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Questions about first Tesla coil
Subject: Questions about first Tesla coil
Date: Sat, 19 Apr 1997 19:56:13 -0400
From: "Owen Lawrence" <owen-at-iosphere-dot-net>
To: <tesla-at-pupman-dot-com>
Hello, fellow Tesla coil builders! I thought I'd finally
introduce myself
after enjoying the banter on this list for a few weeks now. My name is
Owen Lawrence and I live near Ottawa. Like any beginner I'm bustin'
with a
million questions, so I'll pose a few of them now.
When I was in high school I built a coil but was never quite
happy with
the results. However it was entirely my own design so I'm still proud
of
it. I'll describe what I can remember of it and you can tell me how I
can
do better the next time. I've always had this dream of doing it over
again
and getting it right, so here goes. The primary transformer was a small
(brand new) ignition coil. Starting with six volts DC I created pulses
using a relay wired to vibrate. My four capacitors wired in parallel
were
made of approximately one square foot of aluminum foil glued to each
side
of a slightly larger sheet of 1/16" plexiglass. Each capacitor was
spaced
an inch or two from the other. By my calculations at the time I recall
assuming they were 50pF each, so 200pF in total.
My spark gap was made from two small bolts. I filed off the
threads near
the end of one of them to make it round, and sparked to the head of the
other bolt. With a nut on the threads and a wire attached to the nut I
had
an adjustable spark gap. Next, the primary winding was a cylinder of 14
guage bare copper wire, maybe a dozen turns at one foot diameter, height
of
about 8" or 9" inches in total. In the middle of this winding was my
secondary, which was about (and I'm really guessing here since I never
did
count, even back then) 800 turns of 32 guage enamelled wire on a 2" PVC
pipe about 18" long, terminated with a 1/4" stove bolt. I don't
remember
at all how all of these parts were wired together. Does it really
matter?
In the end I was able to get sparks maybe 2" long. I could draw
multiple
fine threads of sparks from anywhere along the secondary by holding my
finger near it. I had hoped for much longer sparks. Now I can see many
ways this coil could have been improved but it has been dismantled for
nearly two decades and I just want to learn what I can from it. I know
it
was functioning as a Tesla coil because I borrowed an oscilloscope and
measured a frequency of roughly 32MHz with the typical damped envelope.
You can't belive how excited I was in the middle of the night when I
first
applied power. Well, maybe you can.
So here's my question: NOW it seems to me that 32MHz is much
too high a
frequency for this setup. Is it the length of the secondary winding
that
really dictates the best frequency for a Tesla coil (assuming no
capacitance, that is)? I've tried out some of the formulas I've found
in
the archives and a more likely frequency seems to be in the
neighbourhood
of 2MHz. Is it possible my coil was resonating at a much higher
harmonic?
If so, could I have improved perfomance by adding capacitance to the
tank
circuit to lower the ring freqency to the same wavelength as my
secondary
coil? (Sorry if I get the terms mixed up; correct me where I'm wrong
and
I'll learn eventually) If my assumptions are not correct, can you tell
me
how you think this coil was performing?
I would like to build a new coil and would like to use an
ignition coil (I
actually have the original one!) for my primary step up transformer.
These
things are limited in the amount of power they can output, so I was
wondering what sort of spark length I should expect from a Tesla coil
properly designed around this limitation. If all goes well, I will use
the
experience to take the step up to a neon sign transformer and see where
that leads. I also have an old microwave oven in the garage with a
blown
transformer, whatever that means. Can anything be done with this? But
I'm
getting ahead of myself.
As far as my primary winding goes, the dimensions I chose were
completely
arbitrary. It just seemed to look right so that's how I did it. Would
I
have benefitted from a smaller diameter? Many of the Tesla coil plans I
see these days (in magazines or whatever) seem to have a tightly wound
primary with a diameter only slightly larger than the secondary. I
never
liked these because they always used insulated wire which severly limits
the choice of where it can be tapped. I can't remember exactly who said
it, but since I've been devouring what I can find on the internet about
Tesla coils I read somebody's remark that they preferred the shape of
the
field produced by a spiral coil over that of a helix. But that's where
it
ended. What's the real explanation here?
Another question: I also own a high voltage power supply that I
built
from an Information Unlimited kit (30KVDC). I bet some of you are
famliar
with it, yes? Two 2N3055 transistors and a flyback transformer. Is
there
any reason why this wouldn't be good for operation in a Tesla coil? I
would definitely want to protect the diodes in the high voltage
rectifier,
though, because they're expensive. I'm not gone on having a spark every
1/60th or 1/120th of a second, so if I have to wait while a capacitor
charges that's fine. Whatever works best with the equipment at hand.
Finally, I've seen some of the suggestions for building high
voltage
capacitors. One problem mentioned was that for rolled capacitors once a
certain length is exceeded inductance starts to become a problem.
Couldn't
this be reduced by tapping off each plate along its length? When it's
all
rolled up the tabs from each plate could all be joined together so you'd
be
drawing off charge from the side of the plate rather than the end.
You'd
end up with terminals at each end of the capacitor. Excuse the artwork:
___|______|______|______|______|______|_______|______|______|__
| + + + |
---------------------------------------------------------------
(dialectric inbetween)
________________________________________________________________
| - - - |
--|-------|-------|------|------|-----|-----|----|-------|------
roll it up: -------------------------------->
| +
/ \
|||||
-----
| |
_____
|||||
\ /
| -
I have other questions (like, I still don't understand
quenching, how much
does a pole pig cost, and why isn't there a tesla builders' usenet news
group) but these are the ones that have been nagging at me. Hopefully I
won't be the only one that would benefit from answers to any of these
questions. If you prefer to refer me to resources rather than answer
questions directly, that's fine. Any response will be much appreciated.
I'm very excited to have found such a cool bunch of people. I've
already
got several offices of people oooh-ing and ahhhh-ing over your pictures
of
operating Tesla coils I've downloaded (my wife is the only holdout :)).
Thanks a million and I'll talk to you later!
- Owen -