Light switch

A light switch is a switch, usually in the shape of a rectangle or compressed prism, which operates an electrical switch, two wires that are pressed together to form a current, to turn on and off a circuit. They are most commonly used to operate electric lights, hardwired equipment, or electrical outlets.

In modern homes most lights are operated using switches set in walls to operate ceiling lamps, while in torches the switch is far nearer the bulb. Home light switches, being in reality a metal box with a switch, commonly have switch plate covers; these are commonly made of plastic or tile, and are purchased to allow the switch to more properly blend in with the style of a room.

A dimmer switch is a kind of light switch that allows a light to be dimmed or brightened continuously. An inefficient method of doing this would involve placing a variable resistor in series with the circuit. More sophisticated methods use a form of pulse width modulation, stopping current flow completely for a fraction of a second and allowing it to flow freely for other periods. Dimmers should be used only with light fixtures and then only with certain lamps, see dimmer for more information.

History and culture

Light switches are usually built into the walls of the house. Surface mounting is also fairly common though is seen more in commercial industrial and outbuilding settings than in houses.

Because of electrical safety considerations in many countries their design and installation is regulated either by law or by widely accepted industrys standards. In the U.S. there is a complex web of local and state laws and building codes. In practice however in most countires any requirements for permits or certification are widely ignored and replacing a light switch is a considered a simple "do-it-yourself" task with the parts being widely availible.

Due to the regulatory issues and the fact that light switches aren't something that people are usually too bothered with the looks of they are usually durable and conservative in design. They frequently remain in service for many decades, often being changed only when a portion of a house is rewired. It is not extremely unusual to see century-old light switches still in functional use (though this should probablly be taken as a warning sign that the building is in dire need of a rewire as the wire is likely that old and was not subject to many elementary safety regulations).

The dimensions, mechanical designs, and even the general appearance of light switches changes very slowly with time. Manufacturers introduce various new forms and styles, but for the most part decoration and fashion concerns are limited to the faceplates. Even the "modern" dimmer switch with knob is at least four decades old, and even in the newest construction the familiar toggle and rocker switch appearances predominate.

The shape and size of the boxes and faceplates as well as what is integrated (for example in the UK it is normal to have the switch built into the plate) varies a lot by country. The direction which represents "on" also varies by country. In the United States it is universal for the "on" position of a light switch to be "up," whereas in the UK and in Australia it is "down."

Design

Light switches must perform the same tasks as other electrical switches though usually at lower currents. In the case of light switches, the circuit to be switched is (typically, in the US) about 120 volts AC with maximum circuit currents of up to 15 or 20 amperes. In the UK the current rating is generally lower at 5A 6A or 10A but the voltage is higher (around 240V) so the overall power per cuircuit is similar. In the US it is considered normal to mix outlets and lighting on the same cuircuit. In the UK putting normal 13A BS1363 sockets on a lighting cuircuit is frowned upon (though not explicitly prohibited) but 2A or 5A BS546 sockets are often put on lighting cuircuits to allow control of free-standing lamps from the rooms lightswitches.

Internal Operation

A switch is vulnerable when the contacts are opening or closing. As a switch is closed, the resistance changes from nearly infinite to nearly zero. At infinite resistance, no current flows and no power is dissipated. At zero resistance, there is no voltage drop and no power is dissipated. However, when contact is made, there is a brief instant of partial contact when resistance is neither zero nor infinite and power is dissipated. During that instant, the contacts heat up. If the heating is excessive, the contacts can be damaged or even weld themselves closed. When contact is broken, if there is an inductive load an arc occurs, and again, power is dissipated and the contacts heat.

When a pair of contacts is badly designed, or overloaded in relation to its design, if the contacts are visible two kinds of "sparks" may be seen. On closure, a few sparks like those from a flint-and-steel may appear as a tiny bit of metal is heated to incandescence, melted, and thrown off. On opening, a bluish arc may occur with a detectable "electrical" (ozone) smell; afterwards the contacts may be seen to be darkened and pitted. Damaged contacts have higher resistance, rendering them more vulnerable to further damage and causing a vicious circle in which the contacts soon fail completely.

To make a switch safe, durable, and reliable, it must be designed so that the contacts are held firmly together under positive force when the switch is closed. It should be designed so that regardless of how the person operating the switch manipulates it, the contacts always close or open quickly. Despite this, you should never hold a switch between its two poles (On or off) this is especially true on older mechanisms.

Variations on design

Push button

Prior to the toggle switch a popular design was the push button switch. The design was two buttons, one depressed, the other sticking out above or next to each other. To operate the switch, you'd push the button sticking out and the contacts whould open or close. The depressed button whould then pop out letting you reverse the process.

Toggle

The traditional light-switch mechanism is a toggle mechanism that provides "snap-action" through the use of an "overcenter" geometry. The switch handle does not control the contacts directly, but through an intermediate arrangement of springs and levers. Turning the handle does not initially cause any motion of the contacts, which in fact continue to be positively held open by the force of the spring. Turning the handle gradually stretches the spring. When the mechanism passes over the center point, the spring energy is released and the spring, rather than the handle, drives the contacts rapidly and forcibly to the closed position with an audible "snapping" sound. The snap-action switch is a mechanical example of negative resistance.

This mechanism is very safe, reliable, and durable, but produces a loud snap or click. (Many people have at some point in their lives made an attempt to reduce this noise by operating the handle slowly or gingerly. Of course this is to no avail, since the very purpose of the mechanism is to insure that the electrical portion of the switch always operates rapidly and forcefully—and noisily—regardless of how the handle is manipulated).

As of 2004 in the United States, the toggle switch mechanism has been almost entirely supplanted by "quiet switch" mechanisms. "Quiet switch" mechanisms do not have any form of snap action, but only a weak detent. They are equipped with large, high-quality contacts that are capable of switching domestic loads without damage, despite the less-positive action.

Mercury switches

Before the 1970s, mercury switches were popular. They cost more than other designs, but were totally silent in operation. The switch handle simply tipped a glass vial, causing a large drop of mercury to roll from one end to the other. As it rolled to one end, the drop of mercury bridged a pair of contacts to complete the circuit. Many of them also whould glow faintly when they were "off" to aid people in finding them when the room was dark. The vial was hermetically sealed, but concerns about the release of toxic mercury when the switches were damaged or disposed of led to the abandonment of this design. In the U.S. has never been any effort to recall or replace existing mercury switches, and millions of them remain in use.

In principle, it is easy to design silent switches in which the mechanical contacts do not directly control the current, but simply signal a solid-state device such as a thyristor to complete the circuit. Many variations on this theme have been created and marketed. Nearlly all dimmers use phase cutting systems based on triacs. "Touch-plate" devices can be operated by touching or merely waving a hand near the switch. Public buildings such as hospitals frequently save energy by using "motion-detector" switches. As of 2004 these remain specialty items, probably because of the greater cost of insuring safety in the more-complex electronic designs. Unless carefully designed, electronic devices are subject to catastrophic failure in circumstances such as lightning-induced power surges.