Re: coil question
Subject: Re: coil question
From: Terry Fritz <twftesla-at-uswest-dot-net>
Date: Thu, 23 Dec 1999 17:06:19 -0700
At 05:21 PM 12/23/1999 -0600, you wrote:
>OK, I'm new on the list... Never posted anything on here before, but I have
>been keeping up with the list lately and it's great... there's a lot of cool
>ideas being thrown about...
Welcome to the list!!
>anyways, on to my question... I'm working on my first coil and having some
>trouble... my primary circuit is working great but the secondary coil isn't
>doing jack... I'm thinking that I might now have enough coils on it but I
>don't know... was looking for some good suggestions... here's my coil
>120V 60hz in, 15KV 30ma out on the transformer
>variable static spark gap.... can get about two inch sparks with no
>trouble... so I know the primary circuit is going good
Two inches sounds too far?? Be sure your gap is not set wider than what
just the bare transformer will arc across. It is very possible the caps
are resonating with the neon transformer causing the voltage to go much
higher than it should. Simply set the gap to where just the transformer
alone will give reliable arcs across it. I would think that would be about
1/2 inch. You may want to adjust that later but wait until everything else
>2 salt water caps... these have 24 bottles in each... I haven't measured the
>caps so I don't know their ratings... I'm a little short on money... but I'm
>guessing their close to what I need
Typically, a single beer bottle is about 0.0009uF or 0.9nF. Depending one
which ones are in series or parallel, you can roughly guess the overall value.
>8.75 loops of 3/8 inch copper pipe, spaced at 1/2 inch with an 8 inch center
>coil for the primary
This should be fine.
>14awg coiled around 4.5 inch PVC, 24 inches high for the secondary
I assume that is enameled wire that is only coated with a thin layer of
>I think I need high gauge wire on the secondary but I don't really want to
>do that if I can avoid it... any suggestions?
>also, what is good for connecting the components? is there anything special
>I should account for in my connections? like thicker wire or anything?
See the "wire" posts that asked this same question.
Ok, an 8 inch inner diameter secondary with 3/8 tubing and 1/2 inch air
space between turns and 8.75 turns, gives an inductance of 32uH max which
should be fine.
The secondary of 4.5 inch dia and 360 turns on a 24 inch length gives only
2.26mH which is pretty small. Rewinding with 24 gauge magnet wire would
make if much more "typical".
You didn't say what the top load or terminal was. However, I "think" that
the secondary wants to run at a much higher frequency than the primary
(secondary around 700kHz and primary around 400kHz). You need to increase
the primary frequency to match. Try removing caps and/or using far fewer
turns (change tap point) on the primary. Also a large top load will reduce
the secondary frequency which should help too.
Long term, I would think about getting a secondary with like #24 wire which
should make things much more "normal". Sounds like the primary is fine but
you should add safety gaps if you don't have them now. If you don't have a
top terminal, get one too.
If you can roughly figure the cap value, describe the top terminal and if
the #14 wire on the secondary is enamel coated or not, we can give a much
better guess. In general, we try to figure the primary and secondary
inductance and the primary and secondary capacitance to solve tuning
issues. Lp x Cp = Ls x Cs. The inductances are always easy and these days
if we know the dimensions of the secondary and terminal we can find Cs very
accurately. Those salt water caps are always a bit of a "big guess" but if
we know the others we can usually work through that.
Once your coil is working, An MMC (or lower cost EMMC) cap will be high on
your wish list and is the definite "next step" so start saving money ;-)
Be vary careful of over voltage on the neon! Yours is working great right
now but they can turn on you quick!
Hope this helps,