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Plasmasonic II build

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indiana

Tue Jul 07 2009, 10:02PM

Registered Member #2009 Joined: Tue Mar 03 2009, 07:07PM
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Posts: 43

good news and bad news on the building front.

I did the homework on the capacitor change, and put a .001uf capacitor in place of c25 (previously 200pf)

Works great at 50 kHz now.

Proceeded to testing of the outputs only to find that the traces are jumping all over the place. They're jumping up and down on the voltage like mad. Not consistent at all. I went back and tested the testpoints for the low voltage side, and found the waveforms to look great still. On the high voltage side, bizarre garbage coming through even with no voltage applied from the variac.

I applied up to about 20V to see if it would change, just a higher amplitude of the same jittery traces. No heating at all on the sinks however.

Any thoughts? I was sure hoping to get this connected to my coil today.

HV Enthusiast

Wed Jul 08 2009, 07:38PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Hook a 100W lightbulb to the output of the bridge. Then using two probes (making sure not to exceed their rating), take a differential measurement of the output. DO NOT CONNECT A PROBE GROUND to the otuput, otherwise IT WILL blow-up the bridge / and /or probe. You should be able to see an output pulse waveform. Crank voltage up just enough so you don't exceed the probe's ratings.

Once you do this, then, you're ready to hook up your coil.

indiana

Thu Jul 09 2009, 02:57AM

Registered Member #2009 Joined: Tue Mar 03 2009, 07:07PM
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Posts: 43

I did follow the instructions in the instructions pdf. I didn't put a bulb in line, as the test without the bulb seemed to be giving me bizarre waveforms. The previous pic was of a differential reading, probe 1 and 2 connected to the outputs, grounds connected together, 2 set to invert, and the purple line is the add function.

I tried the same test as you suggested with the bulb in line, and found the same results. This time for clarity I only show the add function of the reading, turning off the 1 and 2 waves.

I have taken some slow shutter photos of the waveform now. It's the same effect I was experiencing before, but with a slower exposure so you can see more of the movement. It shifts rapidly between different baseline voltages, and apparently the phase is jittering too? Something to that effect. It doesn't seem to be consistent. There is a period that remains constant, but the voltage shifts up and down rapidly, and the phase jumps forward and back a bit.

Doesn't seem like a good pulse to drive a coil cleanly.

I have tried reading it several different ways with my oscope, and get the same results.

When I test the test points on the control side, the signal is as clean as you like. Only on the power side is is jittery.

First pic is two of about four readings that jump around in sequence. second is approximately the other two.

The third is a different exposure. Hopefully you can get the idea from the pics. Video would be better.

Any thoughts?

HV Enthusiast

Thu Jul 09 2009, 11:29AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Maybe its your oscilloscope and the way you have things connected / grounded????

indiana

Thu Jul 09 2009, 05:55PM

Registered Member #2009 Joined: Tue Mar 03 2009, 07:07PM
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Posts: 43

I went back and looked over everything, I had waves stacked on waves stacked on waves.

Why it was triggering so funny. The capacitor I put in for the tests had really long leads (hadn't soldered it in) and I think it was picking up signal for one. Fixed that and it cleaned up quite a bit. It's got me scratching my head for sure. Except for the capacitor change, everything is set up exactly the same, and now the waveform looks fine. This is only the third time I've used my new oscilloscope too... First digital scope I've owned. I'd like to guarantee there's no user error involved....

The spikes are still a bit funny. Seems like it will work great though. It's spiking up to about 250V with a 12V input. Higher than I expected.

Thanks a lot for the help guys. I'm going to hook up the coil tonight. Finally might get a show.

HV Enthusiast

Thu Jul 09 2009, 06:46PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

That looks much better, but yes, there some big spikes there. which is suspect could be the probes. Hard to say if they are real or not. Basically, looks like there is lots of inductance there.

Anyways, i would recommend the following:

1. Light bulb test. (Resitive load) The light won't be real bright, but will light up.

2. Measure EXACT resonant frequency of your primary/secondary/toroid. Set your oscillator to this exact frequency. This is your starting point.

3. Connect your coil.

4. Use a small flourescent bulb and lay next to primary. Start turning up variac very slowly, and use fluorescent bulb to indicate tuning point. (You won't get break out at these low voltqage levels, but the bulb will light) Adjust for maximum brightness.

5. Run at low voltages, check temperatures, etc...

6. Then when confident everything is working and tuned okay. Start turning up voltage until you get break-out. Note, you'll need to retune slightly as you increase voltage.

Hope this helps.

Also, make sure your bias points are set right on your TL494 as indicated in the instructions.

indiana

Thu Jul 09 2009, 09:24PM

Registered Member #2009 Joined: Tue Mar 03 2009, 07:07PM
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Posts: 43

I did that last test with the light bulb. I should have mentioned that.

The spikes show up in the control side wave forms as well. We mentioned them earlier in the thread. Makes sense that they'd show up on the other side too... I just wish they didn't.

The waveforms on your build don't look like that? I wonder what's different here.

I'll set up the coils tonight after work and see how it looks.

Herr Zapp

Thu Jul 09 2009, 10:31PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

indiana -

In your May 9 post, you mention:

"I've lifted the GDT's off the board with a little armature, just to make it easier to connect."

The small photo (why so small??) seems to show your GDTs raised up off the PCBA, and possibly sitting on a metal bracket of some sort. (It's hard to see, since the photo is so small.) What are your "armatures" made out of? If your GDTs are actually resting against a metallic bracket, I'd remove the brackets and re-route and shorten the GDT leads to keep them as short and direct as possible.

Typically, the GDTs on Plasmasonic boards have been mounted with their axes horizontal, not vertical, and with the GDT resting directly on the PCBA surface.

Regards,
Herr Zapp

indiana

Fri Jul 10 2009, 02:07AM

Registered Member #2009 Joined: Tue Mar 03 2009, 07:07PM
Location:
Posts: 43

You know Herr Zapp, I was thinking the same thing when I first noticed the ringing. I unstrapped the GDT's from the bracket and removed them, pulling them as far away from the armature as I could.

I didn't notice any change in the waveform or I would have changed them out right away.

But in looking through the whole build, it's the only thing that would be causing the problem.

I'm going to remove it and see what happens. Maybe the distance I was able to move away from the armature wasn't enough to get it out of the GDT's field.

Put it on there as I was having a hell of a time getting the GDT's mounted to the board.

Looks like I'm going to have to tousle with it again. Grumble grumble. ;)

EDIT: I've removed the armature, and the pulses look so clean you could put them in a dishwasher ad. Thank you for that much needed kick in the ass Herr Zapp. I'm very happy with these results. Next step, strap it onto the coil! Woo!

indiana

Fri Jul 10 2009, 06:37AM

Registered Member #2009 Joined: Tue Mar 03 2009, 07:07PM
Location:
Posts: 43

Ratten Fratten gal dern....

Well, the control side signals look neatened up a bit... but the power side looks exactly the same! Spikes and all. Spoke too soon! lol

Ah well, the only issue is the voltage spikes.

With no power applied to the bridge there is a signal coming through still. A couple photos below. Is this normal?

Just fishing around for some clues. It will be interesting to see what happens when it's attached to the coil. The driver feels very cool up to about 400V outup, which is all my scope can handle. I'll run it up higher voltage and keep testing for heat with the coil. Might be able to get to it tomorrow.

The old juggling act.

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