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Re: Tesla Ground (fwd)



---------- Forwarded message ----------
Date: Mon, 19 Jan 1998 14:37:07 -0700
From: Bill Lemieux <gomez-at-netherworld-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Tesla Ground (fwd)

Tesla List wrote:

>         Today I (attempted) to construct an RF ground for my tesla coil I
> am building. Being the cheapskate that I am, I decided to use some scrap
> aluminum conduit that was laying around. I found a piece about 6' long and
> drove it  into the ground leaving about 6" sticking out of the ground to
> connect to. My question is this: will this poorly built ground be good
> enough for my coil?

Aside from grounds so poor that there is a risk to life and/or
equipment, there isn't really a threshold of ground impedence that
everyone agrees is "good enough".

However, most coilers I know of disdane aluminum as a conductor, as do
I.  One reason is that with TC work, you need something with low
_surface_ resistance, since so much of the current in question will be
on the surface of the conductor due to skin effect.  The higher the
resonant frequency of your coil, the more pronounced this effect
becomes. Aluminum has the unavoidable habit of instantly forming a thin
layer of nearly-non-conducting aluminum oxide on its surface upon
exposure to air.  This is why aluminum house wiring requires special
connectors to be used to avoid overheating of the wire.

8' long copper-clad ground rods are only a $11 US at our local hardware
store.
I should think most amateurs could afford one.  Personally, I use five,
connected
in a ring with very large (4ga) wire.  I would recommend using flat
copper strip
to connect your coil & equipment to the ground.  Don't "daisy chain"
grounds, either- connect each piece of equipment (control panel, coil,
etc) directly to
the ground itself with separate pieces of cable/strip/etc.

My experience with grounds has been: "if more is better, too much is
just right".
Go to as much effort and expense as you can afford, then quit.  If the
effort and
expense you could afford turns out to be inadequate, stop running the
coil/HV gear until you can afford the time/money to improve your ground.

But unless your facility is in an unusually poor soil area (such as very
sandy soil with low mineral and moisture content) or your gear is placed
in odd circumstances, a good ground shouldn't be hard to come by.

Salient points for good RF grounds:

1. Use as much surface area, both conductors and the grounding rod or
material, 
   as is practicable.

2. Keep your ground path length as short as is practicable.

3. More than one parallel connections to "the earth" are better than a
single
   very large one.  Both AM radio transmitter engineering handbooks and
power 
   distribution lineman's handbooks point this out.

> My coil is only about 3.25" and powered a tranny
> that is about 7Kv 30mA. I didn't know weather to connect the bottom turn
> of the secondary to the primary like some people do, or try to construct
> my own ground. Any comments, criticisms are appreciated.

If you are using an unmodified NST, _do_not_ ground anything that either
of it's
HVbushings is connected to, such as your primary.  Since the transformer
is 
center tapped grounded tp the case (and you _do_ have the case grounded,
_don't_
you?), you would be shorting out half the secondary of your NST.

"Floating" primaries work quite well, but are of course, a bit more of a
safety
hazard, at least in theory.

Also in theory, a primary with one end well-grounded is at ground
potential
at 60Hz, reducing (if not eliminating) the chance of electrocution if
someone grabs the primary while the coil is running.  (why or how any
one might do that is left as an exercise for the reader)  Of course, the
chance of a nasty RF burn is still present.  Most people I know of
(including your's truly) ground the primary of any coil using a single
ended or isolated-secondary HV power transformer.



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