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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