Re: secondary questions

Subject:        Re: secondary questions
       Date:    Wed, 16 Apr 1997 18:30:58 -0700
       From:    Bert Hickman <bert.hickman-at-aquila-dot-com>
Organization:   Stoneridge Engineering
         To:    Tesla List <tesla-at-pupman-dot-com>

Tesla List wrote:
> Subject:  secondary questions
>   Date:   Wed, 16 Apr 97 00:46:42 UT
>   From:  "William Noble" <William_B_Noble-at-msn-dot-com>
>     To:  "Tesla List" <tesla-at-poodle.pupman-dot-com>
> I'm dutifully following (most) of the RQ recommendations and the info
> from
> this list.  I've made myself a 14 turn flat spiral out of 1/4 inch
> tubing on
> 1/4 inch spacing with a 6 inch inner diameter, and now I've wound myself
> a 14"
> X 4" coil on PVC, following the instructions to spray urethane first,
> then
> wind, then spray more urethane.  I should have (according to the radio
> amateur's handbook, just under 650 turns (46.3 turns per inch) on this
> secondary (I know, 900 is the recommended number, but I ran out of wire
> at 14
> inches).  Anyway, my questions:
> 1. why is the initial coat of urethane needed??? I can see that it may
> help
> adhesion of later coats, but unless the PVC is electrically conductive,
> a
> layer of urethane ontop of the PVC won't affect dialectric constants by
> a
> measurable amount, or so it would seem - yet the RQ files make a big
> deal of
> this.

PVC absorbs water, and thus will become more conductive during humid
days. The best approach is to dry the PVC by heating it - I used a
couple of high intensity quartz halogen bulbs aimed at the coilform
while rotating it. Once dry, it should be immediately coated with a
couple of layers of polyurethane - the solvent-based stuff, NOT the
water-based. Some coilere also coat the inside as well. A somewhat
conductive coil form will add leakage current to your secondary, and
will rob power from your coil. Even though I've sealed mine (on the
outside at least), and added epoxied plexiglas end-plates, I still get
somewhat reduced performance on humid days.

> 2. The RQ files say NEVER put any wire inside - yet it seems to me that
> the
> ground terminal is best done with a stove bolt through a PVC cap that is
> then
> wired to the secondary - why should wire not be inside the form at the
> ends???
> (of course not in the middle, but why not at the ends).  Same general
> comment
> about the high side of the secondary - another bolt would be convenient
> for
> mounting a HV terminal or capacitor to, yet that would involve some wire
> (or
> at least a bolt head) inside the coil.

The problem is that you never want to get a flashover INSIDE the
coilform, since you may not see it until it has heavily carbon-tracked
the coilform - potentially ruining it. Also, burning PVC fumes are NOT
what you want circulating through you house - they're toxic and REALLY
nasty smelling! This can become a real problem as you increase power
levels on a well designed system, since sparklength can hit 2X - 3X the
length of the coil. 

You might be able to safely use a bolt on the base - but to be on the
safe side, I'd use thicker piece of plastic for the base, and
countersink the hole for a flat-heat bolt. After securing the bolt,
cover it with a piece of plastic to make it tougher for any corona to be
generated directly off the bolthead. Oughta' work!

Safe coilin' to you, William!

-- Bert H --