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In electrical engineering, a switch is an electrical component that may “make” or “break” an electrical circuit, interrupting the present 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 could also be operated manually, for instance, a light switch or a keyboard button, could also be operated by a moving object akin to a door, or may be operated by some sensing element for pressure, temperature or flow.
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
4.1 Mercury tilt switch
4.2 Knife switch
4.4 Reversing switch
Essentially 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 certainly 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) might be either a “toggle” (flip switch for continuous “on” or “off”) or “momentary” (push-for “on” or push-for “off”) type.
A switch may be directly manipulated by a human as a control signal to a system, akin to a computer keyboard button, or to control power flow in a circuit, similar to a light switch. Automatically operated switches can be used to manage the motions of machines, for instance, to indicate that a garage door has reached its full open position or that a machine tool is ready to accept another workpiece. Switches could also be operated by process variables resembling pressure, temperature, flow, current, voltage, and force, acting as sensors in a process and used to automatically control a system. For instance, a thermostat is a temperature-operated switch used to regulate a heating process. A switch that is operated by another electrical circuit is named 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 stop accidental operation of a machine during maintenance, or to prevent electric shock.
A super switch would haven’t any voltage drop when closed, and would don’t have any 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’ve resistance, limits on the current and voltage they will handle, finite switching time, etc. The perfect switch is often used in circuit analysis because it greatly simplifies the system of equations to be solved, but this may lead to a less accurate solution. Theoretical treatment of the consequences of non-ideal properties is required within the design of large networks of switches, as for example utilized in telephone exchanges.
In the best 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 will 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, similar to conductive plastic, are sometimes used. To stop the formation of insulating oxides, a minimum wetting current could also be specified for a given switch design.
In electronics, switches are classified in accordance with the arrangement of their contacts. A pair of contacts is alleged to be “closed” when current can flow from one to the other. When the contacts are separated by an insulating air gap, they’re 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 variety 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 variety of “throws” is the number of separate wiring path choices apart from “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 will be connected to considered one of three other contacts, etc.
In a switch where the contacts remain in one state unless actuated, similar to 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 forms of contact known as 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 opposite.
These terms have given rise to abbreviations for the types of switch that are used within the electronics industry corresponding to “single-pole, single-throw” (SPST) (the only 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).
Switches with larger numbers of poles or throws could 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 text the terms SPST, SPDT and intermediate might be used to avoid the ambiguity.
Contact bounce (also called chatter) is a typical problem with mechanical switches and relays. Switch and relay contacts are usually fabricated from 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’s 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 and logic circuits that respond fast enough to misinterpret the on‑off pulses as a knowledge stream.
The results of contact bounce will be eliminated by use of mercury-wetted contacts, but these at the moment are infrequently used due to the hazard of mercury release. Alternatively, contact circuit voltages could be low-pass filtered to scale back or eliminate multiple pulses from appearing. In digital systems, multiple samples of the contact state could be taken at a low rate and examined for a gentle sequence, so that contacts can settle before the contact level is taken into account reliable and acted upon. Bounce in SPDT switch contacts signals will be filtered out using a SR flip-flop (latch) or Schmitt trigger. All of those methods are known as ‘debouncing’.
By analogy, the term “debounce” has arisen in the software development industry to describe rate-limiting or throttling the frequency of a technique’s execution.
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 because the switch is opened, forming a gas plasma, also known as an electric arc. The plasma is of low resistance and is ready to sustain power flow, even with the separation distance between the switch contacts steadily increasing. The plasma is also highly regarded and is able to eroding the metal surfaces of the switch contacts. Electric current arcing causes significant degradation of the contacts and in addition significant electromagnetic interference (EMI), requiring the usage of arc suppression methods.
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 make use of a quick-moving switch mechanism, typically using a spring-operated tipping-point mechanism to assure quick motion of switch contacts, whatever 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 because the spring tension is released.
As the facility being switched increases, other methods are used to attenuate or prevent arc formation. A plasma is hot and can rise because of convection air currents. The arc may be quenched with a series of non-conductive blades spanning the distance between switch contacts, and because the arc rises, its length increases as it forms ridges rising into the spaces between the blades, until the arc is too long to remain 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 in excess of 100,000‑watt capacity often have switch contacts surrounded by something aside from air to more rapidly extinguish the arc. For instance, the switch contacts may operate in a vacuum, immersed in mineral oil, or in sulfur hexafluoride.
In AC power service, the present 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 circuits.
When a switch is designed to change significant power, the transitional state of the switch in addition to the ability to withstand continuous operating currents should be considered. When a switch is in the on state, its resistance is near zero and very little power is dropped within 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 (or worse if the load is just not purely resistive) is briefly dropped within the switch.
For that reason, power switches intended to interrupt a load current have spring mechanisms to verify 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 are available in two types. A momentary on‑off switch (equivalent to 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 continuing on-off feature. Dual-action switches incorporate both of those features.
When a strongly inductive load equivalent to an electric motor is switched off, the current cannot drop instantaneously to zero; a spark will jump across the opening contacts. Switches for 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.
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 gradual-state value. A switch designed for an incandescent lamp load can withstand this inrush current.
Wetting current is the minimum current needing to flow through a mechanical switch while it’s operated to break through any film of oxidation that will 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 end in switches remaining electrically “open” as a consequence of contact oxidation.
The moving part that applies the operating force to the contacts known as the actuator, and may be a toggle or dolly, a rocker, a push-button or any type of mechanical linkage (see photo).
