Hello,
I'm not sure who the author of this email is, but thanks for your reply. I
put pictures up here: https://imgur.com/a/5ByTOHl
The only picture I had of the MMC seems to have been during testing with
the safety gap. That worked well (and quite loud!). Please ignore the
"tilted" look of the topload; this is due to camera angle. The topload is
symmetric.
> Do you know that the transformer is good?
Yes. I can draw arcs from each terminal to (case) ground, and from terminal
to terminal. The Jacob's ladder from each terminal works fine. I do not
think the transformer is the issue.
> The capacitor voltage ratings are extrapolated from DC ratings
I'm not clear on what you're trying to say here. The CDE 942C20P15K-F caps
are rated for 2kVDC. The AC rating is 300-500 VAC. Given the extremely
rapid voltage changes (dv/dt), the DC rating appears to be the more valid
one; and indeed that method is used by other well-known coilers, eg. see
http://www.hvtesla.com/mmc.html. Lists of "known" good caps (eg.
http://www.tb3.com/tesla/capacitors/good_bad.txt) put the CDEs on the to,
and again use the DC rating. Terry Fritz uses this method; see:
https://hotstreamer.deanostoybox.com/TeslaCoils/MMCInfo/mmcinfo.html, "We
have found that the DC voltage rating can be used as the peak AC voltage
rating.".
In my particular case, 12kV AC is equivalent to 16.8 volts peak to peak,
which should be safely inside my 30kV total rating for the capacitor bank.
I do not think this is the cause of my troubles but would be interested if
others have similar concerns.
> If I'm understanding the setup, you have 3 gaps in the spark gap between
4 copper pipes, plus fan cooling. This sounds excessive for 12kV and one
neon sign transformer.
That's correct; see picture in album. The first image is the gap under
construction. I based this ducted, circular design on one that I found
during research. The fitting for the AC fan exactly matched the 6" PVC
fitting which greatly simplified the building. Is there a reason this might
be excessive? Multiple gaps should spread the heating effect over several
pipes, and the fan should increase quench. The gaps are calibrated with a
variac to fire near peak voltage, and they do fire well.
Thanks,
Joshua Thomas
On Mon, Nov 1, 2021 at 7:45 PM <pupman.com@xxxxxxxxxxxxxxxxxxx> wrote:
> Is there a way to post photos of your setup?
>
> A few things -
>
> Do you know that the transformer is good? Draw arcs off each leg to
> ground with a ground wire on a sissy stick, then try terminal to
> terminal. A yellow glowing arc is what you want.
>
> The capacitor voltage ratings are extrapolated from DC ratings and not
> relevant to AC use. They are rated 300-500VAC at 60Hz, it will be far,
> far less for RF use.
>
> If I'm understanding the setup, you have 3 gaps in the spark gap between
> 4 copper pipes, plus fan cooling. This sounds excessive for 12kV and one
> neon sign transformer. Maybe bypass one if not both gaps, or move the
> fan further away. As for the resonant frequency, I'd ignore any goofy
> simulators and just measure it. Get a frequency generator and
> oscilloscope. Connect the frequency generator to just the entire
> primary coil and measure voltage on the secondary and sweep for maximum
> rise. The resonant frequency will be somewhere near there. You'll then
> have tune the primary to this. The dimensions indicate the secondary is
> real heavy on capacitance. You may need to test with a smaller terminal
> on the top. I don't hear much talk of this, but you can tune a tesla
> coil by listening to the spark gap. A buzzing sound is what's ideal, not
> a source of loud noise. It's similar to MIG welding. A smooth sound is
> what you're after, not harsh noise.
>
> On 10/31/2021 8:06 PM, Joshua Thomas wrote:
> > Hello TCML!
> >
> > I'm having trouble with my first tesla coil. The specifications are as
> > follows:
> >
> > * 12/30 Transco NST, GFCI removed
> > * Primary coil: 0.25" copper tubing in flat spiral, 0.5" between turns,
> > total 12 turns, 12" total radius from center. Height adjustable.
> > * Secondary coil: 3.5" x 16" on PVC form, 28ga magnet wire, ~ 1100 turns
> > * Topload: 4" dia flex PVC ducting torus, total diameter 18", 10" plywood
> > center. Entire form covered with conductive copper tape.
> > * Capacitor bank: 15x CDE 942C20P15K-F caps in series. 30kV rating, total
> > 10 nF. 10M ohm 1/2W bleeder resistors across each.
> > * Static spark gap of 4x 1" copper tube, microwave AC fan used for
> quenching
> > * RF grounding rod of 1 ft, solid 1" copper, with 14 ga connection wire
> > * Safety gap calibrated with variac to be just above normal firing
> voltage
> > * Safety lowpass RC filter (aka ''Terry" filter) of 500 ohm, 100W
> wirewound
> > resistor and 3x 15kV, 1nF ceramic caps (parallel); one RC set per HV
> line.
> > F_cutoff ~ 106kHz
> > * All HV connections made with 14ga GTO-15 wire.
> >
> > I've simulated this with JavaTC and it gives me a matching resonant
> > frequencies around 230 kHz.
> >
> > First test was today. Spark gap works like a charm, banging away. Safety
> > gap occasionally fires. Nothing at all on the secondary or topload. No
> > corona, no arcs, CFL bulb put near the coil did not light up. A breakout
> > point was attached, and a large metal object was put near it, and still
> > nothing.
> >
> > Trouble shooting:
> > * Removed the Terry filter. No change.
> > * Changed tap point on primary coil from turn 8 to turn 7, no change.
> > * Continuity checks on the secondary show normal, from topload to
> grounding
> > strap.
> >
> > Thoughts:
> > * My secondary coil is kind of terrible. First one I ever wound, and it
> has
> > some wire overlaps.
> > * A second check of my simulation suggests I should actually be using the
> > entire primary coil instead of tapping at turn 8 of 12. Dunno why I did
> > that. Would that have this much of an effect?
> > * The RF grounding on this confuses me a little. I have a large metal
> strip
> > attached to the bottom of the secondary. This has connections "in" from
> the
> > safety gap and the terry filter, and "out" to the RF ground rod hammered
> > into the earth. Is this how this is supposed to be wired?
> >
> > Thanks in advance for any advice etc. Glad a list like this exists, as no
> > one in my department at the university (I'm a physics major) has any idea
> > how TCs work.
> >
> > Joshua Thomas
> >
> >
> >
> >
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--
Joshua Thomas
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