ZVS driven Plasma Ball (w/ segmented PTFE Secondary)

[zytra]

I added a sharp wire, closed the globe and refilled with argon.
Here's a oscilloscope video showing a voltage ramp from 12 to 30V. Sorry the potentiometer on that PSU is a bit sticky, so that ramp wasn't super gradual. Hopefully it seems like it starts showing signs of saturation around 75V on the drains (~ 24V input). Perhaps it's time to start adding gaps.

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Does the zero crossing look normal to you though?


about the parasitic inductance, no doubt - right now, wiring' isn't cleaned up at all as I was figuring out capacitor size and such.

[davekni]

Zero-crossing of what?  This is a ZVS.  Switching occurs at zero voltage, which is close to current maximum.

Yes, that's when I see high-frequency oscillation bursts - when clamping on different caps to experiment.  Is this what you mean by the zero-crossing looking questionable - the burst of high-frequency after each zero-voltage point?

[zytra]

Yes, those bursts make it difficult to see what's going on with the switching, but they're likely a consequence of the switching, not the other way around, so I wont' worry too much about them especially as I intend to clean this up a bit.

Too bad I couldn't get those shots yesterday before the failure. The output is significantly lower than yesterday, I really need to find a way to add turns reliably. And of course I need a suitable enclosure that I can pull vacuum from. I wonder if the later alone would be enough to make those discharges reach the inner surface of the globe.

With the reduced output, adding an interrupter doesn't feel as urgent.



edit: I'm attaching a plot of the center tap line in channel 4. the magnitude of the current in that wire is just plain small compared to what's going on in the other legs. That would seem to be the logical explanation for the thermal image recorded a couple days ago, wouldn't it? The average current on channel 4 roughly equates the numerical value displayed by the PSU (3A), and the currents in the 2 legs although with very large peak to peak value mostly cancel out and equate what's measured in that center tap line. The fet's are switching quite the current on that small circuit, I'm glad I went a bit overkill with those.

[davekni]

Yes, outer wire current is much higher, as I'd said in reply #7.

The FETs are not conducting that high current, only the center wire current.  The high outer lead currents are conducted from the transformer to the capacitor, not through the FETs.

Definitely simulate any interrupter design before constructing.  You will need some way to handle energy stored in the supply inductor at turn-off.

At higher frequency (ie. gapped core) you can get more volts/turn.  However, secondary intra-winding capacitance becomes more problematic as frequency increases.  Your UY30 ferrite will allow higher volts/turn at the same frequency.

[klugesmith]

Good detecting.

Meanwhile I couldn't stop doing the skin effect exercise.
Guessing a frequency of 100 kHz, found that an AC ampere in 14 AWG wire
produces almost as much heating power as a DC ampere in 18 AWG wire.

When your outside wires appeared bright in the thermal infrared image,
were they perceptibly warm to the touch?

[zytra]

Yes, outer wire current is much higher, as I'd said in reply #7.

The FETs are not conducting that high current, only the center wire current.  The high outer lead currents are conducted from the transformer to the capacitor, not through the FETs.

Definitely simulate any interrupter design before constructing.  You will need some way to handle energy stored in the supply inductor at turn-off.

At higher frequency (ie. gapped core) you can get more volts/turn.  However, secondary intra-winding capacitance becomes more problematic as frequency increases.  Your UY30 ferrite will allow higher volts/turn at the same frequency.

Yes you called it it really took the waveforms for me to understand it.
I'd like to keep running with this type of frequencies, so the UY30 ferrite will be welcome to continue pushing on this project.
The secondary wound with 100 turns per chamber has not failed, and none of the early signs of failure have appeared yet (bubbles forming in the secondary).

I installed LTSpice last week and I have to spend more time on it. I was trying to simulate the ZVS on circuit lab last night and couldn't; I tried a bunch of things to create a slight unbalance initially to allow the system to start oscillating but none worked, and it seems like I have the same issue with LTSpice; however with LTSpice if I try for example to change the value of the 470 ohm to try to generate an imbalance, the simulation converges immediately (as opposed to crunching for a long time with tiny time steps). edit: the option "skip initial operating point solution" fixed the problem - there must be some other problems still as it doesn't seem to oscillate correctly

Good detecting.