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 instance, is a normally-open “push-to-make” switch. A “push-to-break” (or normally-closed or NC) switch, then again, breaks contact when the button is pressed and makes contact when it’s 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’s held open when the door is closed.
A rotary switch operates with a twisting motion of the operating handle with at least two positions. A number of positions of the switch may be momentary (biased with a spring), requiring the operator to hold the switch within the position. Other positions may have a detent to carry the position when released. A rotary switch may have multiple levels or “decks” in order to allow it to control multiple circuits.
One type 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 might be connected to the rotor. The switch is layered to allow the use of multiple poles, each layer is equivalent to at least one pole. Usually such a switch has a detent mechanism so it “clicks” from one active position to a different rather than stalls in an intermediate position. Thus a rotary switch provides greater pole and throw capabilities than simpler switches do.
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 instance, a radio controlled overhead crane may have a big multi-circuit rotary switch to transfer hard-wired control signals from the local manual controls within the cab to the outputs of the remote control receiver.
A toggle switch is a class of electrical switches which can be manually actuated by a mechanical lever, handle, or rocking mechanism.
Toggle switches can be found in many different styles and sizes, and are utilized in numerous applications. Many are designed to offer 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 type of mechanism or joint consisting of two arms, which are almost according to one another, 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, known as a “positive on-off switch”. A quite common use of this sort of switch is to switch lights or other electrical equipment on or off. Multiple toggle switches could also be mechanically interlocked to forestall forbidden combinations.
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’s 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.
Switches might be designed to answer any type of mechanical stimulus: for instance, 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 instance, in machine tools to interlock operation with the correct 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.
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.
Any such 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 really low resistance bounce-free connection, and movement and vibration do not produce a poor contact. These types can be utilized for precision works.
It may also be used where arcing is dangerous (equivalent to in the presence of explosive vapour) as the complete unit is sealed.
Knife switches encompass a flat metal blade, hinged at one end, with an insulating handle for operation, and a hard and fast 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 applying. Fixed contacts could also be backed up with a spring. Several parallel blades will be operated at the identical time by one handle. The parts could also be mounted on an insulating base with terminals for wiring, or could also be directly bolted to an insulated switch board in a large assembly. Because the electrical contacts are exposed, the switch is used only where people cannot accidentally come involved with the switch or where the voltage is so low as to not present a hazard.
Knife switches are made in lots of sizes from miniature switches to large devices used to hold thousands of amperes. In electrical transmission and distribution, gang-operated switches are utilized in circuits as much as 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 total current during opening, then quickly part to rapidly extinguish the arc.
A footswitch is a rugged switch which is operated by foot pressure. An example of use is within the control of a machine tool, allowing the operator to have both hands free to govern the workpiece. The foot control of an electric guitar can be a footswitch.
A DPDT switch has six connections, but since polarity reversal is a quite 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 be used as intermediate switches in a multiway switching system for control of lamps by greater than two switches.
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 could 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 forms of vandal resistant switches are used to prevent unauthorized use.
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. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching—often a silicon-controlled rectifier or triac.
The analogue switch uses two MOSFET transistors in a transmission gate arrangement as a switch that works very similar to a relay, with some advantages and several other limitations in comparison with an electromechanical relay.
The facility transistor(s) in a switching voltage regulator, comparable to a power supply unit, are used like a switch to alternately let power flow and block power from flowing.
Many individuals use metonymy to call a wide range of devices “switches” that conceptually connect or disconnect signals and communication paths between electrical devices, analogous to the way in which mechanical switches connect and disconnect paths for electrons to flow between two conductors. Early telephone systems used an automatically operated Strowger switch to attach telephone callers; telephone exchanges contain one or more crossbar switches today.
Since the arrival of digital logic in the 1950s, the term switch has spread to quite a lot of digital active devices reminiscent of 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 offer connections between different ports in a computer network. The term ‘switched’ can also be applied to telecommunications networks, and signifies a network that is circuit switched, providing dedicated circuits for communication between end nodes, resembling the general public switched telephone network. The common feature of all these usages is they check with devices that control a binary state: they are either on or off, closed or open, connected or not connected.
Dead man’s switch
Load control switch
Slotted optical switch
Zero speed switch
RF Switch Matrix
^ “Switch”. The Free Dictionary. Farlex. 2008. Retrieved 2008-12-27.
^ “Switch”. The American Heritage Dictionary, College Edition. Houghton Mifflin. 1979. p. 1301.
^ RF Switch Explanation by Herley ?General Microwave
^ a b c “Engineer’s Relay Handbook, 5th edition, Chapter 1.6 by RSIA (formerly NARM)”.
^ Walker, PMB, Chambers Science and Technology Dictionary, Edinburgh, 1988, ISBN 1-85296-150-3
^ “Lab Note #105 Contact Life ?Unsuppressed vs. Suppressed Arcing” (pdf). Arc Suppression Technologies. April 2011. Retrieved February 5, 2012. (3.6 Mb)
^ a b Fardo, Stephen; Patrick, Dale (2009-01-01). Electrical Power Systems Technology. The Fairmont Press, Inc. p. 337. Retrieved 2015-01-26.
^ Gregory K. McMillan (ed) Process/Industrial Instruments and Controls Handbook (5th Edition) (McGraw Hill, 1999) ISBN 0-07-012582-1 page 7.26
^ Gladstone, Bernard (1978). The new York times complete manual of home repair. Times Books.
Gooston provide a large selection of promotional resistance touch switch on sale