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Re First Coil Design (fwd)





---------- Forwarded message ----------
Date: Mon, 30 Mar 98 10:05:48 EST
From: Gary Lau  30-Mar-1998 0949 <lau-at-hdecad.ENET.dec-dot-com>
To: tesla-at-pupman-dot-com
Cc: djquecke-at-worldnet.att-dot-net
Subject: Re First Coil Design

Hi DJ:

The specs for your coil are nearly identical to mine:

>I have one 15k, 30ma, 115v, 60hz neon transformer and intend to add a second
>soon.

Same way I started.  When I got hold of a 15K/60 NST, it worked for a few
weeks, then shorted and I had to de-pot it.  This despite having a good
NST protection circuit (with chokes, bypass caps, series R's).

>With one tranny, I'll need .0053uf and with two, .0106uf for the tank
>capacitors?  I'll try saltwater, poly/flash in oil and I'm working on an
>experimental design (aren't they all <g>) using a product called ALPOLIC
>I've mentioned on the list several months ago.

I use a .01 uF 100KVDC commercial cap from Fair Radio Sales.

>Secondary is approx. 4.2" diameter and I'm figuring 21" tall (5 to 1).  I
>haven't wrapped it yet, though I've built a crude motorized "lathe" (PVC
><g>) and have started sanding on a 36" PVC pipe.  I've got 1000' of 22 gauge
>magnet wire and a gallon of oil based polyurethane.  I'm figuring about 790
>turns of wire at 37.5 turns an inch and about 840' of wire used.

My secondary is nearly identical, but 23.5" long.  Make sure you keep the
secondary turning while applying and drying the polyurethane.

>Primary is 1/4" copper fridge tubing, inverted saucer, 1/4" spacing, 15
>turns (or however far 50' gets me <g>).  I'm not certain about lateral
>spacing of primary and secondary.  I'm taking a shot at 5/8" space between
>secondary and inner coil of primary??  Inductance figures don't seem to add
>up??  Best spot to start tap?

My primary is a 15 degree conical design of 1/4" Cu tubing, 1/4" spacing.
The lowermost turn of my secondary is level with the innermost, lowest
primary turn, and horizontally seperated from the primary by about 1
inch.  The 1 inch is possibly slightly on the short side.  Your 5/8" is
definately too short.  I have to disagree with the advice offered by D.C
Cox, suggesting your secondary should start 1 1/2 inches above the top of
the secondary.  This would be way under-coupled with a conical primary,
and especially with a 15KV/30mA NST, you want to use a conical primary.
Going from a flat to conical primary was the single greatest improvement
I made to my system.

>Secondary base is easily adjusted vertically to optimize coupling.

>I've built a safety gap using 3/4" x 1.5" brass rod on 3/8" threaded brass
>rod in a nylon base.

The safety gap electrode tips should be round, never sharp or pointed.
It needn't be as massive as you describe, but no problem if it is.

>I've built a similar spark gap with attached airblast for main spark gap.
>I'll also try a couple of 6" RQ cylinder gaps.  I've acquired a motor, (a
>1/2hp Dayton with a variable speed drive, 705-4230rpm), bearings, etc for a
>rotating sparkgap but have had trouble locating the G-10 (same as GAROLITE?)
>material for a disk.  The locals don't have a clue so far.  I've seen it in
>McMaster's.  Any other (cheaper) mail-order sources for a 10" - 12" disk or
>a 10" x 10" - 12" x 12" sheet?  Best thickness??

My best results, 43" - 48", were obtained using a 10x.03" RQ gap.
Performance decreased when I started using a sync rotary, still not sure
what's wrong, possibly quenching.

>Toroid constructed from 4" aluminum flex, pizza pans and aluminum tape.  I
>have yet figured diameter.  Would seem that the goal for me would be to try
>to get secondary freq. down from approx. 515kHz range to the 250kHz area??

I started with a 4.5" x 15.5" toroid made from dryer duct and Al tape.
This gave me a secondary Fres of 290 KHz.  When I really started cooking
with my 60 mA NST, I needed a bigger toroid, and I stacked a 6" x 24(?)"
toroid on top of that.  Not sure what that Fres is.

>I have a 5a STACO variable transformer. (Large enough?)  I have access to a
>10a if necessary.

With my 60 mA NST, I draw close to 18 Amps, this with 88uF of PFC caps.

>Four 8' ground rods.  Any problem with cutting them in half and going with
>eight 4' rods?? (Hardpan about 5' below surface)

I can't say, I only have one 8' rod.

>I still need to figure PFC??  Seems to be contrasting arguments over the
>need for this?

If the biggest Variac you can get is 10A, definately use PFC's.

>I need to build chokes.  I saw a thread on using the cores from flybacks.  I
>have several such cores at 4" square by 1/2" diameter.  Is this viable?

This is probably a religious argument that will never be settled, but the
value of chokes in an NST protection network, IMHO, is questionable.  My
simulations and a series of experimental measurements made by Terry
Fritz suggest that an R-C network is superior.  See
www.peakpeak-dot-com/~terryf/tesla/main.html

>What changes will be necessary when going from 30ma to 60ma with a second
>(or third) tranny? (Besides capacitance of the tank circuit...)

You will need a larger toroid to realize the full potential of a larger
NST.

>Cheap source for high voltage wire?

What's this for?  Unless you insist on winding and using ferrite core
chokes, you really don't need anything special for hooking up the
non-tank circuitry.

>dj

Gary Lau
Waltham, MA USA