[TCML] Fixing up an old coil -- tune-up suggestions
Mddeming at aol.com
Mddeming at aol.com
Tue Nov 13 21:39:49 MST 2007
Hi Dr. R, Jason,
I fully agree with the MMC being much more efficient, but these two quotes
show you may be headed for different goals. If the intent is
renovation/modernization, then MMC is definitely the way to go. If restoration is the intent
then stick with the bottle caps.
" I would also recommend
replacing the very lossy glass bottles (20-25% loss in dielectric heating)
with more efficient and modern MMC capacitors."
> "Thanks for any help you can give. Please keep in mind that we're not
> interested in redesigning and rebuilding this from the ground up: we just
> want to get it working to original design spec."
Other comments interspersed below:
> Hi, all, I'm new to this list. I'm a physics professor at a small
> liberal arts college. About 5 years ago, before I was hired, one of our
> physics students built a Tesla coil for his senior project. It's been
> gathering dust in our machine shop ever since. Some of my current
> Electricity and Magnetism students and I are working on putting it back
> into working order.
> We haven't been able to find the original construction plans which the
> student followed to build it; if you recognize the design, a link to the
> plans would be appreciated.
Many Tesla coils are built without a formal plan or blueprint. There are
only a half-dozen parts to a basic coil.
> I've got several questions about getting this thing tuned up and working
> well, so let me describe it first. Here are the vital statistics of the
> coil as we found it. I'm writing this from home, so I'm going by memory
> 125 v->12.5 kV transformer to power primary circuit, toaster-sized white
> box (neon sign transformer?)
If the white toaster has well-rounded top corners and edges, it is probably
an Actown NST. In my experience, these are somewhat more fragile than
Franceformer brand, which have sharply rectangular edges.
> 6 brine-filled glass bottle capacitors made from root beer bottles
You may need more of these on parallel.
> Spark gap: single gap made from two trailer hitch balls mounted on a
> wooden frame
Brass balls of the same size would dissipate heat and quench better. Blowing
air across the gap would also increase performance. If the mounting bolts
screw directly into the wooden frame, this could be a source of power loss and
danger. At these voltages, wood becomes conductive, especially if there is
any humidity present. This can lead to carbon arcing and provide a bypass
alternative to the gap. A piece of plastic pipe as a collar between the bolts
and the wood should improve both safety and performance.
> Primary circuit wiring: Heavy gauge wire (4-gauge? 6?) with pretty thick
> insulation, type unknown. Alligator-clip connectors to primary coil.
> Primary coil: Inverted conical helix made from 1/4" copper pipe, about
> 7-8 turns, smallest diameter 14 inches, largest diameter 30 inches,
> height about 20 inches, mounted on triangular plywood supports.
> Secondary coil: 14-16 gauge wire on white PVC pipe, 5"ish diameter, 36"
> Top load: 30" toroid made from 4" diameter metal ventilation ducting,
> mounted on foil-covered plywood disk.
> There are no safety or protection circuits built into the system as we
> found it -- no safety spark gap across the transformer, no filters or
> switches on the mains side of things, no grounding rail above the
> primary, all stuff I've read about. I've made it very clear to the
> students that the primary circuit is absolutely lethal... I'm not sure
> they took me seriously at first, but they sure did once they saw and
> heard the spark gap firing.
Most physics departments have 10-15 Amp Variacs with fuse,power switch, and
output socket built in. Use one of these to power the coil if you're not
going to build these features in.
> The bottle caps had been sitting filled for five years. Exciting
> chemistry had taken place between the brine, the oil, and the metal bolts
> used as electrodes, leaving an unholy sludgy mess inside. We just threw
> these out, replacing them with a six-pack of Corona (heh) bottles, built
> more or less according to http://www.pupman.com/
> We wired the thing with the supply transformer bridging across the
> capacitor (http://en.wikipedia.org/wiki/Image:Tesla_coil_4.svg ) -- I
> now understand that it's better for the transformer to put the
> transformer bridging across the spark gap (http://en.wikipedia.org/
The spark gap across the transformer will greatly improve the longevity of
your transformer, adding a safety gap would be a big plus. I would not open
the main gap more than 6 mm.
The "ground" end of the secondary was
> attached to a copper pipe providing compressed air throughout the
This could be dangerous by raising the potential of the building's air
system to several hundred volts above ground potential. =:0
> We placed a grounded pipe (galvanized 1/2" steel electrical conduit) near
> the toroid to draw a spark.
Having the base of the coil and the rod grounded to the same place will
reduce ground loops, eddy currents, and shock potential.
> We first ran a test of the power supply without the primary coil in the
> circuit, shorting the alligator clips together. With the spark gap too
> wide, we heard a hum and a quiet snappy crackle. We could see some
> corona discharge from the aluminum foil surrounding the bottles, but they
> didn't seem to be heating up etc. With the spark gap set at 5-7 mm, we
> got a nice loud, bright spark, and the students were impressed.
> When we hooked up the primary coil, a fluorescent tube held in the hand
> lit up from about 6 feet away. We got corona discharge from the end of
> the grounded pipe to the toroid when the end was about 8-10 inches away,
> and occasional dim sparks when the end came within 6-8 inches. When the
> pipe end was raised so the shaft of the pipe rather than the end was
> closest to the toroid, we could draw a steady crackle of bright sparks
> 5-6" long.
> Tuning was done by adjusting the position of alligator clips on the
> primary using trial-and-error, just fiddling to try to get the longest
> spark. Best we could do was 6-8 inches of spark.
Inside connection attached permanently to the beginning of the primary coil
would be an improvement as every temporary connection is a potential power
> So, not too bad, but I feel like this coil is probably capable of more.
> Questions for you:
> 1) What should we expect from this coil?
Double or triple the output spark size.
> 2) If we should expect more, is the problem likely one of LC tuning, or
> should we be looking for power loss elsewhere?
Tuning is probably the biggest problem.
> 3) Is it worth going to the extra effort of measuring the resonance of
> secondary and primary using an oscillator and oscilloscope and tuning
> that way, or is that just lipstick on a pig at this point?
I would get an LCR meter and determine actual values, and use a scope.
> 4) Is a little corona discharge from the bottle capacitors a problem?
> If so, how can we correct it? (Trim sharp edges from the aluminum foil?)
It's always a loss, but looks cool. Eliminating sharp edges would help.
> 5) Do the two circuits mentioned earlier perform equally well, with the
> only difference being protection for the transformer, or does one give a
> better zap?
Hope this helps,
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