Help with Class E plasma speaker Eastern Voltage Research

[davekni]

I spaced out the turns the best I could, PTFE is some slippery stuff! It’s more of a very loosely wound primary than evenly spaced. Here is a scope shot. I see that every other cycle, the amplitude is much lower?

Normal operating voltage is 85v when using audio. When tuning (adjusting the secondary top lead length) it’s supposed to be set to 100v.

Yes, PTFE is slippery.  Great for low loss at high frequency, though that isn't a significant concern for primary at 4MHz.  Would matter a bit more for secondary's higher voltage, but still not that significant at 4MHz for a single-layer coil.

It's that every-other-cycle behavior that needs JaveTC and simulation to understand.  Coil is locking to 2MHz sub-harmonic.  Since you are increasing primary frequency (lowering primary inductance), I didn't expect such behavior.  Must be jumping from upper pole to lower pole.  JavaTC will help understand where the two poles are, and simulation should show what may cause running at one or the other.  Goal is to get 4MHz operation with drain waveform curving all the way back to zero volts within each 100ns gate low period.  That is the way class-E is supposed to work, with no turn-on power loss in the FET.

If you want to see a drain waveform, look at the green traces in this post of my 13.56MHz class-E coil.  Gate drive is sine-wave (resonant), black trace.
https://highvoltageforum.net/index.php?topic=1638.msg12549#msg12549
Of course, there are parasitic inductance rings here too, even more pronounced at this higher frequency, though lower amplitude since running properly in class-E mode.

[FonziDaytona]

One thing that came to mind that I forgot about…L1, the 25T choke, is still part of the circuit. Should it be bypassed for these low voltage, no audio, tests?

-Matt

[davekni]

One thing that came to mind that I forgot about…L1, the 25T choke, is still part of the circuit. Should it be bypassed for these low voltage, no audio, tests?

No, bypassing 25T choke would present a high-frequency short to the drain, preventing operation and likely frying the FET.
Does remind me of a question:  Did the core come from EVR?  Can you tell the coating colors?  Would be worth verifying the core is the correct type and size.  (Unless you have a meter to read its inductance.)  If the core is incorrect, that could be the cause of issues.

[FonziDaytona]

The core came from EVR, it is red and looks like it matches up with Micrometals example.

I have been playing around and noticed a few things. One, if I keep the windings tight, and stay under 20%, the frequency stays steady at 4Mhz. If I add a little spacing to the primary it switches back and forth (2Mhz and 4Mhz). I have been playing around with JavaTC and see that the secondary resonant frequency is around 1.7Mhz…Shouldn’t it have been speced to give closer to 4Mhz? Also, I see that I got my secondary coil dimensions off a little. It should have been a winding length of 3.675” and I wound it to 3.375”. My eyes got the better of me…

Here’s a scope shot of the 3 winding primary, no spacing, at under 20% voltage input.

[davekni]

I have been playing around and noticed a few things. One, if I keep the windings tight, and stay under 20%, the frequency stays steady at 4Mhz. If I add a little spacing to the primary it switches back and forth (2Mhz and 4Mhz).

4MHz is desirable, but the resulting sawtooth waveform is not desirable.

I have been playing around with JavaTC and see that the secondary resonant frequency is around 1.7Mhz…Shouldn’t it have been speced to give closer to 4Mhz?

1.7MHz might explain the 2MHz issue.  However, that seems too low for the coil geometry.  Wondering if some JavaTC entries don't correctly match your secondary.

lso, I see that I got my secondary coil dimensions off a little. It should have been a winding length of 3.675” and I wound it to 3.375”. My eyes got the better of me…

That would imply slightly fewer turns, which would raise frequency.  Not sure if this is enough to be significant or not.  I didn't see a turn count specified in EVR schematic.

Here’s a scope shot of the 3 winding primary, no spacing, at under 20% voltage input.

Not what I'd expected.  Just makes it clear that I'm not being too useful, not until I get a simulation running.

[FonziDaytona]

This looks (I think) like I nailed it! I had to lower the primary to below the secondary bottom winding, so very low coupling, right?

[Mads Barnkob]

This looks (I think) like I nailed it! I had to lower the primary to below the secondary bottom winding, so very low coupling, right?

You can calculate the coupling with JavaTC: http://www.classictesla.com/java/javatc/javatc.html

[FonziDaytona]

JavaTC is suggesting moving the primary even further down (can’t go any lower really). Judging by the scope shot, it looks a lot like Class E operation, no?

What should be my next step? Pump up the voltage (more power) or should I wind a 4-5 turn primary and keep the same coupling?

I’ve noticed that corona is now breaking out. No affect on waveforms unless I draw an arc.

-Matt

[davekni]

This looks (I think) like I nailed it! I had to lower the primary to below the secondary bottom winding, so very low coupling, right?

Yes, that is a great class-E waveform.

JavaTC is suggesting moving the primary even further down (can’t go any lower really). Judging by the scope shot, it looks a lot like Class E operation, no?

