[Prev][Next][Index][Thread]

RE: Variac vs. Fan speed control

Subject:  RE: Variac vs. Fan speed control
  Date:   Sun, 20 Apr 97 05:36:02 UT
  From:   "William Noble" <William_B_Noble-at-msn-dot-com>
    To:   "Tesla List" <tesla-at-pupman-dot-com>


fan speed controls (except for VERY old ones) chop the AC waveform thus 
limiting total power, rather than changing amplitude as a variac does. 
modern 
fan speed controls have a TRIAC (a bidirectional SCR) as the main power 
element.  In the simplest possible circuit, the variable resistor (e.g.
the 
thing connected to the knob) sets a "hold off" voltage so that no
current 
flows until the AC reaches the holdoff voltage, then the triac fires and
full 
current flows until the voltage reverses.  In order to be able to hold
off 
past the voltage peak, one adds an RC circuit that makes up lag (e.g.
1/(tau S 
+1)  if you are familiar with laplace notation) that feeds a diac (a 
bidirectional zener) that can then trigger the traic at any phase angle 
between 0 (e.g. full power) and 180 (no power).

Running this kind of waveform into a transformer, or an inductive load
will 
destroy the triac instantly. That is why your dimmer goes from off to
full on 
with no control inbetween - they were probably OK before you connected
them to 
the transformer.  Further, switching the voltage on at some point other
than 
the zero crossing point causes a huge current surge into the transformer
(and 
a resultant huge voltage spike on the output) that could damage the 
transformer.

short summary - the person who told you they were the same thing was
wrong.  
If you can't afford a variac, you can control the power into your
transformer 
by putting a light bulb in SERIES with the transformer primary - then
just 
increase the lightbulb wattage (e.g. put in a different bulb) to
increase 
input power.  

snip
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