Re: Center fed coil

On Tue, 16 Jan 1996 09:00:49 +0700 you said:

>I have been trying to help David Lawrence design a coil that is best
>described as an Ouden (can't remember how to spell it) or center fed Tesla
>coil.  He wants to experiment with intense electric fields created between
>two plates connected to the ends of the coil.  He has ordered one of the
>Condenser Products .025 mfd capacitors so I thought we shoot for a frequency
>that would allow him to use that.  He has a 4.0 ft x 12" form.
>I did some rough calculations today and came up with some numbers.  For a
>normal 1/4 wave Tesla coil we want about 900 turns of wire.  With this type
>of coil I think we want 1/2 wave - or twice the amount of wire.  Using 1800
>turns of wire (he has #25) we come up with a self resonate frequency of about
>69 khz.  In order to use his capacitor, we come up with a primary that is
>about 20 turns of 3/8" copper tubing on .75 centers wound on a 24" form about
>15 inches long.  It appears there will be no way to adjust the coupling once
>he gets this thing built.  Have you ever built one of these?  Dave was
>thinking about allowing 6" of clearance between the primary and the
>secondary.  Do you have any ideas?


Tesla Coil Secrets (laugh) has plans for almost exactly the coil you describe.
Yes, the plans are way less than Ideal, but since this design was the third
coil I ever built I thought I might say something if you're inclined to
continue on with this 4" x 12" coil form...

I built the coil just like the plans say, except I used 26-gauge wire instead
of 25-gauge, and it was space-wound. (Stop shouting! <grin>)
I had to put many layers of polyethylene sheeting between the primary and the
secondary to prevent arc-over.
I used a salt-water caoacitor made from a large, tin washtub and gallon
The gap was a rotary, 16' dia. with 12 electrodes, driven by a 2HP motor,
no speed adjustment, only dwell adjustment.
The plans called for a ten-turn primary, but that was too much I know now,
because when I would draw the spark, I would be strongly "pulsed", (but I
kinda like that ;)
The output was about 18" from either pole. If I stood on a chair-mat or a
sheet of acrylic, I could draw the arc and have 9"-12" sparks flying out of
my feet and snaking all across the plastic. :)
One time, a friend and I were drawing the arc from each pole simultaneously.
Standing side-by-side, we got too close together, and a hot bolt shot between
our shoulders. The resulting shock almost knocked us both down. We laughed. :)
Another time, a friend and I were taping this coil in action. The cam-corder
was plugged into the wall and the cord from it was hanging down my friend's
back as he filmed. Standing on a chair, I held one of the terminals and
had him draw an arc from a piece of metal held in my other hand. Apparently
the RF-current acted as a conduit for the 60-cycle current powering the
camera, and we both got majorly shocked, the second-worse I ever had. I almost
soiled myself. ;)
Even though the design was way less than ideal, as Richard has pointed out,
we still had a lot of fun with it. :) You could wind it without spacing, and
go to about a 30" dia. primary and get "ok" performance, I think. Not a super
performer, but good for a test to see if more hobby money should be allocated
to a better design. :)

Dan  <klineda-at-univscvm.csd.sc.edu>