Meanwhile I couldn't stop doing the skin effect exercise.
Guessing a frequency of 100 kHz, found that an AC ampere in 14 AWG wire
produces almost as much heating power as a DC ampere in 18 AWG wire.

When your outside wires appeared bright in the thermal infrared image,
were they perceptibly warm to the touch?

Yes, the image was taken after one or two minute only which was more than enough to easily see on the thermal imager the outer wires were hotter. To the touch, you could tell they were warmer but after 10 minutes or so they were hot, while the center tap wire was not even warm.

[davekni]

That LTSpice result surprised me enough that I tried it myself.  With ideal inductors, that does look like a valid result - an oscillation based on the supply inductor L4.  Adding 0.2 ohms of series resistance to L4 damps that mode, revealing the intended oscillation.

Inductors are typically farther from ideal than are resistors and capacitors.  Accurate simulation often requires adding appropriate series and parallel resistances.  Those can be added as explicit resistors on the schematic, or as additional parameters of the inductor.  Right-clicking on an inductor brings up a parameter table.

Leakage inductance in your transformer will eventually be important to model too.  You will need three separate coupling factors to be precise.  The two primary halves are likely a bit more coupled than is primary-to-secondary.  For simplicity, you could start by making the one common coupling factor a bit lower, perhaps 0.95.  (Coupling and inductance both drop as the core saturates, but that is not easily modeled.)

[zytra]

Awesome thank you, I'm glad I wasn't that far off and I actually thought about ideal values but since I noticed the default series resistance for L's were 1 milliohm I assumed it was taken into account and that it was "good enough". Lesson learned, next time I'll play with those values regardless.

I'll use this simulation to do a parametric study on the C1 capacitance. I already noticed that something is happening somewhere between 4.7u and 2.2u. Some other unintended oscillation perhaps.



On a side note, I put together a very crude test with a glass bottle and a couple of fittings. I didn't use argon because I don't have a great seal, but it's good enough to bring the pressure down enough (no gauge right now) to see the real time effect of pressure on the discharge. When I have a gauge hooked up I'll make a quick video as it's pretty interesting.

[davekni]

Yes, it is fun to play with pressure, especially with argon.  My little one (8kV peak) when pumped down causes glow to extend through the evacuation tube.

[zytra]

Oh and I understand what you meant by dealing with the energy store in L4.
I added the most basic interrupter I could think of and here's a with / without side by side comparison.

Edit: Couldn't find the right fitting for the vacuum gauge, but I did capture a video showing power draw as vacuum is pulled.
As opposed to the previous transformers (2250 and 3000+ turns secondary) this one (1600 turns) won't allow discharge to take place at 1atm unless there is a sharp breakout point on the electrode. This video, I only have the smooth brass ball and it will only start discharging once the pressure is low enough.

[zytra]

I wouldn't call that progress, but I added a P-channel mosfet to turn off the inductor and the primary through a interrupter signal. I used a N-channel mosfet off the signal generator but a NPN transistor should do as long as it was withstand 30V.

That didn't fix the oscillation still taking place in the transformer though. What's surprising to me is that outside of the first 5ms after the signal generator goes low, there is no damping at all and for the following ~45ms it's still oscillating with no damping.

I checked that the capacitor and L1/L2 all had series resistors to make sure energy had a way out (was mainly looking for an explanation for the lack of damping.

Edit: I'll plot the current coming out of the power source. I think this is likely where the energy is coming from, although the switch open, current can still flow through the 470 ohm resistor looping through the ZVS mosfets which will continue opening/closing regardless of the signal generator. If this is what's going on, perhaps it's time to power the gates with 12V and a lower R, that won't prevent current from still flowing through the primary but at least it should be significantly less. And even then, that oscillation is not really a problem.

edit: Yes, that was it. Feeding the 2x 470 ohm resistors from the P-Channel drain fixed the issue.

[davekni]

Move the bottom (anode) of D5 to ground and you will be getting close.  Switching the main supply (current through L4) is the best option I know of, even though it requires an additional high-current switching device.  (Look at the voltage across L4 in your previous un-switched circuits.  Ignoring series resistance, the voltage across any inductor must average 0.  Placing a diode across an inductor shorts out one polarity, which will build up high current until the opposite polarity has a correspondingly low voltage-time integral.)

