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Re: Discharge terminal (fwd)
---------- Forwarded message ----------
Date: Sat, 01 Mar 1997 18:01:53 -0800
From: Bert Hickman <bert.hickman-at-aquila-dot-com>
To: Tesla List <mod1-at-pupman-dot-com>
Subject: Re: Discharge terminal (fwd)
Tesla List wrote:
>
> ---------- Forwarded message ----------
> Date: Sun, 27 Feb 1994 15:05:35 -0800
> From: Calle Laakkonen <laakkone-at-icenet.fi>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Discharge terminal
>
> What is a good discharge terminal for a small coil
> sphere or toroid ?
> How can I build one ?
>
> -Calle
Calle,
Although either type will add capacitance to the top of the coil, a
toroid will provide better "shielding" for the top of your coil. As long
as you build a toroid that's significantly larger in outside diameter
than the coilform diameter, and as long as you don't mount it too high
above the top of your coil, it will prevent corona breakout off hte top
of the winding. Most higher power coilers use, and prefer, toroids.
The easiest way to make a toroid is from a couple of pie pans and some
flexible aluminum ducting that's typically used for clothes dryer
ducting. I've copied part of an earlier post that describes how this
type toroid is made:
=======================================================================
An excellent performing, nice looking, and (best of all!) _inexpensive_
toroid can be made from one or two pieces of flexible aluminum dryer
duct, some aluminum tape, a couple of pie pans, a short piece of
aluminum sheet metal ducting, a 2" PVC end-cap, some small screws, nuts,
washers and a wingnut. Use the thicker flexible duct material, not the
very thin, foil-like plastic-aluminum stuff. Drill matching holes
through both pie pans, and bolt the two pans together bottom-to-bottom,
and center and mount the PVC end cap. Drill a hole for a short
round-head screw, two washers, and a wingnut, and mount the screw
pointing downward. This screw will be used to attach the secondary wire
to the toroid. Mount a matching PVC end-cap onto the center of your
secondary top coverplate. Drill and tap if necessary and use a Nylon
screw. Alternatively, this can be epoxied to the top coilform
coverplate. You can now use various lengths of standard 2" PVC pipe to
adjust for the height desired.
Cut out about a 4" long piece of aluminum sheet metal (can be flashing)
and roll it so that it's a tight fit inside the dryer duct. Tape it to
one end of the duct using aluminum foil duct tape so that about half of
it protrudes outside if the duct. Carefully begin stretching and bending
the ducting into a circular shape around the pie-pans. (BTW, if you use
pizza pans, you'll need six more PVC end-caps and 3 short pieces of PVC
to properly space and hold the two pans together). Slip the remaining
open end of the dryer duct over the short aluminum tube you just made,
and, while another person holds this in place, carefully use aluminum
tape to seal and hold the joint together. You'll end up with a
beautifuly performing toroid that's easy to make, looks great, and
performs virtually as well as one costing hundreds or thousands of
dollars.
An ASCII diagram is shown below:
. .
. .\ /. .
\--------------/ Dryer Duct
. . /-----____-----\ . .
. / || || \ .
| |
| | PVC pipe
| |
||__||
|------------|
| |
| |
Secondary
Coil
The reason a toroid protects better than a sphere has to do with the way
the
electric fields form around when excited by high potentials. To be most
effective, a toroid must be significantly larger than the coilform
below. The larger the outer diameter of the toroid, the greater the
distance that it can be above the coilform and still protect the coil
below. Also, a toroid adds capacitance to the top of the coil, which for
a variety of reasons will usually improve its performance. The diameter
of the ducting itself (called the Radius of Curvature, or ROC) will
govern the voltage at which the coil begins to "break out" and form
corona streamers. The larger the ROC, the greater this voltage must be.
It's desirable to "size" the toroid to the power level you're running at
so that it holds off any breakout until near-peak voltages are achieved,
letting the coil blaze into full, streaming, glory. Smaller coils will
typically use 3" or 4" diameter ROC, while larger coils may run with 8"-
12" or more. Roughly speaking, the ROC should be the same or somewhat
less than the coilform diameter for coils in the 3" - 10" range. For
example, I use 8" ROC material on a 10" coil. In general, higher power
levels will support a larger ROC.
You'll need to experiment with the toroid's height adjustment. I'd
recommended you do this while closely observing coil operation in a dark
room. Start out at lower coil power levels, and gradually increase the
powere level closely watching the behavior of the coil . Look in
particular for signs of corona in undesirable locations. A blue
"Christmas Tree" shaped glow extending from the top of the coilform to
the primary tells you that you're near the limit of toroid height,
coupling, and power-handling capability for your coil. Look for evidence
of small streamers or corona brush discharges coming off the top of the
coilform or the wire going to the toroid itself. If this is occurring,
lower the toroid height a bit, or consider going to a little larger
outer-diameter toroid. Changing toroid height may also require that you
retune the primary, since height adjustments will also change the
"effective" capacitance of the toroid a bit.
========================================================================
Hope this helps! Safe coilin' to you, Calle!
-- Bert --