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Topics - zytra

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Hi,

I am creating a new thread before splitting off too deep from the original thread (link: https://highvoltageforum.net/index.php?topic=1374.msg10493#new )

I'll re-post my last message (and David's reply) as it gave a fairly detailed description:

Quote

The transformer is fully custom, I'll post pictures later tonight. It's made of 2x "C" shape ferrite bolted together. The secondary is roughly 26mm ID x 50 mm long (that includes 15 separations that are about 2mm thick). The core has an OD of 20mm, meaning 3mm of insulating PTFE. The segmented secondary was turned on a piece of PTFE. The core is not gapped.

For the primary I have 2x 2.5 turns, which is a bit on the small side which could explain the lack of success with the ZVS driver.

After running the transformer on the SSTC driver, I decided to make my own ZVS driver. And since I ended up using roughly the same components, results were the same outside of the capacitor that was film but with high ESR, it ended up heating a lot so I replaced it with a better quality capacitor with significantly lower ESR but also slightly higher capacitance. Results were immediately better, and after playing with the few caps I had available, found that 4.7 uF was giving me the best results. Note: I did go back to the commercial ZVS and put the same 4.7uF cap across the far ends taps of the primary and results were much better, but as good as mine oddly - perhaps due to the other caps still being there. I say "caps" because that driver used 2 for some reason. The ZVS circuit I am familiar with use 1 cap across the far ends of the primary, and this one uses two. I looked at the PCB and it looks like they use 2x ~0.330F but I am not sure how it is all wired together.

I proceeded with assembly of my 200mm plasma ball, flushed it with argon at 1atm and results were great. Voltage was probably a bit high because I can't run a vacuum on that cheap plastic globe (that has a flat bottom - I tried by the way, and it's not a good idea, haha). I used a grounded striker to test contact and it would definitely shock you as opposed to normal plasma ball.

The goal is to make a bigger one (glass, round, under vacuum) so I don't think I will dial down on the voltage yet, just to make sure I have enough for the larger one down the road.


I did experience a set back with a failure of the secondary. The last chamber, last few turns (hard to tell exactly) arc'ed through the PTFE to the ferrite. Not through the thickness which was 3mm but through the last separation which was only 2mm in my design for some reason.

I've made revisions to the design and will turn it this week, increased the thicknesses separating the windings from the ferrite to a total of 4mm. I had to reduce the thickness of chamber to chamber separation to compensate, but I think this will be fine, as I don't expect any arc to get through 1mm PTFE on two adjacent chamber. If it took 16 chambers to go through 2mm or so on the HV side where the potential difference is maximum, I think I should be fine with 1mm between 2 adjacent chamber.

Another setback today after rewinding a spare V1 core. I wound this one with only 2 differences: I didn't fill the last chamber (HV side), and I tried to compensate by adding 1mm more thickness on all of the other chambers. That one failed with an arc within one chamber, along the surface of the PTFE which darkened, but no penetration. I think what happened is that my layering wasn't great on this way, I kinda rushed it last night, the added layers didn't help as it increased the voltage difference within one chamber.


With all tests I found that I was lacking a direct voltage measurement on the HV side. I actually was able to use a HV probe on my early tests (that didn't work well) and measured 8kV. After tweaking the capacitor on my own ZVS I was able to get intense plasma at ambient air and discharges over 2" long, so there is no doubt that my 10kV probe wouldn't be able to work there.

And here's David's reply:

Quote
It would be interesting to see waveforms for this - primary voltage and secondary with just antenna pickup (reasonable phase, just no amplitude calibration).  ZVS drivers usually aren't used with ungapped inductors.  Inductance is high and saturation current low.  I wonder if your system is running at a frequency determined by leakage inductance and the primary and secondary (intrawinding) capacitance.

Besides the HV end failure, the common tricky problem with segmented bobbins is the wire transition from the top of one segment to the bottom of the next.  Unless there are slots or other accommodations in the separation walls, that wire to the bottom ends up adjacent every layer (including the top) of the new segment as it is wound.  That leaves a full segment's voltage across two thicknesses of enamel insulation.

BTW, the two 0.33uF (1200Vdc, 630Vac) caps in the commercial ZVS are almost certainly in parallel.  They are standard Chinese induction cooktop capacitors, the ones I use for my DRSSTC MMC.  Some ZVS units have 6 or 8 in parallel.

And my response:

- The driver I am using is this one: https://www.amazon.com/gp/product/B07BNZ5HC1/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

- The first HV failure was definitely a hole through the PTFE, extremely small. The second HV failure (today's failure) was exactly as you described. The wire coming from the previous chamber arc'ed with another one from the new chamber. This is a typical difficult to address mechanically. I initially tried to add a tiny hole at an angle through the separation, which was just too difficult to do without at least a 4th axis. Then I thought about just using a groove. I figured that oil fill up that groove, while this could work I think I haven't been able to machine this, it is entirely possible I just need time to actually setup in the CNC machine one of my spares, or even start with one of the damaged ones.

- I haven't hooked up the oscilloscope for relevant tracing yet, but I will do that on my next try. On one early try with the HV passive probe I measured 8kV and about 50kHz.



A few more things:


As you know me by now, I am not familiar with the technical jargon and I tend to describe things at length when in fact there is usually a simple technical term for it. I'm sorry about that; so you'll judge by my pictures but I'm pretty sure my ferrite core isn't gapped, just two "C's" forming a "O" with a centered-tap primary on one cylinder and a 3000 turns or so secondary. Secondary inductance was measured to several dozen henry, and its DC resistance to ~ 370 ohm.

