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A Tesla / Marx Coil (Really!)
I have just tested an experimental Tesla coil with a Marx generator
included in its tank circuit. Results were most satisfying, considering
the hasty, thrown-together nature of my experiment. The tank capacitor
consists of two, 96nF, 8KV MMCs wired in parallel. This presents a 188nF
load to an unballasted pair of 2400VAC MOTs wired in series and driven with
230V-at-20A. The two MOTs develop 4800VAC, which charges the two 96nF caps.
Capacitor isolation is provided by a pair of 100 Ohm, 55W wire-wound power
resistors in parallel with the two caps.
The setup uses two identical spark gaps fabricated from 3/4" copper pipe
fittings and 1" PVC "Tees". The gaps are mounted on a common vacuum
cleaner motor which also serves as the base and provides airflow for
quenching. The gaps are electrically isolated from each other by the PVC
fittings. One gap serves to connect the two tank caps in series as they
approach full charge. The other gap serves as the tank gap and is set a
bit wider so that the series gap can fire first. Once the series gap
fires, the two MMCs are electrically in series, reducing the tank
capacitance to 47nF and placing a total potential of roughly 12KVDC across
the tank gap.
Spark output is pretty good, with solid, hot 48 inch streamers and regular
hits to the strike rail and the ceiling. Unfortunately, I can SMELL the
55W isolation resistors burning. They simply aren't up to the task. I had
to limit my runs to short bursts of a few seconds duration so they wouldn't
melt. I plan to replace them with homemade chokes loaded with ferrite in
the near future. It that doesn't work out, I'll buy bigger resistors.
Still, I'm pretty excited that it worked at all.
So what's the advantage of a Marx / Tesla coil? Well, for one thing, it
lets me use just two series MOTs with 4800VAC output, yet enjoy the
performance of a 12KV bang. It allows me to dispense with the voltage
doublers and their sensitive silicon diodes. It means I don't need to mess
with trying to series four MOTs, with the attendant problems of coil/core
breakdown and second stage saturation, not to mention the bulk and weight.
The marriage of Marx and Tesla isn't for everybody, but it can provide a
way forward for some coilers. For example, if a fellow owns a 7200V PT and
wishes he had a 14,400V PT, this might be a neat solution. Likewise, for
the serious MOT experimenter, this idea could be just the ticket to a good
performing medium-sized coil using only two MOTs for power. This area of
study & experimentation is ripe for the harvest folks. I wish one of you
engineer-types would pick up the mantle and build a real efficient,
butt-kicking Marx / Tesla coil for the rest of us to copy.
I'm having a ton of fun with this. I'll get some images & text about this
nutty experiment and the resulting sparks on my web site as soon as I can
pry myself away from the garage.
Best Regards from Middle Georgia,
Gregory R. Hunter