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Switch

From Academic Kids

(Redirected from Switching)
This article is about electrical switches. For other meanings of the word "switch", see Switch (disambiguation).
Missing image
Switches-electrical.agr.jpg
Electrical switches. 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. style), miniature toggle switch, in-line switch, push-button switch, rocker switch, microswitch.

A switch is a device for making or breaking an electric circuit, or for selecting between multiple circuits.

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. 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. 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).

Contents

Contact arrangements

Missing image
Tpst.jpg
Triple Pole Single Throw (TPST) switch used to short the windings of a 3 phase wind turbine for braking purposes. Here the switch is shown in the open position.

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.

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. 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.


Electronics abbreviation Expansion of abbreviation British mains wiring name American mains wiring name Description
SPST Single pole, single throw One way Two way A simple on-off switch: A is connected to B or not.
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.
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.
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.

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".

Biased switches

A biased switch is one containing a spring that returns the actuator to a certain position. 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. This device makes contact when the button is pressed and breaks when the button is released.

Special types

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 mercury switch consists of a blob of mercury inside a glass bulb. The two contacts pass through the glass, and are shorted together when the bulb is tilted to make the mercury roll on to them. The advantage of this type of switch is that the liquid metal flows around particles of dirt and debris that might otherwise prevent the contacts of a conventional switch from closing.

Other types of switch include:

Intermediate switch

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. They 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

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.

Two locations

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Twowayswitching.PNG
1. First method
2. Second method
3. Labelling of switch terminals

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.

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):

First method:

  • double wire between both switches
  • single wire from one switch to the mains
  • single wire from the other switch to the load
  • single wire from the load to the mains

Second method:

  • triple wire between both switches
  • single wire from any position between the two switches, to the mains
  • single wire from any position between the two switches, to the load
  • single wire from the load to the mains

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 bulkyness of this may require use of a deeper backbox.

More than two locations

Missing image
Morethantwowayswitching.PNG
Three-way switching.
1. First method
2. Second method
3. Labelling of switch terminals

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):

  • first method
    • double wire along the sequence of switches
    • single wire from the first switch to mains
    • single wire from the last switch to the load
    • single wire (neutral) from load to mains
  • second method
    • double wire along the sequence of switches
    • single wire from first switch to last switch
    • single wire from anywhere between two of the switches to the mains
    • single wire from anywhere between the same two switches to the load
    • single wire (neutral) from load to mains



See also

External Links

id:Saklar da:Omskifter (elektrisk) de:Schalter (Elektrotechnik) fr:Interrupteur nl:Schakelaar ja:開閉器 sl:Stikalo

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