Pancake Geometry (was Web page update)

To: "'Tesla List'" <tesla-at-pupman-dot-com>
Subject: Pancake Geometry (was Web page update)
From: Tesla List <tesla-at-stic-dot-net>
Date: Tue, 10 Mar 1998 22:53:19 -0600
Approved: tesla-at-stic-dot-net

----------
From:  Mark S. Rzeszotarski, Ph.D. [SMTP:msr7-at-po.cwru.edu]
Sent:  Tuesday, March 10, 1998 1:51 AM
To:  Tesla List
Subject:  Re: Pancake Geometry (was Web page update)

Hello All,
Bill Wysock said in part:
>There have been a number of posts on the List as of late, discussing
>the possibilities of a "pancake" style Tesla coil, with the high
>potential at the outer-most turns/diameter.  This is not the way to
>go.  Rather, with the ground connection being at the outer-most
>turns, you have the most desirable construction; the most turns
>and winding length, closest to ground potential and at the right
>input impedance, with the primary, coaxially wound around the outside
>diameter of the pancake.  As the voltage rises, going to the inner-
>most turns per layer, there is also an automatic benefit to this
>construction; Faraday sheilding of the higher voltage layers.  This
>is why the [early] electro-medical manufactures built their equipment
>the same way; always, the high voltage end of each secondary winding
>is at the CENTER of the pancake; not at the perimeter!
        I built a solenoidal primary, flat pancake driver, solenoidal extra
coil magnifier a couple years ago.  The driver coil consists of a 62 turn 16
gauge flat pancake (spiral) coil about 17.5 inches in diameter (L=625 uH),
and the primary is solenoidal in shape, constructed of 50 feet of 1/4 inch
tubing, wrapped on plastic sheets which were wrapped around the outer
perimeter of the pancake driver to form a cylinder.  The outer turn of the
pancake coil is grounded, and the center turn becomes the high voltage lead
to the extra coil.  The plane of the pancake coil is in the center of the
solenoidal primary coil which is about 8 inches tall.  The coupling is
greater than 0.5 if all of the primary turns are used.
        When fired, I got good performance out of the system with a few
caveats worth keeping in mind if you build one yourself.  First, I agree
that the outer turn should be grounded so the high voltage is brought out
perpendicularly from the center of the pancake.  Second, the tightly coupled
primary/driver coil system acts more like a transformer than a helical
resonator.  As a result, most of the voltage rise in the pancake coil
depends on inductive coupling, and most of the inductance is in the
outermost few turns of the pancake coil (look at the formula for inductance
of a flat pancake coil).  As a result, the voltage rise on the pancake coil
is rather striking just a few turns in from the perimeter.  Those small
diameter center turns contribute little to the voltage rise.  As a
consequence, you may want to watch the insulation of those outer turns.  I
got flashover between the inner and outer turns of the pancake if the system
wasn't in great tune.  Not wimply little sparks either - hot white arcs as
only a magnifier driver can produce!  Fortunately, this geometry is amenable
to immersion in oil, which would control that a bit.  This I did not try.
        I think this geometry has merit as a driver coil, but the self
capacitance of flat pancake coils is rather high and the voltage rise is
difficult to control if used as a conventional secondary unless you wind the
turns with nonlinear spacing.
Regards,
Mark S. Rzeszotarski, Ph.D.