Subject: TESLA COILS
From: richard.quick-at-slug-dot-org (Richard Quick)
Date: Wed, 19 Jul 1995 05:05:00 GMT
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Quoting Mark Conway:
MC> One last question! In your power controller you use relays
MC> to provide isolation from the high voltage circuitry.
I use contactors (100 amp power relays) remotely activated with
low voltage control circuits to isolate the panel switches from
the 120/240, 60 cycle mains. While I admit that I have in the
past placed high voltage components such as neon xfmrs, RF
chokes, safety gaps, HV buss boxes, etc., in the control cabinet;
I no longer have ANY high-voltage in the control cabinets, and I
strongly advise others to avoid placing high voltage circuits in
theirs. I run no more than line voltage (0-280 volts) through the
cabinet. I use 24 volt AC control circuits wiring the switches in
the heavily grounded cabinet face where the operator places his
hands. A 24 volt AC panel switch closes a small relay. The 24
volt AC relay switches a higher current 24 volts DC from a
separate power supply to the main contactor coils. Both the 24
volt AC and the 24 volt DC supplies have grounded cores, and all
relays and contactors have grounded housings. I use two 100 amp
contactors, one before the variacs and one after.
On my smaller power cabinets I use a single 30 amp contactor
controlled by a simple 24 volt AC circuit. All of my control
cabinets have two sets of reversed line filters.
MC> Do you use them to connect power to the neons and how
MC> necessary do you think they are - I think from your video,
MC> when you were doing most of your experiments you didn't use
MC> the relays at all? Do you use relays with your power pig as
Well I have seen a coiler or two who did not use contactors to
remotely switch their 60 cycle mains. I have operated coils at
1.5 kVA without contactors. But let me tell you about the one
time I had an incident that "resolved itself" inside the power
cabinet. The experience changed my mind.
Everything happened too fast to really give a "blow by blow"
description, but basically what started it was the secondary
discharge struck a primary that was not equipped with an RF
grounded strike shield. The secondary discharge shorted the
primary turns, and the main tank circuit ran away. By "run
away" I mean that the oscillator shifted to a much higher
frequency due to the arc shorted primary. The primary/secondary
field flux collapsed when the tune was lost. All of the input
energy, the field flux energy, and the energy already in the
secondary, appeared to feed back into the tank circuit; in any
case it looked like discharger had become a vacuum cleaner and it
sucked in the five foot streamers. All of the conductors in the
tank circuit sprouted six inch long sparks and corona at the same
time the safety gap and capacitor terminals went to flames.
What happened next I do not rightly know, but just when my finger
hit the switch to shut the runaway oscillator down, there was a
tremendous explosion inside the power cabinet. It was so bright
that I had spots in my eyes even though I did not directly see
the flash. The smell of ozone and electrical burn were strong in
the cabinet when I opened the back to peer inside. At first I
could not see anything wrong. But given the intensity of the
flash, I was determined to disassemble everything if required.
A careful examination of every inch of the wiring revealed a
small hole in the insulation on the bottom side of a 60 cycle
main. The hole was located about 1/2 inch from the first line
filter, closest to the coil. Apparently kickback had made it
inside of the cabinet and the impedance of the filter was
sufficient that the kickback blew out the insulation and left the
conductor entirely. The line filter, having been bypassed, was
undamaged! Looking around some more I found fine arc tracks
scored into the surface of the polyurethane coated plywood mount
board. The arc had left the wire and traveled nearly four inches
across a surface of poly-coated plywood while spreading into a
"fan" of fine arcs that struck all along the main ground strap.
The delicate fan-like spark pattern was nearly the size of a
As it turns out nothing in the cabinet was damaged. I tried to
power up later, but a neon in my bank had gone south. I assume
that the neon broke down internally, and the kickback had jumped
to the 60 cycle primary.
After all that I completely re-thought, re-designed, then re-
wired everything. BTW, I just happened to be standing on an
insulated platform, which I rarely do when I am at the controls.
Had I not, I firmly believe I would have been jolted to my
... If all else fails... Throw another megavolt across it!
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