Re: [TCML] Fixing up an old coil -- tune-up suggestions

[David Dean <deano@xxxxxxxxxxxxxxxx>]

Hi Jason

Welcome to the list. I will try to answer some of your questions.

On Tuesday 13 November 2007 02:40:47 am Jason Goodman wrote:
> 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.
>
> 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 here....
>
> 125 v->12.5 kv transformer to power primary circuit, toaster-sized
> white box (neon sign transformer?)
> 6 brine-filled glass bottle capacitors made from root beer bottles
> Spark gap: single gap made from two trailer hitch balls mounted on a
> wooden frame
> 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" length
> 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.
>
> Repairs:
> 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/
> listarchives/1995/december/msg00040.html
>
> 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/
> wiki/Image:Tesla_coil_3.svg).  The "ground" end of the secondary was
> attached to a copper pipe providing compressed air throughout the
> building.
>
> We placed a grounded pipe (galvanized 1/2" steel electrical conduit)
> near the toroid to draw a spark.
>
> Testing:
> 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.
>
>
> 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?

Hard to say from the info you provided, but  lets say you have a 12kV NST at 
30mA, 18 to 30 inches, if the NST is rated 60mA, 26 to 35 inches. More or 
less, rough guess anyway.

> 2)  If we should expect more, is the problem likely one of LC tuning,
> or should we be looking for power loss elsewhere?

My gut feeling is the tuning. From your description of the primary coil and 
the secondary coil size and topload I don't think you have enough inductance 
in the primary coil or enough capacitance in the primary circuit to be able 
to tune the primary to the same resonant frequency as the secondary.

> 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?

Lipstick on a pig. I like that.  Actually an oscillator and an oscilloscope 
would be the best way to tune the thing. An LCR meter would be nice too, if 
you have one available. Then you could measure the primary inductance and the 
capacitance of your bottle cap, then do the math to find the frequency.

> 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?)

Or get rid of the foil and put the bottles in a plastic bucket with brine in 
it and put some metal flashing or a loop of copper wire or tubing as an 
electrode.

> 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?
>
No difference in performance but the transformer would be a lot happier.

> 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.
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later
deano
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