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Re: New Toroid --> Using off-axis inductance

Original poster: "Justin Hays by way of Terry Fritz <twftesla-at-qwest-dot-net>" <pyrotrons2000-at-yahoo-dot-com>

Hi Shaun,

When you increased the size of your topload, this added extra
capacitance and dropped the resonant frequency of your secondary
circuit (coil and top). Your primary circuit is oscillating at a
frequency higher than your secondary circuit is, so put simply, you
are out of tune.

You have several options. The easiest is to add more turns to your
primary. But maybe you have run out of turns, so in that case:

Wind an off-axis inductor. Take INSULATED copper wire and wrap it
around PVC pipe or some other suitable HV insulator. Connect this
coil between your primary coil and capacitor somewhere, so that it is
in series with the two. It plain doesn't matter which end's get
connected to what; as long as the three components are in line.

Primary --> off-axis L --> capacitor


Capacitor --> Primary --> off axis L

It is called "off-axis" inductance because it's magnetic field is
oriented on a different axis than the primary coil. 

When you have added enough inductance, the frequencies of the two
circuits in your coil will be matched, and your sparks will be a
little longer than they were before -- but much hotter due to the
extra capacitance.

If you are getting 4" sparks with the big top, it sounds like you
don't have far to go. I would make the inductor out of #12
copper...and give it around 15 or 20 turns wound onto a 3" section of
PVC pipe. I like to use the THHN insulated house wire, and cut the
insulation off so the wire looks like this in air:

12" of insulation ---> 1/2" no insulation ---> 12" insulation --->
1/2" insulation --> etc.

That way, when wrapped on a pipe you can easily "tap" the coil like
you would a normal primary. You don't have to stip ALL the insulation
off, and you don't have to worry about turns touching each other.
It's even simpler to tap, when you make a little "loop" out of the
bare copper so it sticks out from the surface of the coil.

It's interesting to note, that I have noticed very little if any
performance decrease when using this method. Over the years, I have
used off-axis with a 270VA twin coil, a 3.5" single coil at 270VA
(the "red" coil), a 4" coil at 1kVA, and also a 6" coil at 3.5KVA.
All coils performed the same with and without the inductor (when they
were tuned, of course).

Good luck,

Justin Hays
Email: justin-at-hvguy-dot-com
Website: www.hvguy-dot-com

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