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

[Barton B. Anderson]

Hi Jason,

Cool! A Tesla Coil lying around is the best kind (it's mostly built). 
The primary is odd. You mentioned an inner diameter of 14", outer of 30" 
and a height of 20" (is that the vertical height from inner to outer?). 
If so that would put a little over 2.5" spacing av between turns.

In any event, this particular coil would need about .012uF with no 
primary changes. That's approximately 12 bottle caps (beer sized 
bottles). Something to consider while your building this is that the 
coil may "never" have been tuned correctly. I think you and your 
students can certainly do a lot better by correctly sizing the 
components and by maybe rebuilding the primary to accommodate other cap 
values (in the future).

Bottle caps are cool (salt, oil, and water, what could be better!), but 
an MMC is just as cool and will outperform the bottle caps. Lower losses 
are handy especially if the transformer you have is a low 30mA size 
(btw, for a 12kv/30mA transformer, in the range of .01uF is ideal for a 
static gap.

This coil can handle a good deal of power (more than you would actually 
want to throw at it in a lab).

Maybe, get the tuning right first with what you have. You can scope 
this. Connect a variable sweep function generator to the bottom 
secondary winding. Hang the o-scope probe a few feet away with a small 
wire (about toroid level). Sweep the generator to find the frequency 
(the scope waveform will remain flat with a few amplitude rises, but 
when the frequency is found, the amplitude will go noticeably high). It 
should be in the range of 290 kHz. This will at least tell you what your 
secondary is resonating at with the all the components in their position 
(toroid, primary, walls, ceiling, etc.). Then it's just a matter of 
getting the primary and cap to oscillate at the same frequency.

Verify your transformer's output current before making any primary 
changes. The transformer needs to have enough power to charge and 
discharge the cap in a reasonable time. If a 12/30, near .01uF cap value 
is good. The primary then just needs to be built to accommodate a 
reasonable coupling between coils and the inductance required for 
resonance. Whoever built the coil did a good job actually. The coupling 
is good at 0.133. I don't like the particular primary geometry, but for 
what it is, it's in the correct distance from the coil for decent 
operation. So, maybe double up on your bottle caps and you'll near the 
needed resonant frequency. It's mainly a tuning problem, but with the 
caps you have, you would need more primary turns to get there. Since you 
don't have those, adding more bottles to the mix is required.

Take care,
Bart



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?
> 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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