Dimmer switch

A wall-mounted dimmer switch controller

A dimmer switch is a device which allows for the adjustment of power supplied to one or more electric lights. In some rare cases, they are also used for other electric devices, but in many cases doing so would harm the device, so this is rare.

Types

Rheostat (variable resistor)

The first kind of dimmer switch made use of a variable resistor. In this kind of device, a semiconductor strip is placed between two contact arms—one is stationary, one is adjustable. The semiconductor permits the flow of electricity, but does resist it somewhat. The stationary contact arm is placed at one end of the semiconductive strip, and the adjustable one is permitted to move across it. When the latter arm is placed right near the stationary arm, electricity flows from one arm through a small amount of the semiconductor, and out to the other arm. Since it only passes through a small amount of semiconductor, the power flow is minimally effected. However, when the adjustable arm is moved down the strip, away from the stationary arm, the amount of semiconductor the power must flow through increases. This prevents progressively more electricity from passing through, as the arms separate.
This simple system is then installed in a switch, where the user can easily move the adjustable arm. The resulting supply is then connected to some device, usually a light bulb. If the operator wished for a higher power supply, they can simply slide or twist the dimmer control to decrease the amount of semiconductor the energy must pass through. If the operator wishes to reduce the electrical supply, the amount of used semiconductor can be increased, by sliding or twisting the control in the opposite direction.
Another kind of Rheostat dimmer used salt water as a semiconductor, with metal contacts which could be adjusted so they were closer or farther away from each other. As the distance between the contacts increased, the electricity needed to pass through more salt water, resulting in more being lost before it reached the other contact. While also inefficient (and unsafe), this kind of dimmer was fairly easy to construct, although frequent maintenance was required due to the salt damage.

Unfortunately, while this setup does allow for easy adjustment of the electrical supply, it is inefficient. Much of the energy which would have gone through the controller is instead converted to heat. This causes the output voltage drop desired, but most of the electricity is still being used, as it is converted into heat energy. In some cases, this can also be a safety hazard since the heat can build up and potentially cause fire or other damage.

Autotransformer dimmer

A Commercial Variac

Variable autotransformers (trademark "Variac") are very tolerant and reliable "dimmer" devices. They and not significantly affected by how much power is drawn through them, so they are also very versatile. The work by reducing the amplitude of the A.C. sine wave. However, this reduction converts all of the unwanted power into heat, so it is inefficient. They are also large and somewhat expensive, so they are rarely used for lighting today. Variacs are still used for some other purposes, however. For example, some musicians use them not as much to reduce the voltage as to stabilize it. These help deal with unreliable power sources (which sometimes provide too little voltage) and help remove electromagnetic interference and "noise."

Solid state (switching circuit)

Rather than resisting the flow of electricity, switching circuit dimmers simply cycle the power on and off. They work on alternating current by shutting of the flow every time the sine wave cycles to zero, then connecting it again a split second later. AC current completes sixty cycles per second, and in every cycle the sine wave comes to zero twice. This means that 120 times every second, the first part of each positive and negative sine wave is "chopped" off. The amount of the wave which is cut off can be adjusted, so that more or less of each wave can pass through.
As portions of the waves are cut off, the net power allowed through is reduced. Although this does cause the output to be inconstant, most people will not notice since it is so quick, the human eye, nerves, and brain can only process visual input at a limited speed. (This speed is not knows precisely, though some approximate that about 60 frames per second (FPS) can be processed, and others simply say that the processing rate is over 43 FPS.) However, some do actually notice this effect, and usually report that it is irritating. Others report that light emissions which have been "dimmed" in the manner seem harsh, but cannot identify why. (For this reason, some dislike the LED Christmas lights, and certain other such products.)
So far, these systems do waste enough power to produce heat, but they are still improving. They also produce radio-frequency interference (RFI), although much of this can be filtered out, as is mandated in some places.
A variation of the solid-state dimmer uses insulated-gate bipolar transistors, which are able to reduce some of the waste and much of the RFI of TRIACS by cutting off the end of the sine wave, rather than the beginning.

CFL/LED dimmers

Fluorescent lights as well as LED (Light emitting diode) bulbs cannot be dimmed by a simple voltage reduction. However, this does not mean that it is impossible. In most cases, the bulbs themselves can (and must) control their brilliance. Therefore, variable brightness bulbs or tubes must be installed into a "dimmer" system designed for them. The user can then control brilliance easily, but the switch is simply telling the bulbs or tubes what to do. The actual light modules then adjust their output as requested. Although this does not help LEDs to last any longer, it can still reduce net power consumption.

Other varieties

Other varieties exist as well, each with strengths and weeknesses. Over time, ways are being found to actually conserve the unwanted electricity, but this can result in other complications. For example, reverse phase control was found to be a fairly efficient method, but only works with resistive capacitive loads. To many electronics such as fans, this kind of dimming technology would be destructive. Other varieties include Coil-rotation transformers and Field effect transistors.

Purpose

Energy saving

Many use dimmers to reduce energy consumption. A reduction of 5-10 volts on lighting may not even be noticed, so this can be done almost anytime. A greater reduction is also sometimes used for certain situations. Perhaps the lights are dimmed after motion sensors detect no motion for a period of time, or perhaps lights are dimmed at night, since less is usually needed. However, power consumption cannot be reduced much in most cases, and since electric lighting consumes relatively little power anyway, there may be little noticeable difference in consumption.
Some will attempt to save power in this way to simply avoid being wasteful, while many others are seeking to save some money. Some also try to reduce energy consumption based on the belief that this may help prevent or delay self-annihilation due to global warming.

Convenience/appearance

In some cases, it is helpful to reduce lighting, without actually turning it off. In conference rooms and theaters, it may be helpful to dim the lights, but not actually turn any off, so that light is still evenly distributed. This is particularly the case if a projector or screen is sometimes but not always used. Dimmers are also useful in stage lighting, since they allow the operators to set lighting specifically as needed, and also allow for the fading on and off of lights. (This is much easier for the audience's eyes than simply flipping on and off lights, with no time for their eyes to adjust.)
Many homes also include lights on dimmer switches, especially in dining areas. This allows for flexibility in the room's purpose, since some purposes may require bright light, while others need very little. In dining areas, fairly bright light is often wanted. However, if a candlelight dinner is desired, it can be convenient to reduce but not turn off the electric lighting.

Extended device life

While reduced voltage can be harmful to some electronics, others—mainly light sources—can actually be preserved by it. In some cases, it helps to preserve lights by simply running them at a slightly reduced voltage. In other cases, it helps them most of all if they are allowed time to warm up with a very low voltage running through them. After doing this with halogen lamps, for example, it is much less harmful to them when full power is applied to them.