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Re: bi-polar (center-fed) TC

Hi
I've built a few centered primary 1/2 wave coils myself, they are quite
handy for attaching other devices and balancing the capacitive load if the
primary is built to be adjustable. This can be difficult with the vertical
1/4 wave type, and is the reason I started building 1/2 wave coils. Tesla
himself used this configuration for his public demonstrations of experiments
with high frequency. My small coil is 3"x16 3/4" wound on a very thin walled
fiberglass coilform, my primary is 10 turns of 1/4" copper tubing 8" in
diameter.  Mine will give a fat 8" spark between points using a 30ma 7500v
neon. I've used a 15kv 150watt dc supply on it but not at full power, it
begins arcing over with the variac set a little over halfway.  I've used
several different primaries on this coil from 6" to 12" in diameter and they
all worked. It all depends on how much voltage and current you want pump
into the thing. .I've never seen a formula for figuring the distance from
the secondary for high voltage coils, if one exists I would like to see it
myself. I think most guys experiment a bit to get that part right at least
thats what I do, there are probably some rules of thumb others can give you.
.The math for Tesla coils is only a guideline anyway. . .I do know one
thing, Your form is too thick, has too many turns and is too long but it
will still work quite well. You don't have to be a rocket scientist to build
one of these things or be all that precise! At least not on the small ones.
I have a coil that breaks all the rules and still works. . 3.5"x36" on a 1"
thick cardboard tube. . .I played with 3/4" solid stainless balls once.
.They burned very quickly and I was only using 25ma at 9000v, I find zink
plated carriage bolts work better for a small coil until you build something
a little bigger. The doorknobs will work but they are a bit large for the
application but should quench well. Correct, the secondary has to resonate
with the primary tank unless the coil is close coupled and running in
transformer mode. Then you would be very surprised at how large of a
capacitance you can run in the primary circuit. I had a 5.5"x10" secondary
with an 8" 8 turn primary I could run with no capacitor at all. I kept
adding capacitors to the primary clear up to 40mfd and it still kept working
although the sparks got pretty raggedy (lower freq.). I had a lot of fun
experimenting with that coil. I never would have seen this effect if I had
been depending on the math alone to design the perfect coil before I
actually built it, I never would have coupled it that close. Don't be afraid
to experiment, these small coils are cheap to build and play with . .As a
result of those experiments I am now building a close coupled oil insulated
bipolar coil which will be ultimately driven by a 50kv 500 watt DC power
supply, that should be quite interesting. On a related subject . .We use
over 100 Bipolar TCs at work to "age" color picture tube electron guns after
pumpdown. They are triggered using some very expensive high voltage SCRs I
bet some guys on this list would love to get their hands on! I hope I have
been of some help.
Mike KB9NZQ
----- Original Message -----
From: Tesla List <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, April 29, 1999 10:26 AM
Subject: bi-polar (center-fed) TC


> Original Poster: Parpp807-at-aol-dot-com
>
> Hi all,
> I have not seen any mention on the list of a center-fed TC, so I want to
try
> to make one. This may provide considerable entertainment for all. I have
> wound 23 inches, 1480 turns, of #28 on a piece of 3-inch pvc. I used a
piece
> of schedule-40 pvc; is that my first mistake? The sked-40 pvc has 1/4-inch
> walls so the diameter of the coil is 3.5 inches.
>
> The resistance of the coil is 80 Ohms.  The calculated inductance of the
> solenoid using
> (rN)^sq/9r+10 x length and the classical equation using the permeability
of
> free-space,
> (mu -naught) give a value for L of 30 mH within 0.1 mH of each other. The
> Wavetek 27XT splits the difference so I have considerable confidence in
the
> value of L. The sig-gen/counter/scope measurements show a resonant
frequency
> of 642 kHz with harmonics within a few kHz. Now, I get into trouble
because
> when I try to use the resonant frequency
> with the value for L to determine the distributed (self) capacity of the
coil
> nothing works. I
> expect just a few picoF; the calculated value is 0.5 picoF. My hand and
body
> capacity and
> the coil itself with its connections offer so much stray capacitance as to
> make any measurement unreliable. Can I assume that the self capacitance of
> the coil is
> just a few picoF?
>
> My next question is how do I determine the ratio of the primary and
secondary
> diameters?
> Will the equations for the coefficient of coupling (k) work? What should
the
> value be? I plan on forming 1/4-inch copper pipe so the copper pipe
primary
> will be freestanding on insulators. I assume the primary has to resonate
with
> the secondary. What range of Q
> values should I expect for these coils?
>
> Other puzzlement's: Is there an electrical difference between white and
green
> pvc?
> Will 3/4-inch stainless steel balls make a good SG? Can I use two highly
> polished brass doorknobs for a SG? The doorknobs are a little more than
> 2-inches in diameter.
>
> Thank you.
>
> Ralph Zekelman
>
>