Light switch
From Wikipedia, the free encyclopedia
A light switch is a switch, commonly used to operate electric lights, permanently connected equipment, or electrical outlets.
In modern homes most lights are operated using switches set in walls, usually 6-10 inches (15-25 cm) away from a door, to operate overhead ceiling lights. In torches (flashlight) the switch is often near the bulb, but may be in the tail, or even the entire head itself may constitute the switch (rotated to turn the light on and off).
Home light switches, being in reality a metal or plastic box with a switch in it, commonly have switch plate covers called wall plates. These are plastic, ceramic or metal, and prevent accidental contact with live terminals of the switch. Wall plates are available in different styles and colours to 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. Conceptually, a variable resistor in series with a lamp would allow adjustment of its brightness, but this would be inefficient and costly owing to power dissipated in the resistance as heat. Historically, and still used for some theatrical lighting, a variable autotransformer can be used to adjust the voltage applied to the lamps, and so, the brightness; but this equipment is too large to fit into a standard wall box. Solid-state thyristor switches allow for control of lighting by blocking part of the alternating current for an adjustable time delay, thereby allowing only part of the current through the dimmer and reducing the power input to the lamp. Nearly all dimmers use phase cutting systems based on triacs. The components are small and low-cost and easily fit into wall boxes designed for on-off switches. Dimmers are intended only for use with permanently-installed lighting fixtures, and generally work best with incandescent lamps. Certain fluorescent fixtures used for commercial lighting can be dimmed, but these have special wiring requirements. Tungsten-halogen lamps may give unsatisfactory service life if operated on a dimmer since the internal redeposition of filament metal may not work properly at lower filament temperatures; see dimmer for more information.
Contents |
[edit] 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 industry standards. In practice however in most countries any requirements for permits or certification are widely ignored and replacing a light switch is considered a simple "do-it-yourself" task with the parts being widely available.
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.
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 toggle switch to be "up", whereas in the UK, Australia, and New Zealand it is "down."
[edit] 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 within 10% of 230 volts at 5A 6A or 10A for all European and most African and Asian countries, whereas Japan and most of the Americas use a supply between 100 and 127 volts with maximum circuit currents of up to 15 or 20 amperes so the overall power per circuit is similar. In the US it was formerly considered acceptable to mix outlets and lighting on the same circuit; however, building codes in effect for the past three decade in most areas have required that lighting and receptacles be on separate circuits. In the UK putting normal 13A BS1363 sockets on a lighting circuit is frowned upon (though not explicitly prohibited) but 2A or 5A BS546. Outlets are often put on lighting circuits to allow control of free-standing lamps from the rooms light switches. In the U.S., this is very common in mobile homes. It is common in American site-built housing for living rooms and bedrooms to have a switched receptacle on the receptacle circuit for the same purpose.
[edit] Internal Operation
A switch is most vulnerable when the contacts are opening or closing. As the switch is closed, the resistance of the switch 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. When the switch changes state however, there is a brief instant of partial contact when resistance is neither zero nor infinite and power is dissipated. During that transition the contacts heat up. If the heating is excessive, the contacts can be damaged or even weld themselves closed.
Thus the switch is designed to make the transition between effectively infinite resistance and effective zero resistance as swiftly as possible. This is achieved by the initial operation of the switch lever mechanism storing potential energy, usually as stress in a spring. When sufficient energy is stored, the mechanism in the switch "breaks over" driving the contacts through the transition from open to close, or close to open, without further input the switch operator.
In addition, during the transition when the contact is broken there is an additional issue that if an inductive load is being switched, the stored energy in the inductor is dissipated as an arc within the switch, prolonging the transition and worsening the heating effect on the contacts. Thus switches are commonly rated by the current they are designed to break, as this is the most stringent constraint.
The arc that results when the switch operates corrodes the switch contacts, in time leading to erosion of the contact surface and fouling of the contact area by corrosion byproducts. A switch therefore has a finite life, again often being rated at a given number of cycles of disconnection at a specified current. Operation outside its design envelope will shorten the switch life very drastically.
To combat contact corrosion a switch is usually designed to have a wipe action such that the contact corrosion is cleaned off the area of the contact that forms the low resistance path when the switch is closed. It's also designed so that the initial point of contact, and thus the majority of the contact corrosion, occurs at a sacrificial part of the contact, rather than the face that is in contact when the switch is fully closed. Depending on the switch rating and price, the contact area of the switch is often a sophisticated construction of brass contact, silver contact button, and plated finish to minimize the amount of contact corrosion and thus extend the life of the switch.
Many higher current switch designs rely on the separation arc to assist in dispersing contact corrosion, and that a switch designed for high current/high voltage use may become unreliable if operated at very low currents and low voltages because the contact corrosion builds up excessively without an arc to disperse it.
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.
The spring that stores the energy necessary for the snap action of the switch mechanism, in many small switch designs is made of a beryllium copper alloy, that is hardened to form a spring as part of the fabrication of the contact. The same part often also forms the body of the contact itself, and is thus the current path. Abusing the switch mechanism to hold the contacts in a transition state, or severely overloading the switch, will heat and thus anneal the spring, reducing or eliminating the "snap action" of the switch, leading to slower transitions, more energy dissipated in the switch, and progressive failure.
[edit] Variations on design
[edit] 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, the button sticking out is pushed and the contacts would open or close. The depressed button would then pop out letting you reverse the process.
[edit] 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.
[edit] 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 would 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. there 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. "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 2006 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.
[edit] Three-way and four-way
Three-way and four-way switches make it possible to control a light from multiple locations, such as the top and bottom of a stairway, or either end of a long hallway. These switches are externally similar to normal, single-pole switches, but have extra connections which allow, in effect, two circuits to be controlled, which can be thought of as the "on" circuit and the "off" circuit. Toggling the switch disconnects one circuit and connects the other.
Electrically, a three-way switch is a single-pole, double-throw (SPDT) switch:
By connecting two of these switches together back-to-back, it can be arranged that toggling either switch changes the state of the light from off to on, or on to off:
supply voltage 
to light
A four-way switch has two pairs of terminals which it connects either straight through, or crossed over:
By connecting one or more four-way switches in-line with three-way switches at either end, the light can be controlled from three or more locations. Toggling any switch changes the state of the light from off to on, or on to off:
supply voltage to light
