned in two closed states
The terms pole and throw are used to describe switch contacts. A pole is a set of contacts that belong to a single circuit. A throw is one of two or more positions that the switch can adopt. These terms give rise to abbreviations for the types of switch which are used in the electronics industry. Electronics refers to the flow of charge (moving Electrons through Nonmetal conductors (mainly Semiconductors, whereas electrical In mains wiring names generally involving the 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. British English or UK English ( BrE, BE, en-GB) is the broad term used to distinguish the forms of the English language used in the Phonology North American English regional phonology In many ways compared to English English, North American English is conservative in its Phonology. English is a West Germanic language originating in England and is the First language for most people in the United Kingdom, the United States
|Electronics abbreviation||Expansion of abbreviation||British mains wiring name||American mains wiring name||Description||Symbol||IEC 60617|
|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. The International Electrotechnical Commission ( IEC) is a not-for-profit, non-governmental international Standards organization that prepares and publishes An example is a light switch. A light switch is a Switch, most commonly used to operate Electric lights permanently connected equipment or Electrical outlets In modern homes|
|SPDT||Single pole, double throw||Two way||Three way||A simple changeover switch: C (Common) is connected to L1 or to L2.|
|SPCO||Single pole changeover|
or Single pole, centre off
|Equivalent to SPDT. Some suppliers use SPCO 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.|
Switches with larger numbers of poles or throws can be described by replacing the "S" or "D" with a number or in some cases the letter T (for triple). In the rest of this article the terms SPST SPDT and intermediate will be used to avoid the ambiguity in the use of the word "way".
In a multi-throw switch, there are two possible transient behaviors as you move from one position to another. In some switch designs, the new contact is made before the old contact is broken. This is known as make-before-break, and ensures that the moving contact never sees an open circuit (also referred to as a shorting switch). The alternative is break-before-make, where the old contact is broken before the new one is made. This ensures that the two fixed contacts are never shorted to each other. Both types of design are in common use, for different applications.
A biased switch is one containing a spring that returns the actuator to a certain position. A spring is a flexible elastic object used to store mechanical Energy. The "on-off" notation can be modified by placing parentheses around all positions other than the resting position. For example, an (on)-off-(on) switch can be switched on by moving the actuator in either direction away from the centre, but returns to the central off position when the actuator is released.
The momentary push-button switch is a type of biased switch. A push-button (often simply " button " or " pushbutton " is a simple Switch mechanism for controlling some aspect of a Machine 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. An electromagnet is a type of Magnet in which the Magnetic field is produced by the flow of an electric current. Changeover push button switches do exist but are even less common.
Switches can be designed to respond to any type of mechanical stimulus: for example, vibration (the trembler switch), tilt, air pressure, fluid level (the float switch), the turning of a key (key switch), linear or rotary movement (the limit switch or microswitch), or presence of a magnetic field (the reed switch). The reed switch is an electrical switch operated by an applied Magnetic field.
The mercury switch consists of a drop of mercury inside a glass bulb with 2 contacts. A mercury switch (also known as a mercury tilt switch) is a Switch whose purpose is to allow or interrupt the flow of Electric current in an Electrical Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum Glass in the common sense refers to a Hard, Brittle, transparent Solid, such as that used for Windows many 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.
Knife switches are a more or less obsolete type of power switch used in the 1800s. A knife switch is a type of Switch used to control the flow of Electricity in a Circuit. The live (hot) parts of the switch are uncovered and uninsulated, and they are unsuitable for use at shock-risk voltages. Knife switches have a relatively large contact spacing when open, so in the 1800s were often used to control power machinery running at high voltage, a use that can only be considered dangerous.
Knife switches are seen in horror films set in the 1800s, especially in underground laboratories, and have something of an association with Frankenstein et al. Frankenstein or The Modern Prometheus, generally known as Frankenstein, is a Novel written by the British author Mary Shelley
Today knife switches are used in demonstrations, where the large size and simple mechanism make for easy and immediate understanding of operation. They are also sometimes encountered in heavy-duty industrial applications.
Other types of switch include:
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. A centrifugal switch is an Electric switch that operates using the Centrifugal force created from a rotating shaft most commonly that of an Electric motor A DIP switch is a set of manual electric switches that are packaged in a group in a standard Dual in-line package (DIP (the whole package unit may also be referred A Hall effect sensor is a Transducer that varies its output Voltage in response to changes in Magnetic field. An inertial switch is a Switch, firmly mounted upon a vehicle or other mobile device that senses shock or vibration A membrane switch is an electrical switch for turning on and off a circuit A toggle switch is a class of Electrical switches that are actuated by a mechanical Lever, handle or rocking mechanism Transfer switches allow switching from a primary power source to a secondary or tertiary power source and are employed in some electrical power distribution systems A time switch is an electric switch with a built-in Clock that plugs into an electric socket, between the socket itself and a power plug 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. Intermediate switches are also an important part of multiway switching systems with more than two switches (see next section).
Multiway switching is a method of connecting switches in groups so that any switch can be used to connect or disconnect the load. This is most commonly done with lighting.
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.
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 NEC and the CEC. 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 outherwise 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.
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.
