Another finished coil

[Carl Willis]

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Knuckle strike. Yeah, I know they tell you not to do this....

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Discharge to air. Notice the fluorescent lamp in the background near the observer.

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A good ground strike arc

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Add me to the list of satisfied OneTesla customers! I've attached some pics of my coil operating outside for Halloween last night, where it attracted some large audiences. The night was breezy, which made performance a little ragged compared to indoor conditions, but I still got a few nice photos. Some show the "banjo effect" in the streamers brought on by the wind.

A little background about me: I have been involved in the Tesla coiling hobby for many years as a small-timer. I was an early innovator in solid-state-interrupted "staccato" tube coils. About 15 years ago, I made some MOSFET-driven solid-state Tesla coils. But a lot has happened in 15 years, and the OneTesla coil exemplifies many of the "new" trends in a package that is elegant and approachable. For instance, IGBTs have made enormous strides, and components are now inexpensively available that can switch the relatively high frequencies of small SSTCs at the relatively high voltages present in off-line drivers. The dual-resonant concept pioneered by Steve Ward is another major development that had yet to see the light of day back when I last touched an SSTC. I've got to say, I had a lot of fun firing up the OneTesla kit. It's a strong performer!

I made a few small mods to my kit before cranking it up full-bore:
1) Added an external ground strap (banana plug connected to IEC ground pin). This seems to be helpful at modestly improving performance (if tied to a reasonable hard ground nearby).
2) Stock kit topload capacitance was quite a bit too low for resonance. I added a stainless kitchen bowl to improve the tune (see photos). I strongly recommend that other builders check for resonance at low power (or operating on a Variac) before going full-bore.

Here are a couple of my own suggestions for construction of this kit:
1) Use dichloromethane or similar solvent cement on acrylic. It gives great strength and great cosmetics (if used carefully) and is the best way to go with acrylic fabrication.
2) I don't like the suggestion of gluing the secondary 1/4-20 bolts to the acrylic endcaps to keep them in place. Instead, put an internal-tooth lockwasher under the bolt heads (under the secondary winding ring contact) and just spin on the wingnuts once the toroid / top acrylic piece are on. I didn't find it to be too tricky and the result is electrically and mechanically much more robust.
3) The enameled wire on my secondary is thermally strippable. Don't bother sanding it; just put it where it needs to go and solder normally, making sure of course that the wire is getting appropriately wetted.
4) Trace the signals through Step 10b in the manual, even if everything looks good before that. As you trace the signals, push down / rock a finger on each IC and make sure the signals remain stable. IC pins sometimes do not make good contact with these kind of sockets. If bad contacts are found, fix them by gently bending the pins of the offending IC inward and then reseating it.
4) There is a misprint in the instructions, p. 39: "Do pins 2, 3, 4 of the 74HCT74 match the waveforms (F), (I), and (J)?" The answer is always going to be no, as pins 2 and 4 are tied to +Vcc. Ignore this instruction.

Here are a few things that I might think about adding, design-wise:
1) RF bypass caps (film, a few microfarads ought to be sufficient) across the bridge's electrolytic bus caps. I worry a little bit about how well these 'lytics will tolerate the the HF currents they experience in the stock circuit.
2) ground the IGBT heatsink. It's not at obvious risk of arc strike or inducing huge potentials from the coil above it, but I don't think it should float. Perhaps some others will comment on this situation.
3) TVSs on the gates / collectors of each IGBT. Cheap additions that will kill off any spikes that may endanger the transistors, such as from primary strikes.
4) Inrush current limiter in series with the rectifiers. The capacitive inrush current when plugging in can potentially be quite high. Maybe the possibilities have been fully analyzed in this design and a current limiter found totally superfluous, but I have noticed quite a big kick while plugging the power cord in and I worry about the diodes.

The one thing I hated about this kit:
1) getting the paper off the acrylic. Hands down the most time consuming and annoying part of the job. I do quite a bit with acrylic and I'm sorry to say I have encountered no good universal solution to depapering it. Sometimes it's just easy. Much of the time it is quite involved and infuriating. If a manufacturer can be found that does not put paper on their acrylic, I am sure it would be much appreciated by future kit buyers!

Anyway, that's about it from me! I look forward to many years of fun from the OneTesla coil, and I commend Heidi, Bayley, and everyone else involved on a very competently-designed, beautiful coil.

-Carl Willis

[Heidi]

Thanks for the detailed post and comments, Carl!

Our next manufacturer may use acrylic with a plastic coating that's a lot easier to remove, sparing your fingernails

[ChriX]

Got to agree with Carl about the paper on the acrylic - easily the worst part!

[stegu]

Could you please give us some information on the resonance frequencies of your primary and secondary circuits in the unpowered state, or when running on minimum power without much of a spark? There are quite a few people who are experiencing problems with too short secondary coils, like yours. You had the experience and presence of mind to fix that on your coil, but others are less fortunate, resulting in blown IGBTs and lots of grinding of teeth. One problem in fixing this without a lot of experimentation is that we do not really have a good grip on how much the secondary shifts down in frequency under the kind of streamer you show in your pictures. Because the oneTesla creates such huge sparks for its size, it seems that the streamer loading has a stronger influence on the resonance frequency than the common rule of thumb "allow for a frequency drop of about 10%".

