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First Light, Full Power? Mini SSTC (Steve Ward)

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Dr. Drone

Thu May 28 2009, 11:36PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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Posts: 1673

coillah

Fri May 29 2009, 06:38PM

Registered Member #1517 Joined: Wed Jun 04 2008, 06:55AM
Location: Chico CA
Posts: 304

OK So a bit of an update.

Everything checked out fine, so I hooked the coil up to full power. Good looking sparks were coming out of the breakout point.

I was experimenting with the ground wire of the secondary and the antenna of the driver, trying combinations of looping and whatnot to hopefully improve feedback. Everything was going fine, until some sort of transient developed somewhere in my circuit. A MOSFET blew. There was a large gash in the case. Those things pop loudly...

I replaced the FET, and went to turn the coil on. My antenna was not wrapped with the ground wire, so a good feedback signal was not being recieved (Im guessing), and another mosfet blew. The same side of the bridge. So this triggers the breaker in my house. I turn the breaker back on, and a second something in the bridge blows up, now Im feeling pretty noobish...

Its like sometimes I use my head, and sometimes I just don't....

Anyways, I call it a night. I took some time this morning to test my diodes and fets, after replacing the blown fet. One of the diodes allowed a reverse current, so its kaput. Replaced that. Tested the rest of the fets and diodes, and everything seems good. Now... I reinsert the lightbulb in series, and power on the coil... nothing, my driver board is powered on, but no sparks... so something else in my bridge circuit is dead. I suspect the rectifier. So my question at this point is, what is the best way to test the rectifier?

Also, just to be sure, does it make sense that because my antenna was not picking up a feedback signal the mosfet died?

thanks in advance

FireBird

Sat May 30 2009, 03:33AM

Registered Member #1104 Joined: Tue Nov 06 2007, 07:38PM
Location: Wisconsin, United States
Posts: 34

coillah wrote ...

...
my driver board is powered on, but no sparks... so something else in my bridge circuit is dead. I suspect the rectifier. So my question at this point is, what is the best way to test the rectifier?

You could probably just check for continuity with a voltage meter across the bridge. Are you sure your driver board is still working properly?

Also this may be a little OT but Iâ€™m almost done with my mini SSTC and I was wondering if I should power up the bridge or the driver circuit first. With the 9 volt battery test would it be a big deal how I power it up?

SeanSpark

Sat May 30 2009, 05:00AM

Registered Member #2082 Joined: Fri Apr 17 2009, 03:59AM
Location: USA
Posts: 3

coillah wrote ...

Very nice sean!

You have stuff crammed in there tighter than japanese people in a train at rush hour. :D

I like the spinning mobile thing you got for a topload, its like the sparks themselves are pushing it around.

Thanks for the tips dude, it looks like I am going to take a hacksaw to the heatsink and ensure that the fets are isolated.

LOL, Thanx ,
Ya, and there IS smoking aloud on board ! Must be from my mobile audio stuff and RC plane building, and well it is a "mini" SSTC.. nyuck nyuck..
I found with my sec coil "4in x 24in" my ant is not so picky, with all the RF comming outta that thing, it turned my monitor off across the room once, lol.
Do you have a scope? To see if you got the right stuff in the right spots. That stove idea sounds very interesting, cheap variac, plp throw them out all the time, high current capable, and heck you could cook lunch while fire up the ole TC. I got a variac online from All Electronics I think, $199, the red one in the vid, and it has been priceless, a good preformer.

coillah

Sat May 30 2009, 05:09AM

Registered Member #1517 Joined: Wed Jun 04 2008, 06:55AM
Location: Chico CA
Posts: 304

Yeah I have a scope, dont really know what to look for though... Actually that's not true...

Here's what I have been doing:

I programmed an audrino to produce a ~100Khz square wave, and simply connected the antenna to this output. I power on the driver circuit and observe the GDT input, a nice square wave is displayed on my scope. However, when I probe the outputs of the GDT, the signal is significantly degraded, almost to the point of not having any signal at all...

On top of all this, the scope is old. I believe that sometimes the issues arise from the scope itself and not the circuit under test.

I am really bummed I was so foolish and blew my bridge... I am placing an order with digikey tonight for replacements for all the components on the bridge. Although, absolutely no power draw on the bridge is highly suspect. I believe once I return to Chico I will have more of the tools required for troubleshooting (ie. my only multimeter). So for now I shall have to resign to reading Feynman's lectures...

