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Re: Variac vs. Regular Dimmer (fwd)





---------- Forwarded message ----------
Date: Fri, 15 May 1998 14:00:30 -0600
From: Gomez <gomez-at-netherworld-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Variac vs. Regular Dimmer (fwd)

Tesla List wrote:

> Can a regular modern household dimmer switch be used to changed the
> output levels on a 15kV/30mA neon?  Or are the power requirements of the
> neon too high for the dimmer switch?  (I believe those dimmers are
> electronic now eh?)

Theoretically, a standard 600W dimmer could handle the average power
level
of an unmodified neon, since it's only going to draw about that much or
a bit
less.  And there are 1200W dimmers available as well.

But there is another problem or two.  Cheap wall dimmers use triacs to
do
their phase switching.  When triacs (or SCR's for that matter) switch,
they
create spikes, and the sharp cut-off of the sine wave (especially around
90
degrees phase angle, or 1/2 power) creates harmonics.  Both of these
phenomena
can create additional heating and/or saturation in your NST core and
windings,
and interfere with efficient energy transfer.  It is possible that a
high-
power-factor NST (one with a cap added) might help, but I don't know for
certain.

Another issue is that cheap wall dimmers use triacs for switching,
whereas
higher quality (entertainment industry) dimmers and motor controllers
use
back-to-back SCR's, one for each polarity of the sine wave.  The
difference
is that SCR's have one less junction than Triacs, and therefore _much_
higher
surge current capability.

And finally, realize that even the most heavily filtered and
well-grounded
Tesla coil puts voltage spikes, current surges, and RFI back into the
power
line.  And if the discharge hits anything electrical that relates to the 
dimmer in any way, it will kill it.

I've seen a dimmer get blown right out of the wall, including the utter
destruction of the steel box it was mounted in, due to a secondary
strike
to a _grounded_ metal conduit that ran to the dimmer..
  
In short, you'd be much better off finding a small (5amp) variable
transformer
in a surplus store, flea market, ham swap-meet, etc.  That size will run
a
small coil (a single NST) easily, will take a lot more abuse, and won't
cause
switching spikes and harmonics.

-Gomez

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