A dimmer is a device
that allows you to control the brightness of a lamp
in an analog fashion. The most common type of lamp controlled by a dimmer is an incandescent
lamp, but gas discharge lamps like flourescents can also be dimmed using different technology.
By far the most popular kind of dimmer uses an SCR, or Silicon Controlled Rectifier. Normal household current is AC, or Alternating Current, where the voltage varies in a sine wave a number of times per second (50 or 60Hz are the most common frequencies). An SCR circuit works by stopping the current flow when the sine wave reaches a certain point. This point is set by a control voltage fed into the SCR; for instance, between 0 and 10 volts. There are usually two SCR's wired together, one for each half of the sine wave. (positive or negative)
To adjust the SCR control voltage, the user turns a potentiometer. This results in the light bulb becoming brighter or dimmer as more or less of the AC waveform is allowed to flow.
SCR dimmers themselves are very efficient, but a dimmed lamp is not. When operating at half power, a lamp may be putting out a quarter of its rated light output, cutting efficiency in half.
Dimmers can greatly extend the life of the light bulbs they control. There are two reasons for this; first of all the lamp filament warms up gradually instead of nearly instantly, reducing thermal stress. Second, if the lamp is not operated at full brightness, the filament will take much longer to wear out, also extending lamp life.