ZVS driven Plasma Ball (w/ segmented PTFE Secondary)

[zytra]

Quick updated, I submerged in oil the secondary as is (all 28 chambers but 1 filled with about ~ 100 turns each).
It's been really stable compared to tests in ambient air.

I also connected to my custom ZVS driver and was able to push the voltage up to 32V (Max of that PSU).

I also performed brass ball separation testing and the maximum arc length was obtain just a little over 22mm.


And then I had the probably stupid idea of using the brass balls as (short" jacob ladder). So I pulled in the two brass balls to about 10 mm. It works wonderfully for about 5seconds before the 1K 1A diodes diodes (1N4007) along with the 2x 15V gate zener's. I repeated this 3 times, by replacing the diodes and experienced the same result every time. There is nothing fundamentally different than the separation test I did earlier order than that was test was just a on/off test, rather than a continuous test (although the failure occurs rapidly around the 3-5s mark). No oscilloscope was hooked up but the current at failure read 8-10A on the PSU.

Before the separation test, I ran the system at full voltage (~32V) for several minutes with no failure whatsoever, but the secondary ground was hooked up to the DC ground as I normally do when I want to get the secondary electrode to shoot plasma (which is the goal of that setup). But I wanted to have a remote idea of voltage so I did that separation test, and I now I'd like to understand the failure. I wish I had a video recording. As opposed to a normal jacob's ladder, this experiment ran a very short period (1 to 3 Hz I would say), so perhaps this created unusual stress on the circuit.

The mosfets being largely oversized did not seem to get damaged on either of the 3 back to back failures.

Any idea what might be happening? Would it be possible that my 1N4007 diodes are too slow?

edit: As it turns out, it was a diode speed issue. I replaced the 1N4007 by BYV26E and that fixed the issue immediately.
That allowed me to redo the separation test and confirmed the 22mm value. The transformer ran for a good 30-45 mn (not continuously) for various tests/experiments and no failure so far. So definitely some progress with this new design despite the poor initial results; removing that last chamber and more importantly submerging it in oil made really shine when compared apple to apple with the previous machined unit. I know I can improve it further, but the highest voltage I'll reach will definitely be on the UY30; Speaking of which, I've received them and with these results I should be able to implement these lessons to the bigger ferrite. If the 22mm value is correct, the output could be in the range of 60 kV, which seems a little high. I don't have a precise figure for the number of turns but I think at least 100 turns per chamber, but it could be upward of 150 turns. On the next one I'll use the counter for sure. But there might be something off, because 60kV and 2700 turns would yield over 22V/turn which seems a bit high.

[davekni]

Yes, 1N4007 diodes are much too slow for ZVS oscillators.  They are for line-frequency use.  There is a fast-recovery variant UF4007, and many other fast diodes that will work fine.

[zytra]

I think I edited my post as you were responding; Sorry about that.

Yes, too slow of a diode. That was it!
I'm a bit surprised the 1N4007 worked for a while but it's only that ladder test that made them fail, and really fast too. I wonder what in that test stressed the diode so much more? I mean, the 1N4007 never failed until today when it was running the smaller secondary (1600 turns) at the same input voltage. The only things that changed are the secondary with more turns (~2700+) and the test (before, the secondary ground was tied to the DC ground and the electrode was either discharging through a sharp breakout or through a brass ball under 1atm of argon).

That ladder test was a bit different than traditional ladders I built in the past; this one was short (limited by the radius of each ball) and because of that it would pulse with a much faster period, kind of like a spark gap.

[davekni]

The slow diodes may have been operating on the edge of failing before.  It doesn't take much change to move from almost to actually failing.

Concerning output voltage, I'd guess that high frequency (relative to line) might jump the sphere gap at slightly lower voltage, but wouldn't expect any large discrepancy.  Sphere roughness will allow sparks at lower voltage.  It may be that you are getting 50-60kV peak.  Running the core into saturation makes non-sinusoidal waveforms with higher peak voltage for a given RMS voltage.  You could add an additional winding of a turn or two around the core at the ground side of the HV bobbin.  That will allow scoping something closer to accurate volts/turn for the secondary including at least some of the effects of leakage inductance between primary and secondary.

[zytra]

That's very interesting - Are you suggesting some kind of feedback winding (i.e. a second secondary of some sort)? I could use some better estimates of voltage on the secondary.

[davekni]

I'm suggesting a winding for the sole purpose of connecting to your scope.  (The term "feedback" defines how a winding is used - to control oscillation.  The geometry of this winding might be identical.  If you want to name it, it could be called a "scope" winding or "monitor" winding or "measurement" winding.)  The goal is to get this winding as close to the secondary winding as possible without dielectric breakdown issues.  Another option would be to wind a turn around the first segment of the secondary using wire insulated for 2kV or whatever you are getting per segment.

BTW, even the labels "primary" and "secondary" refer only to the intended use.  Transformers are occasionally used in reverse of their design intent.

[zytra]

I appreciate your patience; I didn't realized the physical proximity to the secondary winding mattered; I might be able to get around with a couple of turns on the first chamber. If the goal is to mimic the diameter/shape of the secondary winding this is the way to go. The ferrite itself is not even round on the 2 joining sections; and I have quite a bit of resin thickness around the round section (over 4mm). I am probably close to 2000V per chamber, but I can dial that down easily by reducing the input voltage and allow for measurements.

[davekni]

One turn would be enough if that is easier.  Would allow wire with thicker insulation to fit in the first section.

Another option is to notch the inside diameter of the bobbin at the low-voltage end of the secondary.  That would allow a turn or two around the ferrite with the bobbin fitting over this small monitor winding.

Yes, the closer this monitor winding is to the secondary, the more accurately matched is the field through the secondary and through the monitor winding.  That way the voltage/turn of the monitor winding will more accurately match the voltage/turn of the secondary.  Magnetic field lines are not all inside the ferrite core.  Field lines extend outside the core, especially when the core is saturated or when leakage inductance is significant (coupling factor significantly below 1).

[zytra]

Thanks, perhaps on the next print I will use the first chamber specifically for monitoring. These "housings" (no idea what is the right technical term for that "blank" on which the wire is wound around) are symmetrical in my design and they shouldn't be. Well at least not for my plasma ball project since one end is grounded. That end doesn't need as much "flap thickness" as the voltage is not that high (as opposed to the other end/flap that sees the maximum voltage). So I could make that end close to ground much thinner for additional chambers or to host a monitor winding.

The same applies to the thickness of the material around the ferrite cylinder. That thickness could increase with chambers. Not sure if this could be leverage to improve the design much.

[davekni]

The terms I see most often are "bobbin" and "coil former".

I've seen asymmetric segmented bobbin designs on this forum.  Symmetric designs seem more common.  That could be for simplicity or to allow either end to be grounded.