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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 rocker, a toggle or dolly, a push-button or any type of mechanical linkage.

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 normally closed contacts are 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 the following abbreviations.

• SPST = single pole single throw, a simple on-off switch.
• SPDT = single pole double throw, a simple changeover or on-off-on switch. SPDT switches are sometimes called "3-way" switches, especially when two are used to control a single load (e.g. one at the top and one at the bottom of a flight of stairs).
• SPCO = single pole changeover, equivalent to SPDT.
• DPST = double pole single throw, 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.
• 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, equivalent to DPDT.
• TPST = triple pole single throw. TPST switches are on/off switches for 3 phase power.

Switches with larger numbers of poles or throws can be described by replacing the "S" or "D" with a number.

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 tilt 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.

4-Way 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. Four-way switches are also used for alternating current, for example, when it is desired to have multiple light switches to affect one load, such as the lights in a stairwell that can be switched on or off from any floor. For example, a "3-way" switch may be used in the basement, and one at the top floor, while a 4-way switch is used on every floor in between. A change of state of any switch will change the state (on or off) of the lights in the stairwell. Leviton manufactures 4-way switches, which have been found to be defective. Other companies, such as Pass & Seymour manufacture more reliable 4-way switches.

See Also

• http://www.networkdictionary.com/telecom/class5.htm - Class 5 Switch
• circuit breaker
• contactor
• relay
• analogue switch
• railroad switch

• Electrical components