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Registered Member #79
Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673
UPDATE: 5/22/07 Ok guys, here’s the story with this coil.
**Actual measured values** Rules of significant digits are followed, example “150 ohms†has 2 sig digits.
SECONDARY: AWG 30 Form diameter = 4.13†Wire wrap length = 16.2†Turns = ~1600 L = 53.3mH (with LCR meter) R = 150 ohm (LCR meter)
TOPLOAD: Upper: 3†x 7†center-center diameter (10†outer diameter) Lower 1†x 5.75†(outer diameter)
Fo of secondary, uncoupled with topload = 180.6 Khz (sig gen & o'scope) Ctotal of secondary = 14.57pF (calculated from Fo and L of secondary) Ctopload = not calculated, but estimated at 7.32pF?
PRIMARY Diameter = 8†to centers 0.25†refridgerator tubing Height of winding = 3.4â€
Max L = 14.0uH (LCR meter) Min L = 0.23uH (calculated from resonant Fq; sig gen & o'scope)
TANK CAPACITOR: 6 CDE .15uF 2000V caps, two parallel strings of three. C = 0.099uF (LCR meter) Vmax = 6kV (calc from datasheet) ESR = 40millohms (calc from datasheet) ESL = 21nH (calc from datasheet)
Primary circuit: Max Fo = 331.0 Khz (sig gen & o’scope) Min Fo = 128.1 Khz (sig gen & o’scope)
Secondary has been raised to 2†IIRC, for coupling ranges around 0.16 Q for primary OTTOMH was somewhere around 30? Or was it 10? Q for secondary was like 3! That number bothers me a bit. You can figure the value if you wish.
BUS Capacitors are 2700uF 200V ‘lytics in a doubler fashion IGBTs are whatever I had laying around, generic 600V Ufast IGBT. I’ll let you know when I pick something.
CHECKLIST: 1. Get the darn thing working reliably and tuned 2. Design soft start 3. Build a decent interrupter 4. Populate VU meter boards 5. Redesign front panel AGAIN, paint, and attach all that brass hardware I bought. 6. Put the sides back on and display her in my room!
That's probably more than anyone here wants to know, but It's exactly what I used to look for.
5/21/07 Hey guys! Great to be back browsing the forum on a regular basis!!!
Ok, so I have been working on this for a few months now. School and all have been really getting it down, but I don't have school for a while and I plan to finish it!
Here's a pic of the assembly. Since taking this picture, I had a bridge failure because of primary strikes and have raised the primary about 1". I don't see any more problems thus far, and do not see any signs of any in the future.
This is the DRSSTC controller. (Duh.) It is Steve Ward's DRSSTC-3 schematic. By far the best board I have ever designed, it went together like a dream, and the signals are better than every other controller board I have designed as of yet. I have not had a single problem with it. (I think this is revision #6 of this circuit board? And that's MAJOR revisions. LOL! Hey, I didn't know nothin' when I started! My old designs look disgusting to me now!)
The toroid is basically 1" foam board that protected the furniture I assemble at work. I glued it together with spray contact cement, it was the only thing I could find that would work. Other kinds simply wouldn't stick or wouldn't dry. It was important that I didn't get too much cement on it either, otherwise the contact cement would not stick either. Then, I covered it with paper mache (just white glue and water + newsprint), dried with a heat gun (impatence) and then sanded. Then I did the usual gluing Al foil and planishing with a wallpaper roller.
Primary supports.
Assembled secondary/toroid. I used the dual toroid system because simulations suggested that it would project streamers up and outward, and the dimentions were determined both by the desired capacitance, and the EMF around the toroid(distance between toroids especially). I attempted to design it so that any strays were directed towards the strike rail.
The jeweling for the front panel. This is a test piece I did, a later test came out even better! When I started on the full panel, however, I ran into some issues with the dowel "dying" and not knowing about it. So they'res some dark spots...
Laying out the dimentions. I attempted to make it so the primary circuit did not have much loop in it and was directed towards the primary coil. I think it turned out pretty well. The primary wire coming out of the capacitor and through the CT's connects to a 'C' shaped ring that runs beneath the primary coil. I then have 6 posts coming up through to base to the primary, which not only looks really cool, but provides a neat apperance and shortest route possible to connect to the primary coil.
Here is the G-S waveform on the IGBT while the coil is running. All the crap in the signal is generated by the coil and my redneck scope probe.
