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Re: Arc'ing in my variac

Original poster: "Christoph Bohr" <cb-at-luebke-lands.de> 

Hello Pat.

I experienced similar problems in two cases:

First one was, that the contact surface of the pretty old variac I bought
had some dirt on it that stuck
to the surface but left the edges of the wire, where the wiper really
contacts ist clean, so I left it on, afraid I would ruin the
Variac by cleaning it the wrong way.
This worked for some minutes very well, then the flashing started. The
winding got warm during operation and the dirt was released
and caused the whiper to jolt above the small fragments of dust and whatever
else was in it. Sometimes small pieces would
catch fire from the arcs of the whiper. Looked very scary but could be
removed by cleaning the surface carfully.

Second was as missing RFI filter. I had it hooked up between the wall outlet
ant the variac, just as you have and so the RF got back into the variac.
I changed this and set the filter between the MOT stack and the Variac,
problem solved.
Ony might say that the filter should be directly on the input to surpress
any possible RF, even that generated by the variac when once in a while
small arcs accour on the whiper. But this is much less RF than your coil
produces, so I think it would be OK for home use. If you have another one
you could of course hook up 2 RFI filters in both places to play it safe.

Do you use a Terry style filter? If not, this could be another reason. I
believe I saw many setups with Terry filter and the RFI filter on the input
side of the
power controller without the problem you describe.

Hope this helped a bit, tell me how it turned out.

best regards

Christoph Bohr

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, June 17, 2004 4:46 PM
Subject: Arc'ing in my variac

 > Original poster: Pat Masterson <bat-at-grumman-dot-com>
 > I was tuning my small coil (15/30 NST) and noticed little sparks dancing
 > around the windings of my variac. Is there something fundamentally wrong
 > with the setup, or do I need an RFI filter between the variac and the
 > NST? The variac is an older model, and has no 3rd wire (ground) on its
 > AC line.  -pat