Hello Dick,
Thank you for your kind reply.
My 12/30 Transco came with a ground-fault unit which I removed. My first
use of the Transco was as a Jacob's ladder and it worked excellently,
running indefinitely. This was from both high-voltage terminals. I can not
tell if it has an internal shunting resistor, as the entire rest of the
unit is potted.
A check of the 10M ohm, 1/2W bleeder resistors shows them all giving
correct resistance values. Could you elaborate on the negative effect they
have on Q? It would be a shame if I had to remove them as they provide a
useful safety feature, and they've appeared in all the other designs I've
run across.
I calibrated my main gap in the same manner as my safety gap, using a
variac. The current set gap distance seems to be just at the limit, as
increasing the distance or decreasing the voltage causes failure of sparks.
The safety gap occasionally (once every 3-4 seconds) fires. This suggests
to me I have the safety gap fairly close to the main gap in terms of
breakdown voltage.
Thanks again,
Joshua Thomas
On Mon, Nov 1, 2021 at 10:12 AM <hooverrl@xxxxxxxxxxxxxx> wrote:
> Mr Thomas,
>
> I would check bleeder resistors. They lower Q of tank circuit. Check to be
> sure they are not shorted. May not need them.
>
> Does NST have internal shunting resistors? They may be causing issues.
> Older
> ones did not have them, but newer ones do.
>
> NST usually doesn't exceed 12 or 15 kV rms. Rule of Thumb is 1" gap per 10
> kV peak is used for safety clearance that ensures NO arcs (or 100 Kv/ft by
> IEEE #4). You may need to adjust tank circuit spark gap.
>
> Have you experimented building a Jacob's ladder with your NST? Verify that
> both ends of the high voltage output are each separately working correctly.
> NST's are notorious for getting a short in one end (as the ones I've used
> usually have grounded center-tap on the high voltage output).
>
> Use a safety grounding stick to ensure caps are discharged before making
> adjustments to any part of the circuit.
>
> Thanks,
> Dick Hoover
>
>
> -----Original Message-----
> From: Joshua Thomas [mailto:joshuafthomas@xxxxxxxxx]
> Sent: Sunday, October 31, 2021 9:06 PM
> To: tcml@xxxxxxxxxxxxxxx
> Subject: [TCML] First coil - need some help
>
> 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
>
>
>
>
> --
> Joshua Thomas
>
> My new email address is: joshuafthomas@xxxxxxxxx
> Please update your information if you have not already done so.
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>
>
--
Joshua Thomas
My new email address is: joshuafthomas@xxxxxxxxx
Please update your information if you have not already done so.