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Re: Beginner's coil safety
At 09:54 PM 11/19/98 -0700, you wrote:
>Original Poster: Ed Phillips <evp-at-pacbell-dot-net>
>
>" I keep hearing that someone has a system where two GM coils are
>putting
>out 3 foot arcs! "
>
> BULL ROAR!!! The insulation must be stressed severely at 3" sparks,
>and I'm sure secondary breakdown is not far away from that. Your
>experiments have given the longest sparks I have heard of, and I
>strongly suspect you will get shorted secondaries before long.
>
>Ed
I'm just guessing, but I suspect that the legendary (perhaps mythical?) two
coil system consists of two parallel GM (induction) coils being driven by a
(solid state?) bistable pulse generator. The GM coils then power a
conventional Tesla tank circuit.
When I was still a school kid, I found a copy of "GADGETEER's GOLDMINE" by
Gordon McComb in the school library. I built my first Tesla coil from the
plans in the book. The plans describe a table top Tesla coil which was
powered by a circuit in which a 555 IC controls a pair of 2N3055 power
transistors which drive an automobile ignition coil. Tuning is achieved
with trimming potentiometers which vary the amplitude and frequency of the
driving circuit's output voltage.
The coil was a poor design with a tall and narrow secondary, a tiny
primary, no top load capacitance, and a common circuit ground (which was
connected to the house mains ground!). However, on good days I managed to
coax 2.5 inch streamers and my interest in coiling was sparked.
Allthough not 'true' Tesla coils, I feel that a similar but well designed
table top Tesla coil could be a good way of introducing new coilers to our
hobby. I have read many posts here describing the hard work that
prospective coil builders have in obtaining NSTs, especially outside of
North America. An automotive ignition coil is far easier to find and is
nearly always easier to replace than a burnt out NST.
Safe coiling,
Gavin Hubbard