I have very little footage with that first secondary design, I should have recorded more, it actually worked for a few hours before failing, it actually only failed because I hooked up a larger PSU (I was using a 60V capable PSU but limited to ~3A, so voltage input was around 13V @ 3.2A) and it failed around 17V and 10A or so. So here's the only video I have, sadly with all lights on. It doesn't show it "starting up", which at times can be tricky (I need to use a switch and perhaps use a large bulk cap). It worked pretty well overall and without the need for a sharp breakout point. Also the video was taken as I was experimenting with the Amazon ZVS with the 4.7uF which I found to be optimal with my home made ZVS. So the video shows a little bit less performance than I had with the home made ZVS.

/>
Specs:
- roughly 3000 turns of 34AWG enamel wire
- U shape cores I used: link
- a (unnecessarily) large oil container (I only had 1ft tubing and nothing to make a cut straight enough for a good seal)
- all parts for the container and globe base/globe seal were CNC'ed
- the globe itself is a plastic flat neck (not sure really the material) and sadly a flat bottom (not vacuum/pressure happy) - from amazon. Took this one for its flat neck, which would allow me to design/make a simple seal. The goal is use a 350mm glass globe eventually, that will of course not fail under vacuum.
- the globe base a bunch of fittings for gas fill up and vacuum (I can bring the pressure down by 0.1-0.15 bar safely but that's enough to see a significant improvement on the arcs/plasma).
- 1.5" Brass ball


What's next:

- Short term: rewind yet another secondary, this time I'll add one layer of kapton tape to stick each new wire coming in a new chamber against that separation and keep it there, all the way down. I'll wind a bit slower too, to ensure I layer those turns as evenly as possible. I'll also try to not wind all the way to the top like I did on my first attempt which was overall much better considering it failed at much higher voltage and with a failure mode that can be completely avoided by skipping that last chamber (like I did on the second attempt).
- I am making an improved PTFE core, with thicker ends, and thicker body around the ferrite.
- I am redesigning everything else as well, larger diameter container but also much shorter. I'll also get the HV ground to come down from the bottom of the container rather than the top. I am designing this around a much bigger 20L boiling flask (that was the initial idea), and will be using vacuum valves (2 of them) for fill up and vacuum. I am also going to be using a large test tube (25mm diameter, only those are long enough to reach the center of the flask) as HV electrode support. All of those 3 tubes will go through the rubber cap of the boiling flask.
- I'll probably stick with my home made ZVS driver, unless I can figure out a way to use my SSTC driver (Full bridge, TO247 IGBT's). But that would only possible if I can get anywhere close to soft switching. And if I do stick with the ZVS driver, I need to research more what David mentioned about ZVS driver working better with gapped transformer which is not my case.
- I'd also like to add some kind of interrupter to the system (which would be trivial if I used the SSTC driver).

Pictures:

- Secondary Rev1.0 before failure
- Secondary Rev1.0 after failure: the black area has a pinhole pierced by the arc that went from the HV'most enamel wire through the PTFE to the ferrite
- Secondary Rev1.1: freshly wound secondary, this time with the last chamber skipped. Notice how, out of greed I added more turns to compensate for one less chamber? haha
- Driver pictures: I CNC'ed single sided Clad (4 or 5 oz I think). I used SOT227 Mosfets mainly for practical reasons; also thought I'd easier to kill them, especially considering I'm using current limited PSU's. I need to upgrade the 470 ohm resistor to higher wattage.

2
Solid State Tesla Coils (SSTC) / First SSTC build - some questions
« on: December 27, 2020, 04:21:29 AM »
Hi guys,

I've been building my first TC, which initially was going to be powered by a static spark gap.
I steered away from the SG/NST but kept the secondary, which in retrospect isn't a great idea considering SSTC are typically not as long (relative to their diameter).

I went with Loneoceans' SSTC2 circuit. I changed my primary design to a 12AWG wound tightly around the secondary.

The way I got the system working was by disconnecting the secondary feedback and feeding the input of the UCC27425 with a square wave from a signal generator. FYI The rectifier and the AC transformer are both running off an isolating transformer.

With no top load I got the coil to resonate at 299 KHz. With the two large half bowls (stainless steel) mounted, that frequency dropped to 174 KHz.

With roughly 60-65VAC on the output of the variac, discharges were roughly 5-6" long.

As interrupter I am using the second channel of that same signal generator (feeding the base of a transistor that turns an IF optic fiber on and off).

I am not quite sure why the feedback isn't working. I did try to reverse polarity on the feedback. But I haven't checked what the signal looked like on the oscilloscope, yet. I did read a post yesterday that suggested a 1K resistor should be placed before the clamping didoes, but I thought I would try the original circuit before "fixing" it. So I'll probably give this a shot tomorrow.


One question (the main reason I am posting this) though:

I read that TC's without feedback are not running as optimized because the resonant frequency tends to change depending on several factors. On my tests, I didn't seem to observe that behavior off the 10-15 minutes I was playing with it.

However, at some point the nice discharges I was getting (1ms pulses maximum, frequency being varied from 10 to 200 Hz roughly), stopped a heavy plasma came out. Current draw on the wattmeter (on the wall) increased significantly so I turned the variac down and turned off the system. Waited a little bit before turning it back on and it was still outputting plasma rather than discharges. I waited a little more, and after a while it was back to normal.

Can anyone help me understand what that behavior might be? Although I didn't try changing the frequency (as it was definitely outputting something, so it was at least fairly close to resonance), it did feel like something else was at play.

I didn't capture this on video, but if it helps I am sure I can eventually reproduce this tomorrow.

Cheers

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