When a switch is designed to switch significant power, the transitional state of the switch as well as the ability to stand continuous operating currents must be considered. When a switch is on 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 briefly a quarter (or worse if the load is not purely resistive) of the load's rated power is dropped in the switch.
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 laser pointer is a portable pen-sized Laser designed to be held in the hand and most commonly used to project a point of light to highlight items of interest during a A regular on-off switch (such as on a flashlight) has a constant on-off feature. A flashlight (also called an electric torch or torch) is a portable electric spotlight which emits light from a small incandescent Lightbulb Dual-action switches incorporate both of these features.
Domestic light switches are generally moved up to switch on in the US, yet down in most of Europe.
The reason for the difference remains a bit of a mystery. A few hypotheses are often put forward, 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.
Contact bounce (also called chatter) is a common problem with mechanical switches and relays. A relay is an electrical Switch that opens and closes under the control of another Electrical circuit. Switch and relay contacts are usually made of springy metals that are forced into contact by an actuator. When the contacts strike together, their momentum and elasticity act together to cause bounce. The result is a rapidly pulsed electrical current instead of a clean transition from zero to full current. The waveform is then further modified by the parasitic inductances and capacitances in the switch and wiring, resulting in a series of damped sinusoidal oscillations. In Electrical circuits, any Electric current i produces a Magnetic field and hence generates a total Magnetic flux \Phi acting Capacitance is a measure of the amount of Electric charge stored (or separated for a given Electric potential. Oscillation is the repetitive variation typically in Time, of some measure about a central value (often a point of Equilibrium) or between two or more different states This effect is usually unnoticeable in AC mains circuits, where the bounce happens too quickly to affect most equipment, but causes problems in some analogue and logic circuits that respond fast enough to misinterpret the on-off pulses as a data stream. Analogue electronics (or analog in American English) are those electronic systems with a continuously Variable signal A logic gate performs a logical operation on one or more logic inputs and produces a single logic output
Sequential digital logic circuits are particularly vulnerable to contact bounce. In Digital circuit theory sequential logic is a type of logic circuit whose output depends not only on the present input but also on the history of the input 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. Metastability in electronics is the ability of a non-equilibrium electronic state to persist for a long (and theoretically unlimited period of time (see Asynchronous circuit A race condition or race hazard is a flaw in a System or process whereby the output and/or result of the process is unexpectedly and critically dependent In Digital circuits a runt pulse is a narrow pulse that due to non-zero rise and fall times of the signal does not reach a valid high or low A glitch is a short-lived Fault in a system The term is particularly common in the Computing and Electronics industries and in Circuit bending
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. A system with hysteresis can be summarised as a system that may be in any number of states independent of the inputs to the system
Mercury wetted switch contacts do not suffer from bounce, as once the connection is made the mercury keeps the contact conducting during mechanical bounce.
Mercury wetted switches are not a popular option today, primarily due to mercury's toxicity.
Timing based techniques rely on adding sufficient delays that the extra transitions introduced by bounce are ignored. Their big advantage is they do not require any special design on the switch side and so are generally cheaper. However for good performance they must be designed to suit the switch (too much delay and the response will be needlessly sluggish, too little and bounce will not be eliminated).
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. In Electronics, a Schmitt trigger is a Comparator circuit that incorporates positive Feedback. 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.
A finite state machine or software running on a CPU can be designed to wait a fixed number of clock cycles after any transition before registering another one. This provides a cheap option for debouncing when a microprocessor, microcontroller or gate array is already in use but is unlikely to be worthwhile if constructing with single logic gates CLPD's. A microprocessor incorporates most or all of the functions of a Central processing unit (CPU on a single Integrated A microcontroller (also MCU or µC is a functional Computer system-on-a- chip. A gate array or uncommitted logic array ( ULA) is an approach to the design and manufacture of Application-specific integrated circuits (ASICs
Alternatively, it is possible to build in hysteresis by making the position where a press is detected separate from that where a release is detected. A system with hysteresis can be summarised as a system that may be in any number of states independent of the inputs to the system As long as the bounces are small enough not to take the switch between these positions, bounce problems will be eliminated. Hysteresis can be mechanical or electronic (e. g. a Schmitt trigger). In Electronics, a Schmitt trigger is a Comparator circuit that incorporates positive Feedback.
A changeover switch provides two distinct events, the making of one contact and the breaking of the other. These can be used to feed the inputs of a flip-flop. In Digital circuits a flip-flop is a term referring to an Electronic circuit (a Bistable Multivibrator) that has two stable states and thereby This way the press will only be detected when the pressed contact is made and the release will only be detected when the released contact is made. When the switch is bouncing around in the middle no change is detected. To get a single logic signal from such a setup a simple SR latch can be used. In Electronics, a latch is a kind of Bistable Multivibrator, an Electronic circuit which has two stable states and thereby can store
Normal switches are designed to give a hard on-off but it is also possible to design one that varies more gradually between the hard-on and hard-off states. This keeps the output changes caused by bouncing small. Then by feeding the output to a Schmitt trigger the effect of those bounce based changes can be eliminated. In Electronics, a Schmitt trigger is a Comparator circuit that incorporates positive Feedback.