Lighting technologies have developed rapidly in recent years, and their drive electronics are also changing. Incandescent lights used to provide the vast majority of residential lighting, but now LEDs, compact fluorescent bulbs and halogen bulbs are becoming common. Home automation systems are also becoming popular, providing convenience and energy savings.
Before high-power electronics became available, several approaches were used to control lights. Light dimming was typically limited to large installations such as theaters due to the size and cost of the required equipment. At the time, most lighting was provided by incandescent lamps which, being primarily resistive, presented no particular electrical challenges.
Rheostats (variable resistors) have been used for adjusting low-power lights, but they are impractical for large loads due to the heat they generate. Variable autotransformers, commonly known by the trade name Variacs, were an efficient way to dim lights before high-power electronics were available. Being transformers, they only work with AC power, but produce a clean sine wave output. Motor-operated variacs were used for automated control of lights but are large and noisy by modern standards.  These were common in theater lighting until solid-state designs became viable. Variacs are still used as electrical test equipment and are a low-cost way to dim lights in environments such as recording studios which are very sensitive to electrical noise.
The most common way to dim incandescent lights is with phase control. With this technique, the AC waveform is interrupted so that only part of each half-cycle reaches the bulb. The brightness is controlled by varying the time after a zero-crossing at which the waveform is turned on or off.
Residential dimmers are common and low-cost. These devices typically use a triac to turn on the output a variable amount of time after a zero-crossing. The triac cannot be turned off before the next zero crossing, where current through it falls to zero. These dimmers are only suitable for use with incandescent lamps. Using such a dimmer with a reactive load like the transformer of a low-voltage lighting system can cause overheating in the dimmer or the load. Some high-power dimmers use a pair of SCRs instead of a triac but operate on the same principal.
Since SCRs and triacs turn on quickly when the line voltage may be high, significant electromagnetic interference (EMI) is produced. This can be reduced by large inductors in series with the line, but this is not normally done for household dimmers due to size and cost. A side effect of this fast rise time is that the bulb "sings", producing a high-pitched whine at a harmonic of the line frequency when the brightness is adjusted to a certain range.
EMI and "singing" can be considerably reduced by switching the line on or off slowly with a MOSFET or IGBT, but at some reduction in efficiency because of the power dissipated in the device while switching.  With this design, the switch is turned on at the zero-crossing and turned off slowly some time later. This technique is called "reverse phase control" and is also only suitable for resistive loads.
Modern high-power MOSFETs and IGBTs can switch large loads much faster than the 60Hz line frequency. This allows a dimmer to produce an output which, after filtering, is a high- quality sine wave with adjustable amplitude. Such a dimmer, like a variac, can operate a wide variety of loads including transformer-based low-voltage lighting, although it may still be unsuitable for lights which are designed only to work at the nominal line voltage (120V or 220V). This type of dimmer would provide the output quality of a variac with the reliability and automated control possibilities of an electronic system.
Compact fluorescent lamps (CFLs) are now rapidly displacing incandescent bulbs.  A CFL contains driver circuitry in its base, and most CFLs are not designed to operate on dimmers at all, and may be damaged by such operation. Some CFLs with special drive circuitry can detect the output waveform of a phase control dimmer and adjust their output accordingly. 
LEDs are currently more expensive than CFLs but have the opportunity to be much more flexible. They may be assembled into strips, sheets, or other forms, and may be made in a variety of colors. White LEDs are available in several color temperatures, providing the possibility of an efficient light source which changes its spectrum with brightness much like an incandescent bulb.  For best efficiency, a string of LEDs can be powered by a switching power supply which monitors the current through the string. The power supply can then control the brightness itself, without significant additional circuitry.
© Ben Johnson. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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