Switch - Wikipedia. In electrical engineering, a switch is an electrical component that can . It may be operated manually, for example, a light switch or a keyboard button, may be operated by a moving object such as a door, or may be operated by some sensing element for pressure, temperature or flow. Description. Top, left to right: circuit breaker, mercury switch, wafer switch, DIP switch, surface mount switch, reed switch. Bottom, left to right: wall switch (U. S. Each set of contacts can be in one of two states: either .
The mechanism actuating the transition between these two states (open or closed) can be either a . Automatically operated switches can be used to control the motions of machines, for example, to indicate that a garage door has reached its full open position or that a machine tool is in a position to accept another workpiece. Switches may be operated by process variables such as pressure, temperature, flow, current, voltage, and force, acting as sensors in a process and used to automatically control a system. For example, a thermostat is a temperature- operated switch used to control a heating process. A switch that is operated by another electrical circuit is called a relay. Large switches may be remotely operated by a motor drive mechanism.
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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 . The ideal switch is often used in circuit analysis as it greatly simplifies the system of equations to be solved, but this can lead to a less accurate solution. Theoretical treatment of the effects of non- ideal properties is required in the design of large networks of switches, as for example used in telephone exchanges. Contacts. The contact material is chosen for its resistance to corrosion, because most metals form insulatingoxides that would prevent the switch from working. Contact materials are also chosen on the basis of electrical conductivity, hardness (resistance to abrasive wear), mechanical strength, low cost and low toxicity.
Sometimes the contacts are plated with noble metals. They may be designed to wipe against each other to clean off any contamination. Nonmetallic conductors, such as conductive plastic, are sometimes used.
To prevent the formation of insulating oxides, a minimum wetting current may be specified for a given switch design. Contact terminology. Here the switch is shown in the open position. In electronics, switches are classified according to the arrangement of their contacts. A pair of contacts is said to be . When the contacts are separated by an insulating air gap, they are said to be . The number of . For example, a .
The number of . A single- throw switch has one pair of contacts that can either be closed or open. A double- throw switch has a contact that can be connected to either of two other contacts, a triple- throw has a contact which can be connected to one of three other contacts, etc. A switch with both types of contact is called a changeover switch or double- throw switch. These may be . In electrical power wiring (i. An example is a light switch.
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SPST- NOForm A. The two terminals are normally disconnected (open) and are closed when the switch is activated. An example is a pushbutton switch. SPST- NCForm B. The two terminals are normally connected together (closed) and are open when the switch is activated. An example is a pushbutton switch. SPDTForm C. Some suppliers use SPCO/SPTT for switches with a stable off position in the centre and SPDT for those without. Some suppliers use DPCO for switches with a stable center position and DPDT for those without.
A DPDT/DPCO switch with a center position can be . The positions of such switches are commonly referenced as . PST, SP4. T, etc.) or in some cases the letter . In the rest of this article the terms SPST, SPDT and intermediate will be used to avoid the ambiguity.
Contact bounce. The switch bounces between on and off several times before settling. Contact bounce (also called chatter) is a common problem with mechanical switches and relays. 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 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. Alternatively, contact circuit voltages can be low- pass filtered to reduce or eliminate multiple pulses from appearing.
In digital systems, multiple samples of the contact state can be taken at a low rate and examined for a steady sequence, so that contacts can settle before the contact level is considered reliable and acted upon. Bounce in SPDT switch contacts signals can be filtered out using a SR flip- flop (latch) or Schmitt trigger. All of these methods are referred to as 'debouncing'. By analogy, the term . 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 is also very hot and is capable of eroding the metal surfaces of the switch contacts.
Electric current arcing causes significant degradation of the contacts and also significant electromagnetic interference (EMI), requiring the use of arc suppression methods. 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 use a fast- moving switch mechanism, typically using a spring- operated tipping- point mechanism to assure quick motion of switch contacts, regardless of 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 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 in excess of 1. 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. Power switching. 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 a state where a quarter of the load's rated power. A momentary on. A regular on. Dual- action switches incorporate both of these features.
Inductive loads. 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.
Incandescent loads. A switch designed for an incandescent lamp load can withstand this inrush current. Providing a sufficient amount of wetting current is a crucial step in designing systems that use delicate switches with small contact pressure as sensor inputs.
Failing to do this might result in switches remaining electrically . 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 . Each key of a computer keyboard, for example, is a normally- open .
An example of a push- to- break switch is a button used to release a door held closed by an electromagnet. The interior lamp of a household refrigerator is controlled by a switch that is held open when the door is closed. Rotary switch. Any number of switching elements may be stacked in this manner, by using a longer shaft and additional spacing standoffs between each switching element.
A rotary switch operates with a twisting motion of the operating handle with at least two positions. One or more positions of the switch may be momentary (biased with a spring), requiring the operator to hold the switch in the position. Other positions may have a detent to hold the position when released. A rotary switch may have multiple levels or . It has an array of terminals, arranged in a circle around the rotor, each of which serves as a contact for the . 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 .
Thus a rotary switch provides greater pole and throw capabilities than simpler switches do. Other types use a cam mechanism to operate multiple independent sets of contacts. Rotary switches were used as channel selectors on television receivers until the early 1.