As an example, I first had a resonance of 250 kHz for the primary, and 330 kHz for the secondary, more than 30% out of tune. I blew several sets of IGBTs and diodes before I sorted it out. Right now I am running at 300/330 kHz with a 5 turn primary, with satisfactory results. I think I need to tune the primary lower still to get the really impressive sparks, but I am fumbling in the dark, and I hesitate to bring the primary resonance back to lower frequencies, because I had an expensive, time consuming and quite frustrating experience with the stock configuration. Some numbers from you would help a lot.

[Carl Willis]

Could you please give us some information on the resonance frequencies of your primary and secondary circuits in the unpowered state, or when running on minimum power without much of a spark?

I'm happy to provide these quantities to the best of my ability to measure them.

The primary capacitance in my coil is measured on an impedance bridge at 0.0704 uF.
The primary inductance is measured (from cap lead to feedback CT) at 5.274 uH. There is more uncertainty in this figure owing to challenges controlling the impedance bridge lead geometry to calibrate.
On this basis, the primary resonant frequency is calculated to be 261 kHz, slightly higher than your value.

My kit secondary has an inductance (@f << SRF) of 22.96 mH. It is self-resonant at 494 kHz. I measure resonant frequency by hanging a neon lamp near the top of the coil while driving the bottom of the coil from a 50-ohm sinusoidal signal generator output. I simultaneously sample the field a few feet from the top with an small capacitive pickup tied to an digital oscilloscope with a frequency calculator. With the kit topload installed, the secondary system resonates at 328 kHz. Again, this is very consistent with your observation. With my own ad-hoc topload in place, it resonates at 275 kHz. This is within about 5% of the calculated primary resonance value, and obviously when the driving power is <1W, there are not going to be streamers loading the coil. I agree that the streamers will load it in a complicated and variable manner, but as you point out these effects are bounded within a few percent.

I hope this is helpful.

Best regards,
Carl

[stegu]

but as you point out these effects are bounded within a few percent

My own experience seems to indicate that the effect is quite a bit more than a few percent. It seems that the frequency drops more than 10% with a strong streamer present. I don't know by how much, but my coil has a 10% difference (pri/sec 300/330 kHz), and it seems to hit the sweet spot with matching frequencies in primary and secondary at around 50% power and 10-12 inch streamers. If I bring my coil into perfect tune at low power, it creates a remarkably strong spark and a startlingly loud noise at minimum power, but fails to produce any strong output at higher power. Your coil seems to behave differently, and that puzzles me.

[Tetsula]

Hello. I have appreciated your beautiful pictures.

3) TVSs on the gates / collectors of each IGBT. Cheap additions that will kill off any spikes that may endanger the transistors, such as from primary strikes.

Do you mean to add TVS between the gate and the collector, not between G and E ?
For the protection of the gate from unexpected high voltage, is it reasonable?

The one thing I hated about this kit:
1) getting the paper off the acrylic. Hands down the most time consuming and annoying part of the job. I do quite a bit with acrylic and I'm sorry to say I have encountered no good universal solution to depapering it. Sometimes it's just easy. Much of the time it is quite involved and infuriating. If a manufacturer can be found that does not put paper on their acrylic, I am sure it would be much appreciated by future kit buyers!

I have a good idea. Next time you peel, just moisture the paper. It's get surprisingly easy to remove the paper.
viewtopic.php?f=31&t=1160#p2804

[Carl Willis]

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A ground strike.

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Discharge to air

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Another ground strike

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I made some adjustments to the coil and the results are visible above.

(1) I improved tuning by adding some series capacitance in the primary, in the form of a 0.34 uF / 500V film cap. Total primary capacitance is now measured at 0.0579 uF. Primary resonant frequency is calculated at 287.9 kHz. The secondary system resonates at 328 kHz. The coil operates quite well with the stock topload now, although it still seems to want a bit more capacitance on the secondary.
(2) Added a rail-to-rail 5 uF / 400V film cap to help eliminate transients and protect the transistors and diodes.
(3) Tapped the upper hole on the IGBT heatsink for a 6-32 thread and tied it to ground via a solder lug & screw.
(4) Added a "Danger: HV" sticker (thanks Heidi) to take the place of the previously ruined one.

So it was smooth cruisin' for a few hours of fun and then the IGBTs blew. There was no obvious etiology, but the heatsink was quite warm, as were the tank caps, as was the primary winding. If one thing could have prevented this, it's probably a more aggressive spike protection scheme. I am still running without the TVS protection across each emitter to collector and the 30V TVS or zener clamp on the gate to emitter. I would like to add that on the next go. With all that high current circuitry in such close quarters to the gate drive components, loss of the transistors is probably bound to be quite regular. I'd be interested in hearing what other people experience regarding loss rate on the IGBTs in this design.

Finally, I am considering getting rid of the added primary capacitor and instead tuning to a higher frequency by winding a new primary with one less turn. This lowers tank impedance, but it's unclear if that will have a significant effect on peak current. I have also thought about winding the primary out of 4mm litz just to see if that would give any added kick.

-Carl