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64-435: In electricity, a switch is a device that can connect, disconnect, or divert current in an electrical circuit. Switch or The Switch may also refer to: Switch Switches are made in many different configurations; they may have multiple sets of contacts controlled by the same knob or actuator, and the contacts may operate simultaneously, sequentially, or alternately. A switch may be operated manually, for example,

128-407: 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. When a strongly inductive load such as an electric motor is switched off, the current cannot drop instantaneously to zero; a spark will jump across

192-640: A light switch or a keyboard button, or may function as a sensing element to sense the position of a machine part, liquid level, pressure, or temperature, such as a thermostat . Many specialized forms exist, such as the toggle switch , rotary switch , mercury switch , push-button switch, reversing switch , relay , and circuit breaker . A common use is control of lighting, where multiple switches may be wired into one circuit to allow convenient control of light fixtures. Switches in high-powered circuits must have special construction to prevent destructive arcing when they are opened. The most familiar form of switch

256-460: A multiway switching system for control of lamps by more 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 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

320-500: 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. A toggle switch or tumbler 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

384-417: 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. This type of switch performs much better than the ball tilt switch, as

448-401: 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. Contact resistance Electrical contact resistance ( ECR , or simply contact resistance ) is resistance to the flow of electric current caused by incomplete contact of the surfaces through which the current

512-468: 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

576-431: 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

640-413: A drop in hydrostatic pressure (analogous to electrical voltage ) occurs when fluid flow transitions from one channel to another. Bad contacts are the cause of failure or poor performance in a wide variety of electrical devices. For example, corroded jumper cable clamps can frustrate attempts to start a vehicle that has a low battery . Dirty or corroded contacts on a fuse or its holder can give

704-445: 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

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768-562: A given switch design. 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 "

832-700: 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; as

896-447: 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

960-513: 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,

1024-535: A series of non-conductive 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. Extremely large switches often have switch contacts surrounded by something other than air to more rapidly extinguish

1088-425: A simple two-terminal measurement made with an ohmmeter. Specific contact resistance can be obtained by multiplying by contact area. For experimental characterization, a distinction must be made between contact resistance evaluation in two-electrode systems (for example, diodes) and three-electrode systems (for example, transistors). In two-electrode systems, specific contact resistivity is experimentally defined as

1152-438: A state where a quarter of the load's rated power (or worse if the load is not purely resistive) is briefly dropped in the switch. For this reason, power switches intended to interrupt a load current have spring mechanisms 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

1216-545: 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. 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). A switch normally maintains its set position once operated. A biased switch contains

1280-404: 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 combinations. In some contexts, particularly computing , a toggle switch, or the action of toggling,

1344-407: Is Fermi wavevector of the conducting material, Ohm's law does not hold anymore. These small devices are called quantum point contacts . Their conductance must be an integer multiple of the value 2 e 2 / h {\displaystyle 2e^{2}/h} , where e {\displaystyle e} is the elementary charge and h {\displaystyle h}

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1408-400: Is a common problem with mechanical switches, relays and battery contacts , which arises as the result of electrical contact resistance (ECR) phenomena at interfaces. Switch and relay contacts are usually made of 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

1472-436: 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)

1536-487: Is 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 data stream. In the design of micro-contacts, controlling surface structure ( surface roughness ) and minimizing the formation of passivated layers on metallic surfaces are instrumental in inhibiting chatter. In

1600-419: Is also very hot and is capable of eroding the metal surfaces of the switch contacts (the same true for vacuum switches). Electric current arcing causes significant degradation of the contacts and also significant electromagnetic interference (EMI), requiring the use 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

1664-457: Is applied for asperities to penetrate the oxide layer, forming metal-to-metal contact patches. If a contact patch is sufficiently small, with dimensions comparable or smaller than the mean free path of electrons resistance at the patch can be described by the Sharvin mechanism , whereby electron transport can be described by ballistic conduction . Generally, over time, contact patches expand and

1728-441: Is carrier mobility, and V gs {\displaystyle V_{\text{gs}}} and V ds {\displaystyle V_{\text{ds}}} are gate-source and drain-source voltages. Therefore, the linear extrapolation of total resistance to the zero channel length provides the contact resistance. The slope of the linear function is related to the channel transconductance and can be used for estimation of

1792-426: Is flowing, and by films or oxide layers on the contacting surfaces. It occurs at electrical connections such as switches, connectors, breakers, contacts, and measurement probes. Contact resistance values are typically small (in the microohm to milliohm range). Contact resistance can cause significant voltage drops and heating in circuits with high current. Because contact resistance adds to the intrinsic resistance of

1856-488: Is plotted as a function of the channel length: where R c {\displaystyle R_{\text{c}}} and R ch {\displaystyle R_{\text{ch}}} are contact and channel resistances, respectively, L / W {\displaystyle L/W} is the channel length/width, C {\displaystyle C} is gate insulator capacitance (per unit of area), μ {\displaystyle \mu }

1920-542: Is the Planck constant . Quantum point contacts behave more like waveguides than the classical wires of everyday life and may be described by the Landauer scattering formalism. Point-contact tunneling is an important technique for characterizing superconductors . Measurements of thermal conductivity are also subject to contact resistance, with particular significance in heat transport through granular media. Similarly,