This is a pic of the G-S waveform on the IGBT's dualed with the CT signal. Actually, I guess this trace is pretty much useless, but I had a look at the voltage across the bus vs. the CT signals all the way up to 80V and 150A and my maximum deviation from ZCS was 400nS. After tuning towards the upper pole, my current went down, sparks went longer, efficency up and I could barely get a reading on the ZCS variance. Somewhere in the region of ~100nS or less. Boy was I thrilled about that!
Not too much later, I had a failure from my decoupling cap arcing to my heat sink and shorting out the IGBT's. I had inadvertently jammed it up against the insulation when I screwed the heat sinks in. :P Instead of being safe, this capacitor has been in the way the whole time, and I was wondering how I would fare without it? After all, Steve C was mentioning doing away with his. So I tried it. Up until 70V across the bus I was getting voltage spikes exactly the same as with the decoupling cap. Something like starting at 1.7x the rail and dropping to 1.3x the rail... Then when the bus was ramped up to 80V (from 70), the spikes reappeared in all their glory at >2x the rail! Why? I dunno, but it was about this point where the coil seemed to be really coming to life, and I was seeing a greater increase in my spark length. After having a brief look, (the spikes were still in the voltage rating, so I figure I was ok, but as I was deciding to turn it off, I heard a FWOOSH, and my GDT stared glowing... Then the usual sound of the variac bogging down... *breaker*...etc. D-A-N-G!!! Those were the last IGBT's I had on hand too. I don't have any more CAT-5E either. Looks like this is stuck for a week or so.
***QUESTIONS***
Does anyone have any idea why the spikes reappeared all the sudden?
What on earth causes GDT's to short out? This is the second one that has failed on me, and I was careful this time when I made it. ???
If I was getting 14" sparks @ 1/4 the expected voltage and current, that means I can at least expect 40 something inch sparks at full blast right? My goal was 40, and I hadn't even tuned for length yet.
Also, with the primary current fb, you're pretty much not going to stress your IGBT's if you are out of tune right? Especially if I'm checking the ZCS and voltage spikes and they're ok?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If your GDT burns out, it's probably because you have a lot of RF voltage appearing between the line conductors (that your H bridge is referenced to, because they power it) and the ground connection that your driver board is referenced to.
I see you're using an EMI filter, so is your driver board grounded to the ground terminal on this filter? If the filter is asymmetrical, which side is facing in towards the coil, the side with chokes or the side with capacitors?
On my DRSSTC, I put a small capacitor between the (-) rail of the DC bus and the chassis ground that references the other side of the GDT, to try and limit the high frequency nastiness that might appear across the GDTs.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
What on earth causes GDT's to short out? This is the second one that has failed on me, and I was careful this time when I made it. ???
CAT5 GDT *can't* fail unless it severely overheats...? That's an really extreme GDT failure over there, such GDT's worked well with 800V over them and this is really first time for something like that to happen!
Looking at your pics, you used a tantalum cap for DC blocking? If your DC blocking fails your transformer can saturate and get hot, but probably not enough.
And, what does the third set of CT's do?
Your board is absolutely beautiful, did you order it or etched it yourself?
If I was getting 14" sparks @ 1/4 the expected voltage and current, that means I can at least expect 40 something inch sparks at full blast right? My goal was 40, and I hadn't even tuned for length yet.
I wouldn't expect much more than 15..20inch sparks with a halfbridge of tiny IGBT's.
40 sounds much more like a figure for a full bridge if you are still using those IRF IGBT's...
Registered Member #79
Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673
CAT5 GDT *can't* fail unless it severely overheats...? That's an really extreme GDT failure over there, such GDT's worked well with 800V over them and this is really first time for something like that to happen!
That's just it... I couldn't figure that out either. I mean, the only thing I could come up with were voltage spikes related to the D-S spikes (which at the time were just a little over 160V) or I happened to smash off the insulation when I built the GDT. The latter is much more probable, but I was careful this time 'cause it had happened once before. Still, I'm not believin' that 15/30V ate through two thicknesses of even smashed insulation. I'll bet it has something to do with Steve C's comment, I guess I'll be doing the Chris Hooper epoxy bit on the next series.
Oh, and BTW, Steve, I did not connect the board's ground to the EMI ground. I large amount of nievity on my part. I thought adding it to the RF ground would cause more trouble than it saved, but then again I remember being able to pull 1/2 sparks off my first driver board... :O That should have been a clue right there! I'll bet it is the same effect that killed the GDT.
Looking at your pics, you used a tantalum cap for DC blocking?
Yes, I did. I stole that idea from EVR... it seems to work quite well. It's a 10uF 35V IIRC.
If your DC blocking fails your transformer can saturate and get hot, but probably not enough.
