Re: Coil Grounding

To: tesla-at-pupman-dot-com
Subject: Re: Coil Grounding
From: "Jan Florian Wagner" <jwagner-at-cc.hut.fi> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Tue, 15 Feb 2000 07:33:29 -0700
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme


> I am in a small rented Duplex - my coiling work is done in the basement.
> How do I ground my coil? any suggestions?

Here's a (maybe bad) idea that I've succesfully used with my smaller coil.
Maybe it is not advisable for your planned huge 8" dia coil, but what I
did was the following: encase the whole primary and mains circuit in a big
and spacey metal box or in a dense chicken wire cage (with proper
insulation between cage and innards), then mount the secondary on a
platform a few inches above this and connect the base to the cage. With
400VA input and a 10nF cap and at 700kHz it runs without any additional
grounding. The cage is something like  30"x15"x15". 
You could cover the basement floor with roof flashing, and use that for
ground. Or as you're up in the basement and if you've a metallic roof, you
could connect to that one too. Just don't get the idea of touching the
streamers in any of these cases, and don't work too close to the coil.

> Also, I plan to use beer-bottle capacitors because of parts availability
> where I live (small town - capacitors: hard to find, beer bottles: very
> easy ;)

Someone correct me if I remember wrong, but wine bottles have about
1200-1900pF and beer bottles around 600-900pF capacitance. Corona can be a
problem, at those 10kV you're using. You need to put at least 3 bottles in
series to reduce the voltage&corona at each bottle. Then add these
"strings" of three bottles up in parallel to get the wanted capacitance.

Then C total = (0.9nF / 3) * StringsInParallel
For 3nF you'd have to get 30 bottles in this manner, to have a "reliable"
and corona free capacitor.

Placing all bottles in parallel, you'd need just 3-4 bottles to get 3nF.
But, at least my first bottle array with bottles only in parallel produced
massive corona at just 8kVAC. You better place some in series and make a
series&parallel cap array.

It would be very much better if you could spend some additional $$$ and
build a MMC capacitor, or a self-rolled one. Glass & beer bottles is a bad
material because of its high losses in radio frequency range. To get a
nicely operating 8" dia TC you'd better go for appropriate capacitors.

greetz,
Jan