Online Encyclopedia
Switch
- This article is about electrical switches. For other meanings of the word "switch", see Switch (disambiguation).
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).
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Contact arrangements
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. |
| SPDT | Single pole, double throw | Two way | Three way | A simple changeover or on-off switch. |
| 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. | ||
| 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. | ||
| 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
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 DPDT 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.
The second method is to join all three terminals of the two switches and take the incoming supply and the wire out to the light to the L1 and L2 terminals.
The first method saves cable if the live and switched live connect to different boxes. The second method saves using a terminal block in the backbox if the live and switched live both arrive at one of the boxes.
More than two locations
For more than two locations, the two cores connecting the L1 and L2 of the switches must be passed through an intermediate switch wired to swap them over. Any number of intermediate switches can be inserted, allowing for any number of locations.
See also
- Circuit breaker
- Contactor
- Relay
- Analogue switch
- Microswitch
- UL, CSA, VDE .