Variac vs. Fan speed control

Subject:  Variac vs. Fan speed control 
      From: wb8jkr-at-juno-dot-com (Mark S Graalman)
        To: tesla-at-pupman-dot-com

>I received some advice recently which didn't work at first try, but 
>curious what the list wisdom has to offer.  I was shopping for a 
>(don't know where to find a new one) when I was told to use a fan 
>control instead - it would do the same thing.  The one I bought,
>however, I
>burned two neons with - when I finally thought to check it, it turns 
>my dimmer-type switch moves immediately from "OFF" to "120 Volts".  So
>isn't surprising that I burned out the two Xformers.
>   Obviously the switch I bought doesn't control the fan speed by
>varying the
>voltage input to the fan.  1) How does it work?  2) Is there a variety
>of fan
>speed control which would work in this application (replacing a
>Variac)?  If
>so, it would certainly be cheaper and more easily available.  :)  
>answers from anybody who's already explored this area. 
>              Aaron Datesman
Aaron, these types of "speed controls" work by controlling the duty
cycle or the "on time" during the ac sinewave using a threshold
bidirectional trigger diode (diac) which in a controlable manner gates
a bidirectional SCR called a triac. They are totally unsuitable for 
voltage control, could be use to vary the speed of a SMALL ac motor
running a rotary gap but not the low side of a drive transformer.