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Re: Variac
From: Bill Lemieux[SMTP:gomez-at-netherworld-dot-com]
Sent: Wednesday, January 07, 1998 5:29 PM
To: Tesla List
Subject: Re: Variac
Tesla List wrote:
>
> From: Edward V. Phillips[SMTP:ed-at-alumni.caltech.edu]
> Sent: Monday, January 05, 1998 1:11 PM
> To: tesla-at-pupman-dot-com
> Subject: Re: Variac
>
> "Ed,
>
> Thanks for the warning. I found their web site.
> http://www.signweb-dot-com/diversified_components
> Could you take a look and see if anything about what they say leads you
> to think differently, and if not, could you explain the reasoning behind not
> using these devices in a TC tank circuit?
> Thanks.
>
> Regards,
> Alfred Erpe"
>
> Alfred:
> These are TRIAC dimmers, which work by varying the conduction
> angle of the waveform.
For one thing, triacs are only used in _cheap_ dimmers. In professional
dimmers and high powered motor controls, back-to-back SCR's (sometimes
in discrete packages, sometimes in pre-combined packages) are used.
The reason is that a triac has four junctions through which current must
pass, but an SCR only has three. Believe it or not, this results in a
well-designed SCR having ten times as much surge current capability as a
triac with the same RMS current rating. One of the few things Tesla
supply equipment has in common with high powered dimmers and motor
controls is the need to handle surges. I wouldn't waste my money on
those neon dimmers.
Also, I have blown up at least two dimmers (literally- exploded right
out of the wall box) just by hitting the "well-grounded" conduit which
fed them with the discharge of a medium-size coil. Such cheap devices
don't handle high frequency RF well, either, apparently...
Regards,
Gomez
--
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