131005-0854 EDT
A/A:
I classify dimmers into two categories, two wire and three wire. The EGC (green wire) is not part of the wire count. A two wire dimmer only has hot in and out. A three wire dimmer requires neutral plus hot in and out, and may have more than three wires.
A two wire dimmer uses current thru the dimmer to provide power for the electronics in the dimmer. It is of some sort of phase shift, usually a time delay to turn on of a Triac. A simple rheostat could also be a two wire dimmer.
A three wire dimmer uses hot in and neutral to power the electronics in a phase shift dimmer, or the primary of a variable transformer. A Variac is three wire dimmer that adjusts the amplitude of the input sine wave by a variable tap on the transformer to obtain an output sine wave of variable amplitude.
A two wire electronic dimmer works fairly well with an incandescent load, can provide a wide dimming range, but may have startup problems when left in a low position when turned off. Or put in a low position when off. This problem seems to be corrected by the three wire dimmer.
There are no satisfactory dimmable CFLs in my opinion. Thus, no comments on dimmer operation.
The sample Cree 40 W equivalent LED I have previously discussed in other threads dims fairly well with a sine wave or phase shift type of input. But does not dim as low as an incandescent. This bulb dims with both a two wire or three wire dimmer. Same startup problem with a two wire dimmer as the incandescent. The three wire phase shift dimmer does not dim the Cree low enough because the maximum delay to turn on of the Triac is too short. The curve of light output vs applied average voltage for an incandescent provides better dimming characteristics than an LED from standard phase shift dimmers.
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