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Re: Variac vs. Fan speed control
Subject: Re: Variac vs. Fan speed control
Date: Wed, 07 May 1997 08:57:12 -0500
From: David Huffman <huffman-at-FNAL.GOV>
To: Tesla List <tesla-at-pupman-dot-com>
----------
>
> On Fri, 25 Apr 1997 12:25:29 -0600 (MDT) Chip Atkinson
> <chip-at-XiG-dot-com> wrote;
>
> > > Hay Chip, ever hear of a Faraday shield? Seriously, Chip I
currently
> > > use phase shift light dimmers to control my rotory spark gaps
{I use
> > > universal motors} and I have no problems BUT I totaly enclose
the
> > > dimmers in a grounded metal box. Apparently what happened in
your
> > > case is the triac was picking up some of the radiated R.F and
being
> > > driven into continuous conduction, try it again but this time
enclose
> > > the control in a grounded metal enclosure.
> >
> > Hmm... That may be. I believe that the case was non-metal, so
that I
> > guess I could shield it. However I now have three variacs
available,
> > and they go from 0 to full, whereas the fan control only went to
about 20
> > volts or so (just guessing). I didn't measure it. It was from
my olde
> > coiling days, before I knew about shielding, etc.
>
Does someone know what purpose the diac or avalanche device on the
gate of a triac serves? Does it have something to do with slamming
the triac on instead of gradually?
DL Huffman
> Variacs are always nice if you've got em! There is some lag
inherent
> to triac phase shift controls. Like a Tesla coil they need a
voltage
> avalanche device to run this used to be a diac which would conduct
> at a few volts, some companies substituted a neon -2 lamp which
> would conduct at something between 75 to 85 volts (really bad - you
> get half on to full on) now a lot of units use two silicon diodes
in
> parallel each conducting in the opposite direction, these will
start
> conducting at a few volts! So they've improved over the years and
> they are a lot cheaper now too! It used to cost over $5.00 just for
a
> 200 V 10 amp triac let alone the potentiometer , resistors, mylar
> capacitor and diac (you can't even bye a diac any more!)to build
the
> whole unit! Now you can get a whole working unit for close to $5.00
> at your nearest K-Mart's.
>