What should be my next step? Pump up the voltage (more power) or should I wind a 4-5 turn primary and keep the same coupling?

Is the scoped waveform with your 3-turn primary?

As you increase voltage, drain capacitance drops, so optimum at high voltage will likely be slightly back towards your previous "bad" waveforms.  Perhaps slowly raise voltage and slowly increase coupling to see if you can maintain class-E as voltage increases.

Edit:  Thinking through all the experiments so far, perhaps the 3.375" vs 3.657" secondary winding height difference is important.  Secondary appears to be fairly high Q.  With fixed 4MHz drive, a shift of ~5% in secondary resonant frequency may be the cause of issues.  May make sense to add the 8-10 turns needed to reach 3.675" before trying to tweak coupling and primary coil parameters more.

[FonziDaytona]

That’s with the 3 turn primary with close spacing. I turned it up to 50% and here’s the scope shot. Looks like the gate signal is deteriorating as I go up.

I’ll see if I can cleanly splice in some more wire and wind on some new windings. If that fails, I’ll rewind the coil with some Essex 22awg ultrashield that I have.

Edit: Wound on another 10 turns of wire. No change in waveforms.



-Matt

[davekni]

That’s with the 3 turn primary with close spacing. I turned it up to 50% and here’s the scope shot. Looks like the gate signal is deteriorating as I go up.

Looks like primary resonant frequency is still just slightly low.  Drain voltage is dropping to around 30-50V before gate turns on, not quite all the way to zero.  That is likely why gate waveform gets worse.  I'm not sure whether to worry about the gate waveform directly.  FET power dissipation is likely low enough at 30-50V turn on to avoid frying.  (Had been ~250V in your early plots.)

Edit: Wound on another 10 turns of wire. No change in waveforms.

Interesting.  Thank you for the update.  Perhaps I can figure out why eventually with simulation.

[FonziDaytona]

I added the little loop that EVR recommends and this allowed me to raise coupling before getting the distorted waveform. Here is the latest run at 50%. Looks a little improved over the last one.

[davekni]

I added the little loop that EVR recommends and this allowed me to raise coupling before getting the distorted waveform. Here is the latest run at 50%. Looks a little improved over the last one.

Yes, looks like clean class-E operation now!  Hope you manage to keep that up to 80V operating point.

[FonziDaytona]

Took it up to 85v in (the EVR recommended setting for audio modulation mode) and it still looks good to me. I zoomed in to show the switching points. Looks like MOSFET turns on when voltage is near zero volts.

If this looks pretty good, I’ll install the audio modulation MOSFETs (Q8) tomorrow to see if everything is still looking good before trying some audio again.

[davekni]

Took it up to 85v in (the EVR recommended setting for audio modulation mode) and it still looks good to me. I zoomed in to show the switching points. Looks like MOSFET turns on when voltage is near zero volts.

If this looks pretty good, I’ll install the audio modulation MOSFETs (Q8) tomorrow to see if everything is still looking good before trying some audio again.

Yes, looks good.  Zoomed-in capture is nice, showing detail better.  Are you getting a reasonable plasma flame with this configuration and 85V?

[FonziDaytona]

It’s putting out a very bushy 1” discharge. Turns out it’s only 49v, I have another 36v to go till I get to the EVR recommend level. I may have to increase primary turns as I’m blowing my 5A fuse. At least I’m not blowing MOSFETs anymore! I’ll report back tonight.

-Matt

[FonziDaytona]

Here is the Tesla coil running audio again. You’ll have to excuse the teddy bear and the glass of whiskey, they are for emotional support!

https://youtube.com/shorts/6GjAaDhPUnA?feature=share

[davekni]

I may have to increase primary turns as I’m blowing my 5A fuse.

Great to see it working.  Increasing primary turns may lead to the initial issue of drain voltage not returning to zero before gate voltage turns on.  Not certain.  Your added 10 secondary turns might have fixed that.  If added primary turns are necessary to prevent fuse blowing, you might need to reduce the value of the 500pF resonant capacitor to compensate.

[FonziDaytona]

Looks like the blown fuse was just a fluke. I replaced it and haven’t blown any others, so I’ll keep experimenting before making any changes. So far, so good! I appreciate all of the help and will follow up later on.

-Matt

[FonziDaytona]

Figured I was due for a follow up:

No more blown MOSFETs! I tried going up to 4 turns on the primary but couldn’t keep the Class E operation waveforms. I rewound with some much nicer Essex ultragrade wire (to the correct dimensions) and still found that 3 turn seems to be the sweet spot.

I can push up to over 3 amps and 70v rms but my 5A fuse blows after awhile. Audio is very nice and arcs/sparks/discharge is very bushy and hot.

It’s too bad that EVR didn’t suggest going down to a 3 turn primary, using a variac, and watching the waveforms. It would have saved me $100 in MOSFETs.

Thanks to you guys (Dave especially), I’m rocking and rolling! Yanni really shows off through this thing.

-Matt