1984-468: 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

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2048-513: 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. 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 ),

2112-423: Is 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

2176-406: Is usually assumed that contact resistance is a major component. William Shockley introduced the idea of a potential drop on an injection electrode to explain the difference between experimental results and the model of gradual channel approximation. Because contact resistance is usually comparatively small, it can be difficult to measure, and four-terminal measurement gives better results than

2240-485: The Hammond organ , multiple wires are pressed together under the piano keys of the manuals. Their bouncing and non-synchronous closing of the switches is known as Hammond Click and compositions exist that use and emphasize this feature. Some electronic organs have a switchable replica of this sound effect. The effects of contact bounce can be eliminated by: All of these methods are referred to as 'debouncing'. When

2304-436: The contact resistance , and wetting current of a mechanical switch. Sometimes the contacts are plated with noble metals , for their excellent conductivity and resistance to corrosion. 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

2368-420: 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.,

2432-405: 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 switches consist of

2496-606: The Kelvin probe force microscopy (KFM) and the electric-field induced second harmonic generation. In the semiconductor industry, Cross-Bridge Kelvin Resistor(CBKR) structures are the mostly used test structures to characterize metal-semiconductor contacts in the Planar devices of VLSI technology. During the measurement process, force the current ( I {\displaystyle I} ) between contacts 1 and 2 and measure

2560-551: The arc. 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

2624-461: The arc. For example, the switch contacts may operate in a vacuum, immersed in mineral oil , or in sulfur hexafluoride . In AC power service, the current 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 switch significant power,

Switch (disambiguation) - Misplaced Pages Continue

2688-416: The complication of contact resistance. In practice, direct current methods are more typically used to determine resistance. The three electrode systems such as transistors require more complicated methods for the contact resistance approximation. The most common approach is the transmission line model (TLM). Here, the total device resistance R tot {\displaystyle R_{\text{tot}}}

2752-424: The conductors, it can cause significant measurement errors when exact resistance values are needed. Contact resistance may vary with temperature. It may also vary with time (most often decreasing) in a process known as resistance creep . Electrical contact resistance is also called interface resistance , transitional resistance , or the correction term . Parasitic resistance is a more general term, of which it

2816-579: The contact resistance at an interface relaxes, particularly at weakly contacting surfaces, through current induced welding and dielectric breakdown. This process is known also as resistance creep. The coupling of surface chemistry , contact mechanics and charge transport mechanisms needs to be considered in the mechanistic evaluation of ECR phenomena. When a conductor has spatial dimensions close to 2 π / k F {\displaystyle 2\pi /k_{\text{F}}} , where k F {\displaystyle k_{\text{F}}}

2880-409: The current decreases to a steady-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 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

2944-407: 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

3008-438: 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

3072-413: 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

3136-448: 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. A rotary switch operates with a twisting motion of the operating handle with at least two positions. One or more positions of

3200-813: The potential deference between contacts 3 and 4. The contact resistance R k {\displaystyle R_{\text{k}}} can be then calculated as R k = V 34 / I {\displaystyle R_{\text{k}}=V_{34}/I} . For given physical and mechanical material properties, parameters that govern the magnitude of electrical contact resistance (ECR) and its variation at an interface relate primarily to surface structure and applied load ( Contact mechanics ). Surfaces of metallic contacts generally exhibit an external layer of oxide material and adsorbed water molecules, which lead to capacitor-type junctions at weakly contacting asperities and resistor type contacts at strongly contacting asperities, where sufficient pressure

3264-424: 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 as the switch is opened, forming a gas plasma , also known as an electric arc . The plasma is of low resistance and is able to sustain power flow, even with the separation distance between the switch contacts steadily increasing. The plasma

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3328-482: The result of roughness and oxide films, they exhibit contact 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. In

3392-581: 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

3456-636: 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. The formation of oxide layers at contact surface, as well as surface roughness and contact pressure, determine

3520-449: 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

3584-489: The slope of the I–V curve at V = 0 : where J {\displaystyle J} is the current density, or current per area. The units of specific contact resistivity are typically therefore in ohm-square metre, or Ω⋅m . When the current is a linear function of the voltage, the device is said to have ohmic contacts . Inductive and capacitive methods could be used in principle to measure an intrinsic impedance without

3648-437: 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 as the spring tension is released. 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

3712-413: 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

3776-415: 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

3840-451: The terms two way and three way are used with different meanings). Form A 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 (also called chatter )

3904-420: The transitional state of the switch as well as the ability to withstand continuous operating currents must be considered. When a switch is in the on state, 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

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3968-407: 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

4032-466: 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 use a fast-moving switch mechanism, typically using a spring-operated tipping-point mechanism to assure quick motion of switch contacts, regardless of

4096-476: The ”contact resistance-free” carrier mobility. The approximations used here (linear potential drop across the channel region, constant contact resistance, ...) lead sometimes to the channel dependent contact resistance. Beside the TLM it was proposed the gated four-probe measurement and the modified time-of-flight method (TOF). The direct methods able to measure potential drop on the injection electrode directly are

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