Another finished coil
Posted: Fri Nov 01, 2013 1:59 pm
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