Yes, gate resistors provide enough power to keep oscillation running at low level.  That has a key advantage:  Makes startup of full-power oscillation faster, minimizing the startup current spike in L4 and following voltage spike on the FETs.  So, I suggest not "fixing" that unless you really need it to turn completely off.

Once mostly working, go through the circuit looking at device voltages and currents, comparing them to allowed values in device specifications.  LTSpice models usually don't show you that a 0.1A diode is really running at 100A automatically.  You need to look.  (There are tools that will examine Spice output to check for device parameter violations, but I've seen those only for specific semiconductor processes.  I've used them in designing analog integrated circuits, but not for board design.)

[zytra]

Thanks Dave, I'll try simulate with the anode to ground.

I'm actually thinking I'll add a switch to use/bypass the interrupter. The interrupter on that circuit for a plasma ball is more of an experiment than anything. I have not seen anyone tried that and it's probably for the good reason that it won't create any nice effect; at best it will probably kill the tendrils prematurely. It won't hurt to try.

On the other end, the adjustable power supply combined with the gates being powered by a 12V Buck through 100 ohm's should give me the power adjustment I am looking for.

Thanks for the tip on checking all voltages and current; I had a hint that it probably wouldn't give give warnings on simulations with errors where the results happily reported thousands of amps in the primary with no warnings, as they say GIGO

[zytra]

Dave: Would you have any advice in sizing that diode (anode to GND)? Also I was wondering if with the anode to ground it would make sense to use a TVS instead.

I am separating the interrupter circuit with a rocker switch so I can run un-interrupted. The gates are powered by a 12V buck and the main power input is DC 12-60V or so; So technically, when running with the switch on the interrupter position, the voltage supplying the inductor can range from 24 to 60V, and this is giving me a hard time picking the right diode...

[davekni]

I happen to know someone in Germany designing a similar circuit at higher voltage (rectified 220V line) for an induction cooking appliance.  He is using a FET in series with the supply inductor both for on/off and as a buck regulator to adjust average power.  If you notice, that FET, diode to ground, and inductor is exactly buck-converter topology.  Just in case you ever want to switch the FET faster (ie. >20kHz) as a buck converter, I suggest a fast diode.  Schottky would be best, especially at your relatively-low voltage.  A fast-recovery diode (ie. <=75ns reverse recovery time) would also work.  If you are interrupting at lower frequency, so the inductor current ramps to zero before the next enable pulse, then diode speed isn't important.  Even a TVS would work for such slow operation, but no reason to use a TVS diode.

If interrupting infrequently, then the diode's peak current is what matters.  Even a 1A schottky such as DSS110 or NTE642 (2A) would be fine.  If interrupting rapidly for buck-converter use, then the diode's average current needs to be over half our max DC supply current, higher to be safe.  For 5A parts, perhaps SK520B or MBRD5100 or VSSAF510.  There are literally hundreds of options.  Obviously you need >60V, so at least 80V to have margin.

Edit:  Rather than specific part numbers, here's what I did to find them:  For part searching, I find digikey.com most useful.  Often not lowest cost, but best parametric search capability I've found.  (Would love other suggestions, as "best" is far from ideal.)  Select "Discrete" under "Products", then select "Diodes-Rectifiers-Single".  In the menus, pick "Schottky".  Highlight all currents from 5A and up.  Highlight all voltages above 60V.  Select "Active" status unless you want obsolete parts included.  After "applying", sort by price (or whatever parameter you want).  Price sorting is more useful if you also enter quantity.  The cheapest ones were surface-mount, so that is what I listed above.  You can include "Through Hole" in the menu selections if you want only those package types.

To find the lowest price (and availability) in normal distribution, enter the specific part number (or at least initial part of it) in oemstrade.com.

[zytra]

Thanks Dave, 5A should be fine for what I'm trying to do; after all the goal would be to pull as little current as possible.

[zytra]

Quick update, I started playing with 3D printed resin (LCD printer) and it looks pretty promising. I found a resin with good dielectric properties and I'm currently fine tuning settings to get my design to print correctly. The cured resin doesn't seem to be less rigid than PTFE I used on the current working transformer. And as far as my design is concerned the angled cutoff joining two adjacent chambers is getting printed without trouble. I am pretty hopeful that this design will allow greater output voltage, based on design parameters and my experience with the PTFE housing, I should be able to reach 3000 turns which almost double the current working secondary.

