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penneyeugene:
The type of light you want to dim is more important than whether to dim or not.
An incandescent light bulb, carbon or tungsten filament, is nicely dimmable from full to zero brightness with a Variac. Not so with standard commerical phase shift dimmers. For whatever engineering or marketing reasons commericial residential phase shift dimmers do not provide phase adjustment from 0 to 180 degrees, the range is more limited, especially with 2 wire dimmers.
Non-incandescent lights present problems relative to dimming whether by Variac (variable autotransformer), pulse width, or phase shift dimming. In my opinion there is no screw in CFL that has good dimming range compared to an incandescent on the low end. Some Cree LED bulbs dim fairly well with either a Variac or phase shift dimmer.
Not related to bulb type, but based on function I recommend a 3 wire dimmer, one that requires or can make use of neutral. The reason is because power to operate the dimmer is available whether or not the dimmer has a load. This means that when power input is applied to the dimmer that whatever was the dimmer setting at the time of power loss the setting will be restored on power up. Example: I have a 2 wire dimmer for my front hall lights. For normal operation the dimmer is set as low as it will operate. When the lights are turned off or there is a power interruption, then on power restoration the lights remain visibly off. The dimmer setting has to be raised to restore dimmer operation, and then again lowered. The dimmer is a 48 year old GE rotary knob type using a Triac. There is not much change in this type dimmer in present units.
Three wire dimmers are much more expensive than two wire dimmers, out of proportion with the cost of the added circuitry. You have to consider cost vs function when you decide what type of dimmer to use.
Do bench experiments with lights and dimmers that you might use to determine your direction. You can use a Kill-A-Watt EZ for power input measurements.
At my web page
http://beta-a2.com/EE-photos.html photos P9 thru P18 are various graphs of some characteristics of incandescent and CFL bulbs. The low end of my incandescent curves did not go to zero because the wattmeter used had low voltage limitations. The power in my curves is the power to the bulb, and not power input to the dimmer.
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