Small VTTC project, PL500 based

[tefatronix]

Hello. Here is my small VTTC project based on the PL500 tube. I have built a few SSTCs and played with VTTCs before, but this is the first VTTC I finished and properly documented.

The Tesla coil runs directly from 230 V~ mains through a voltage doubler. It consumes 15-16 watts idle (filament only) or 40-80 watts depending on the power level set. The sparks are ~4 cm long. The secondary coil is about 8 cm tall, 3.2 cm thick and resonates at ~2.4 MHz.

Here is the schematic diagram:


It is a classic Class C  Armstrong-style oscillator.  Both L1 and L2 are wound on a 5 cm diameter PP pipe. L1 has 15 turns with several taps between turns 10 and 15. L2 has about 8 turns. C8 is built from three WIMA FKP1 1 nF/2 kV= capacitors in series (333 pF, 6 kV). Another 33 pF 3 kV capacitor is connected in parallel, giving a total capacitance around 370 pF.  I used a 13 V transformer with a voltage doubler (measured voltage 26-28 V with load) as I didn't have a proper reasonably sized 27 volt transformer.

The MOSFET circuit regulates the power. It's regulated by (dis)connecting the cathode. The mains voltage is half-wave rectified by D1. R5, R6, RV1 form an adjustable divider. If the voltage on the output of this divider gets high enough, neon lamps X1, X2 (strike voltage around 80 V per lamp) turn on and their voltage drop decreases, increasing G-S voltage of Q1, turning it on. If the voltage on the divider output drops, the lamps turn off and Q1 also turns off. This limits the operating duty cycle, decreasing heating while maintaining spark length.

Here is the entire device:


It is mounted on a pertinax (phenolic resin filled, I think) board. The circuit board is uncovered (a bit of a safety hazard). The PCB was designed and laid out by hand with an etch-resistant marker, as I was too lazy to make KiCad footprints for several parts that weren't there.

And here it's operating:


Bigger sparks could probably be achieved with a more powerful (PL504) or less worn tube, but it works OK.  Hopefully I'll get hold of some 6P45S or PL509 tubes for my next VTTC project (maybe) to be able to push the power higher without damaging the tube.
Video link: https://www.youtube.com/watch?v=BtGX9NkrPT8[/attachment]

[Mads Barnkob]

Nice little coil, all fitting on a small board.

VTTC sparks are just something special.

What is the reason for making a MOSFET/Neon bulb interrupter compared to the "standard" staccato controller that almost everyone uses for their VTTCs to drive a SCR/TRIAC?

I made a PL36 junk VTTC once, poor little thing got abused and could only do short runs with serious red plating Was also doing CW. Schematic and such can be found here: http://kaizerpowerelectronics.dk/tesla-coils/kaizer-vttc-iii/

[tefatronix]

Thanks, Mads! There is no particular reason why I used the MOSFET circuit, except the simplicity and the fact that I just quickly put it together during the original tests and kept it, I think it could be replaced with a SCR with very minor changes. I wanted it to work more as a phase cut regulator than a regular staccato circuit.


This way I also get it to turn off before the actual half cycle ends, but I doubt it plays any role... I just had those parts on the work table from previous experiments and wanted to see how such a circuit would work.


Wow, that tube got hot quickly in that video. I also got some red plating here after longer CW runs without the regulator circuit.