Re: secondary diameter vs. toroid minor diameter

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi Bill,

Bill wrote:

> > Someone once recommended to me that the toroid minor diameter be
> > slightly smaller than the secondary diameter, the idea (roughly) being
> > that breakout is more likely to occur where radius of curvature is less,
> > and you obviously don't want breakout from the secondary.
> >
> > Make sense?

Yes, if the toroid is raised high enough above the top turns of the
secondary where
toroid shielding is ineffective. If the toroid is providing enough
shielding to the
top of the secondary, then a larger radius minor toroid diameter can be
used. If
the top of the coil reaches breakout potential before the toroid, it "will"
breakout and can produce racings sparks (thus, lower toroid or reduce mutual
inductance). If there is adequate shielding, the top turns will not reach
breakout
potential before the toroid does.

> > For my next coil (slow in progress), I have a 9" by 30" spun aluminum
> > toroid (the last bulk buy), and am planning on using a 10.75" diameter
> > PVC secondary form.  Reasonable?  What diameter secondaries are others
> > using with those toroids?
>

Sounds good to me. I'm using a 9" x 30" spun al toroid (as well) on an
8.4375" x
39.75" coil (1789 turns). Sec base height is 28.75" and the top turn of the
coil is
at 68.5". The 9" x 30" toroid center plane height is 77". Just below the toroid
(and particially inside) is a smaller 4.5" x 16" toroid (plane height at
72") which
I use to shield the top turns. This simply allows me to run the main toroid
as high
as I do without problems. All sparks protrude off the main toroid.
(all heights stated are measured from ground plane).

So there's an example of a smaller radius secondary with a (slightly)
larger toroid
minor diameter where the main toroid is rather high and the smaller toroid
provides
added sheilding.

Take care,
Bart