Variac vs. Fan speed control

To: tesla-at-poodle.pupman-dot-com
Subject: Variac vs. Fan speed control
From: Tesla List <tesla-at-stic-dot-net>
Date: Sun, 20 Apr 1997 16:05:26 -0500
Approved: tesla-at-stic-dot-net

Subject:  Variac vs. Fan speed control 
      From: wb8jkr-at-juno-dot-com (Mark S Graalman)
        To: tesla-at-pupman-dot-com
References: 
           1

>
>
>I received some advice recently which didn't work at first try, but 
>I'm
>curious what the list wisdom has to offer.  I was shopping for a 
>Variac
>(don't know where to find a new one) when I was told to use a fan 
>speed
>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 
>out
>that
>my dimmer-type switch moves immediately from "OFF" to "120 Volts".  So
>it
>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.  :)  
>Thanks
>for
>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.