Neither the core or the wire was hot immediately after the failure. There was a light pop, then a fwoosh sound and a small but bright yellow glow where the failure occurred.
And, what does the third set of CT's do?
They run the front panel VU meter and some other intended "extra features". I figured it was best to keep additional stuff out of the OCD feedback to keep it as accurate as possible. I'm not as adept as EVR and the others that just run everthing off the came CT... And because I couldn't rectify the 'scope CT, I decided it was worth the extra effort to avoid the hassle of figuring all that mess out.
Your board is absolutely beautiful, did you order it or etched it yourself?
Why thank you! Nope, I designed it in ExpressPCB, and etched it myself! I was very happy with it too. It's double sided, drilled, and then covered with three coats of polyurethane. That coating works GREAT so far. It does not seem to get in the way of the solder, and is not difficult to solder through. Plus, no more greenish gray boards! LOL.
The driver is kind of a mix of EVR's power layout and Steve W's feedback circuit. I must admit I stole alot of layout ideas from EVR's ExpressPCB files on his site. It's really nice of these guys to leave so much handy information just lying around...
I wouldn't expect much more than 15..20inch sparks with a halfbridge of tiny IGBT's.
Boy I really hope not. Steve W's DRSSTC3 made 36" IIRC. I would be happy with that. I have hope though, I got 27" with my last coil on this exact bus, and it wasn't tweaked out or full blast yet (~300W at the time). EVR's Microbrute got well over 40", is a half bridge and will be running at the same power level. He has more bus capacitance than mine though and is using 40n60 minibricks, but I don't see why my transistors won't work.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
He has more bus capacitance than mine though and is using 40n60 minibricks, but I don't see why my transistors won't work.
Those capacitors do look awfully weeny for the power you want to process... Just be sure they dont overheat enough to pop open and make a mess :P.
BTW, its not the 15-30V that breaks down the GDT, but the several hundred volts across the bridge. Or perhaps you have some really low quality cat 5 wire?
Registered Member #79
Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673
Big edit uploading to first post!
Question. I have not seen anything, but is there a way I can run my Half bridge off one big cap? I've got a few that would be perfect and provide way more power, but to my knowledge, it won't work that way... I'd have to go fullbridge.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Yeah, you can do one cap. It would need 240VAC input (or a doubler just feeding the one cap). You can tie the primary return to either the + or - terminal of the capacitor. I did this with my smaller coils before, works fine. Also, you can feed the big cap with a doubler consisting of smaller lytics. Again, tested this idea with my DRSSTC-2. As a backup for my boost converter, i have a doubler consisting of lytics that are maybe 1/3 of the energy storage of my main cap bank.
Registered Member #79
Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673
Gee Whiz, I forgot to thank you Steve! I actually saw that on one page and the way they described it, I decided it didn't work. The simulations actually look better, than the sims with my other caps, and it turn out I have a cap that is PERFECT!
Off to redo my H-bridge again! LOL
How did I do it? I printed the ExpressPCB file out on photo paper with my laserprinter (MUST be something that uses toner) and ironed it on. To line up the sides, I had a makeshift lightbox, I think it was a microwave door or something... Then you kind of tape it up into an envelope and slide the board in. Then you iron it on. There are several different methods, but I like to use a wooden wallpaper seam roller. Heat, roll, heat, roll until it's stuck good. "Good" is realitive, it's kind of an art. After a few boards, you'll know. Then soak the paper off in warm water, rubbing the back of the paper under water makes it go faster. Try not to pull straight up on the paper, though if you've ironed it right it shouldn't matter. If some of the traces get messed up you can redraw them with a sharpie. Then I drop it in a mixture of HCl and H202...1 part Muratic Acid, 2 parts Hydrogen Peroxide...faster, cheaper, cleaner than Ferric Chloride. It stays transparent the whole time too, which is REALLY nice compared to FeCl. After rising the etchant off, clean the toner off with Acetone. Then drill the board. Finally, I put a few coats of poly on the board, drying in between with a heat gun. Then populate!!! Once you get the hang of it and everything in order, it's really not all that hard to make really nice boards. Goldmine's the cheapest place for blanks that I know of.
Thankyou so much for the help I am making a new self resonant bord that is a combonation of the minibrute and the DRSSTC II from EVR. I am also making some places surface mount so you can get the parts for free. I have been spending up to now 8 hours recreating the bord from the pics of the top of the mini brute on ExpressPCB Ill post my progress and when I finesh ill start a new thred. The method you discribed is something that I was thinking of doing. Once you see the board you can tell me if its reasonable.
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DRSSTC, yes another!