I'll post an update soon, I have not yet finished making all the parts for the 370mm plasma globe, so I'll likely start testing the SSTC getting converted to DRSSTC before I can test the 370mm plasma globe.

[zytra]

Well bad news, I performed a quick test on one of those printed housing wound with about 100 turns per chamber (28 chambers) with 9 turns (2x 4.5) on the primary and I experienced corona through the the final shoulder which is over 4mm thick.

This is not submerged in oil, so the oil would likely help, but this results a bit inconsistent with the 15kV/mm dielectric strength I was given for this resin; that statement could be off and is purely based off experimenting with PTFE which I thought had similar dielectric breakdown.

It appeared at the very end so I will probably try to unwind the last chamber and test again. At that the point the weakest link will be the first to last chamber but this time on the ID of the winding. I'll continue unwinding chambers one at a time to get somewhat of an approximation of the actual dielectric strength. From there I'll be able to tell if that concept is worth pursuing. Perhaps, I'll try to find a better resin too.

Rough estimation from the LTSpice model, I should not be much higher than 15 kV (I was using 12VDC input). So I have a feeling that the resin has a significantly lower dielectric breakdown than my PTFE stock.

It's a bit disappointing because the mechanical design of that secondary housing worked as expected. Last improvements on the groove joining two adjacent chambers helped a lot, the wire sits fully recessed in the groove as I wanted and changing the angle of the groove to make it land tangent to the winding cylinder no longer makes the wire want to come out of the groove; it now stays in here perfectly. It tool me half a dozen tries.

I think Formlabs has higher end resin (which I am not sure are compatible with this LCD printer) but I doubt that they will cure at the wavelength of the LCD printer.
Here's an interesting doc: https://drive.google.com/file/d/1UdcHkNFkwhVlOhqBHQZhIcoYr1_m5VnL/view and I think I am really far from the numbers on slide 17

edit: removing the last chamber helped, but there is still somewhat of a corona forming in the same general area, hard to tell why precisely. The resin is semi-clear and it glows under corona making it difficult to pinpoint its origin(s). I am building some sort of spark gap with 2 large sphere to try to estimate the voltage roughly.

[davekni]

Dielectric breakdown through solids is a complex phenomenon.  Dielectric strength is commonly stated as voltage/distance (ie. kV/mm).  That is valid at only one thickness.  Breakdown voltage is closer to a sqrt(distance) function than to a linear function.  Thin films can withstand much higher field than can thicker films.  That is why HV film capacitors are internally multiple series layers of dielectric and aluminum (rather than a single thicker dielectric layer).  Check the thickness used to define 15kV/mm for that resin.

Is the resin specification for DC or AC?  AC dielectric strength is generally lower than DC.

Just as with Tesla-coil breakout points, field is higher at edges and corners, such as at the top or bottom of a wound segment.  The field at these corners is higher than just dividing total voltage by total insulator thickness.

Finally, does this resin specify strength with some ideal curing conditions?  I have little experience with such, but could imagine that dielectric strength changes based on cure light intensity, build layer thickness, humidity, air entrapment, etc.

Yes, I agree that performance will be better in oil.  Good luck!  Thank you for keeping us informed.

[zytra]

Thank you Dave, it took some time before I realized the non linearity, that I would expect it is linear but the dielectric strength be having a Voltage/Distance unit initially led me to believe that it was (linear) when it's obviously not.

I used a x2 multiplier when I put this design together (so if I x2 voltage, I would x4 the thickness).

LTSpice is a good tool for early estimation, but I think it would be great if I could at least estimate voltage just to validate my model. My HV probe is limited to 10kV so that won't work. I did connect the secondary to a spark gap (made of two 1.5" brass balls), and the longest arc I could create off these 2 smooth balls was 13mm, at 20VDC. That was done with the amazon ZVS with an additional capacitor (4.7uF) across the primary. I am not sure this is a good test, I need to do more research.


The information on the resin is limited but it was measured under D149-97a which suggests 60Hz I think.