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

[resonance]

If you contact me off-list I can help you out.  I will need to gather some 
more specific data on your pri and sec coils.  I would also recommend 
replacing the very lossy glass bottles (20-25% loss in dielectric heating) 
with more efficient and modern MMC capacitors.

I'm using a computer program called JAVATC designed by Bart Anderson that 
will allow me to run all parameters of your coil system and produce a "best 
result".

Dr. Resonance

Resonance Research Corp.
www.resonanceresearch.com


----- Original Message ----- 
From: "Jason Goodman" <goodman_jason at wheatonma.edu>
To: <tesla at pupman.com>
Sent: Tuesday, November 13, 2007 1:40 AM
Subject: [TCML] Fixing up an old coil -- tune-up suggestions


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