Re: Variac vs. Fan speed control

Subject:  Re: Variac vs. Fan speed control
  Date:   Mon, 21 Apr 1997 01:21:34 +0300 (EET DST)
  From:   Kristian Ukkonen <kukkonen-at-cc.hut.fi>
    To:   Tesla List <tesla-at-pupman-dot-com>

On Wed, 16 Apr 1997, Tesla List wrote:

> 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,

It _won't_ do the _same_ thing, like you've perceived. 

> 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.

Look at the _wave-shape_ again with a 'scope. If you don't have one then
I'll just tell : that "dimmer" works by a "switching element" 
that stops conducting at some phase-angle (voltage). The design is
quite simple. Original sine-wave (ascii art:) :

            /   \
  -\     /-/     \-\     /- etc.
    \   /           \   /
     \_/             \_/

now, start the dimmer..

            / |
  -\  |----/  |----\  |--- etc..
    \ |             \ |
     \|              \|

now, if you turn the dimmer to a lower position :

  -\ |-----/ |-----\ |----
    \|              \|

until finally it stops conducting at 0 and you get


A fancier design would turn the triac back on when the voltage
is lower than set-point. That would mean like:

             /| |\
  --\ |-| /-/ |-| \-\ |-| /- etc..
     \| |/           \| |/

>    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

I hope 1) was answered above.

It MAY be some fancier PWM controller but usually not. Fans are not
worth the fancier tech.

> of fan
> speed control which would work in this application (replacing a

No. They are solid-state devices that will fry with a TC.

> Variac)?  If
> so, it would certainly be cheaper and more easily available.  :)  Thanks

Variacs are readily available. Just spend the money and you'll have one.
You'll be able to use the variac for many other things besides TCs.

It won't take long acquiring dimmers and frying things that you've
either killed someone or spent more money than the value of a variac.

> answers from anybody who's already explored this area. 

I acquire all variacs I meet if the price is correct (about 2usd/5kg). 
already have 3 1-phase and 4 3-phase variacs ranging from 220W tiny one
a 50kW definitely-non-portable monster. :)

I hope this helps,

  Kristian Ukkonen.

| Kristian Ukkonen       |  Do as thou wilt shall be the whole of the
law |
| kukkonen-at-alpha.hut.fi  |_____  Chance favours the prepared mind 
| http://www.hut.fi/~kukkonen |  Fear is the mind-killer  |---------