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12 avril 2011 2 12 /04 /avril /2011 13:56

In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another.The most familiar form of switch is a manually operated electromechanical device with one or more sets of electrical contacts   HP 448007-001 battery .

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 nonconducting.

A switch may be directly manipulated by a human as a control signal to a system   HP Pavilion DV7 battery ,

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 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   HP DV6-1120SA battery .

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   HP DV6-1210SA battery .

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 pad-locked if necessary to prevent accidental operation of a machine during maintenance, or to prevent electric shock Compaq CQ50 battery .

In circuit theory

In electronics engineering, an ideal switch describes a switch that:

  • has no current limit during its ON state
  • has infinite resistance during its OFF state  HP Pavilion DV8 battery
  • has no voltage drop across the switch during its ON state
  • has no voltage limit during its OFF state
  • has zero rise time and fall time during state changes
  • switches only once without "bouncing" between on and off positions  Sony VGP-BPS13 battery

Practical switches have loss and limitation. The ideal switch is often used in circuit analysis as it greatly simplifies the system of equations to be solved, however this can lead to a less accurate solution   HP DV9700 battery .


In the simplest case, a switch has two conductive pieces, often metal, called contacts that touch to complete (make) a circuit, and separate to open (break) the circuit. The contact material is chosen for its resistance to corrosion  Compaq CQ35-100 battery ,

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.

Sometimes the contacts are plated with noble metals   Sony VGP-BPS11 battery .

They may be designed to wipe against each other to clean off any contamination. Nonmetallic conductors, such as conductive plastic, are sometimes used.

Contact terminology

A pair of contacts is said to be "closed" when current can flow from one to the other  HP Pavilion DV6-1223EO Battery .

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.

Switches are classified according to the arrangement of their contacts in electronics. Electricians installing building wiring use different nomenclature  Sony Vaio VGN-CR190E/L Battery ,

such as "one-way", "two-way", "three-way" and "four-way" switches, which have different meanings in North American and British cultural regions as described in the table below.

In a push-button type switch, in which the contacts remain in one state unless actuated, the contacts can either be normally open(abbreviated"n.o." or "no"Sony Vaio VGN-CR190E/P Battery

until closed by operation of the switch, or normally closed ("n.c. or "nc") and opened by the switch action. A switch with both types of contact is called a changeover switch. These may be "make-before-break" which momentarily connect both circuits, or may be "break-before-make" which interrupts one circuit before closing the other Sony Vaio VGN-CR190E/R Battery .

The terms pole and throw are also used to describe switch contact variations. The number of "poles" is the number of separate circuits which are controlled by a switch. For example, a "2-pole" switch has two separate identical sets of contacts controlled by the same knob   Sony Vaio VGN-CR190E/W Battery .

The number of "throws" is the number of separate positions that the switch can adopt. 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  Sony Vaio VGN-CR21/B Battery .

These terms give rise to abbreviations for the types of switch which are used in the electronics industry such as "single-pole, single-throw" (SPST).

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