Re: Getting Started (was Re: Arcs off the fingers... )
> Original Poster: Parpp807-at-aol-dot-com
> In a message dated 2/10/00 9:09:53 AM Central Standard Time,
> << Hi Robert,
> My first hint of a Tesla Coil came when I was about 8 and
> read about a 1/2 wave horizontal model in a book on spark coils. It
> was many years before I got around to building one. Much time and
> wire was wasted at that stage in learning about tuning rules. I had no
> idea then that sparks could reach many feet or come out white hot.
> The description in the book was of "purplish sparks about 3 - 4"
> long). The book came from a library and I have no idea who wrote it.
> Malcolm >>
> Hi Malcolm,
> >From the tone of your note I think I can guess, but I would like to read
> you have concluded from your early attempts at coil building.
I didn't try building one until about 18 years later and then it was 1/4
wave model. With not a shred of data to go on I wound several
secondaries and tried wire length formulae which I obtained from
several sources after my attempts met with failure. It was then I had
the idea of feeding the base with a sig gen and examing the e-field
with a scope and realized that tuning was involved. My first working
coil was an absolute dog. Small spherical topload etc. although with
the benefit of hindsight, I realise now that a couple of the
secondaries weren't too bad. I eventually got 20" from a transformer
that now gets to five feet and could possibly do more.
During this period I discovered for myself that bigger
toploads worked better. That 20" (spark) coil was a spacewound low
inductance benchtop model. The primary was a single turn (sort of
like Tesla's ;) and after snaffling a large ceramic cap in return for
helping somebody else, I discovered that winding a standalone tank
using this cap and placing it directly above the primary improved
coupling and performance to the point where it would drive a 9"
sphere and give the 20". Prior to that, it couldn't break out.
Occasionally the cap would flashover with 3" sparks (tank had 11
turns). My main primary cap was two oil filled transmitter smoothing
caps in series. The overall tank value was 0.12uF which turned out
to be almost perfect for the transformer but heating and the demise
of the internal connecting wires showed that these were a bad
choice. I was also determined to find out how to wind a secondary
to a predictable frequency. A spreadsheet showed none of the
wirelength formulae converging at which point I realized they were
wrong. It was another year before an exhaustive hunt of old radio
books turned up Medhurst which met with immediate success in
predicting the frequency of all the coils I had wound. The amazing
thing to me was that turn-turn C didn't even figure in the final
analysis. I tested it on clowewound and spacewound coils. It worked
every time. The realization that none of the writers I had read
understood the basic principles of what the secondary was spurred
a flurry of research on my part.
In 1993 in a fit of boredom one August holiday, I designed the
coil that now currently lives at work (the 5'er job). I designed a dry
fired low ESR poly cap for it. I was stunned to see three+ footers on
first firing. The coil was scoped-tuned prior to power-up. Since then,
refinements to the transformer, cap and topload have got that figure
to the occasional 5 footer. That coil plus 6 months of research
(incomplete I am sorry to say) formed the basis of a magazine
article I wrote. I figured that if available literature was misleading me,
it would also be misleading others and I had a recipe that I knew
A few months later, I discovered email and the usa-tesla list
and the rest is history.