Re: Mini Tesla

Hello Nick,

A few comments and pointers about your coil:

1.) That 3kV xformer is awfully small. It is very hard to design a spark gap
that will operate properly at that low a voltage. I would try to get an OBIT
(~10kV and 20-50 mA) before experimenting further. If you are experienced you
might want to go for a voltage doubling (getting diodes and caps for 3kV
should be easy) setup. (6kV at half the current). HOWEVER as this deals with
DC be VERY CAREFUL. HV DC is DEADLY !!!! Be (careful)^2 around this kind of

2.) The 100 nF (?!? you sure ?!?) is way too big for this transformer. Your
xformer will never be able to fully charge the cap. Sure, it depends on the
output amperage of your xformer, but there simply is a limit to what a 3kV
transformer can charge as it only has ĩsecs charge time.

3.) Is your spark gap a single gap? Judging from the xformer voltage I would
guess so. I have always found going from a single gap to a multiple gap WILL
increase performance regardless of coil, spark gap type (e.g. pressure/vacuum
aided quench gap), or input VA. Your problem will be designing a multiple
segmented gap for 3kV.
4.) Even though I donīt have all your coil data, I would be willing to bet
going from a flat primary to helix in your case will also improve output
dramaticly because the flat spiral doesnīt provide enough coupling for your
input VA.

5.) Your resonant frequency of  1457 kHz seem to pretty high for a 1.5 x 6"
secondary. (or do you really mean 6" diameter and 1.5" high, the way you wrote
it??) Are you sure you didnīt make a calculation mistake? A friend of mine
built a "Vitamini" (the coil form being an old vitamin bottle) which had about
1" x 5" coilform specs and it had a lot lower FRes (somewhere around 300kHz,
if I remember correctly). Just judging from the frequency I would expect a
coil with a true Fres of 1457kHz (about half way towards a microwave
magnetron) would be around 3"(d) x 1"(h) maybe. You would never get any sort
of interesting sparks out of such a coil. You could hook a sig gen to your
secondary coil and measure the Fres with a Oscope and a pickup antenne. As
(Cp*Lp) has to be (Ls*Cs) your primary must be very small (size and turn wise)
with your 100nF cap (in order to get the primary LC circuit to resonate at

6.) Your toroid. You say it is 4 by 1". I hope you mean 1 by 4". You couldnīt
bend a 4" tubing diameter into a 1" circle. If your secondary really does
resonate at 1457kHz this toroid would have a capacitance being many times that
of the secondary and would load it down way too much.

Designing a true microwave coil (if you really mean 6" diameter by 1.5" high)
is very difficult, if not to say impossible. This was discussed on the list a
while ago. You might want to search the archives (at www.pupman-dot-com) for more
info. Plus donīt forget the electromagnetic radiation in the microwave range
is dangerous. Microwaves can fry your eyeballs in seconds. You would need a
shielded and grounded case around such a coil in order to play it safe (which
means you wouldnīt be able to see it anyway). If on the other hand you mean a
1.5" diameter by 6" high coil you will need to redo your calcīs because you
probably reversed the numbers in the TC Cad program.

I hope this helps you a little.

Coiler greets from germany,

 To All,
           I have built a little 6in by 1.5in tesla with a flat spiral
 I have calculated the windings etc. with tc-cad and they all match (resonant
 freq. 1.457Mhz).  It has a 4in by 1in toroid and runs off a 3kv transformer
 with a 0.1 uf cap and a 4mm spark gap.  I would appreciate any advice on
 teslas as the performence is not quite up to scratch.
                                              Nick Field