If you have any advice or tips on what to examine I would be glad to hear it.

Current set up:
40W light bulb in series with mains and bridge circuit. Light bulb does turn on, but does not show any signs of variable load from the bridge circuit....

SeanSpark

Sat May 30 2009, 05:29AM

Registered Member #2082 Joined: Fri Apr 17 2009, 03:59AM
Location: USA
Posts: 3

What GDT are you using? I got some from Coilcraft, free samples, they seem to work ok. The sqare wave goin in looks good, on the output no load, looks good lil ring at rise n fall, but looks good, but when I hook to IGBTs, they look more like camel humps, lol. Your scope is not showing output from GDTs? Try disconnecting check em, with the gates shorted or what ever, it's probly damping the outputs? I got fat fast diodes 25 amps, went to 50 amp IGBTs, and still blew the thing.. rats, lol.

StevenCaton

Sat May 30 2009, 05:30AM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

Hi Colliah. About 4 months ago I built Steve's mini, and my story was much like yours. The coil worked *great* for a few runs, and then the next thing I knew it was dead. I never really ended up fixing it, because I just had other projects going at the time, but now I think I am going to rebuild it. After thinking about that coil for a few months now, when I rebuild it I am not going to put a toroid on it. I have a suspicion that using a toroid for a halfbridge of IRFP260's just loads the coil down way to much. Yes, I know you are using a breakout point, but I still feel that the coil would be more reliable in the long run without a toroid. (but probably not perform as well) My suspicion really isn't backed up but when I rebuild that coil soon I'm not going to put a toroid on it. (your toroid is relatively large)

Also, I never put back to back zeners on the MOSFETS. I suggest you do this. Also, get a nice 1uf ceramic DC blocking cap on the GDT. In the mini sstc schematic I think its only a .1uf. If I recall, this helped keep the GDT waveforms from sagging too much.

I programmed an audrino to produce a ~100Khz square wave, and simply connected the antenna to this output. I power on the driver circuit and observe the GDT input, a nice square wave is displayed on my scope. However, when I probe the outputs of the GDT, the signal is significantly degraded, almost to the point of not having any signal at all...

Just insert your 100KHz square wave into the Hex inverter chip.

Good luck

coillah

Sat May 30 2009, 06:16AM

Registered Member #1517 Joined: Wed Jun 04 2008, 06:55AM
Location: Chico CA
Posts: 304

Just finished checking the GDT. Looks good. Signal in and out look normal. Square wave and camel humps...

Must be the mosfets, every component in the bridge has been tested and checked out OK... now its a waiting game...

Mads Barnkob

Sat May 30 2009, 06:43AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Coillah

Make sure that the gate resistors are not dead, they would always die along with a blown fet in my sstc, and if one mosfet blows, the other (halfbridge) is most likely shorted out too.

FireBird wrote ...

Also this may be a little OT but Iâ€™m almost done with my mini SSTC and I was wondering if I should power up the bridge or the driver circuit first. With the 9 volt battery test would it be a big deal how I power it up?

When powering from the wall, drivers first, then bridge.

Arcstarter

Sat May 30 2009, 06:50AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Hmmm. Was the mosfet that blew heating more? Check your gate resistors. If one is higher than the other, even by a bit, you might be getting extra deadtime, causing shoot-through that can kill the mosfet from overheating before the heatsink temperature changes at all. I know this happens,it happened to me. I accidentally used a 3 ohm resistor for one, and 10 for the other. When i changing this, it ran fine, no more overheating on one mosfet and no more deaths.

At high frequencies the slightest mismatch can really suck :P. Check for shoot-through. Make sure one mosfet does not conduct while the other does. Some shoot-through is tolerable, but too much and it actually shorts the supply out through both mosfets. You will always have at least a tiny bit, but the less there is the better performance and less heat.

For feedback, i would suggest trying a CT. I did and the sparks grew a bit, and the best part, i could draw the sparks all the way to the toroid without the oscillations changing or blowing mosfets. Also, i could use a much bigger toroid. This is because the CT will use the current from the base of the secondary so when the topload is shorted, there is still current flowing through the CT. If you use an antenna and short the toroid, there is no electric field which will kill the oscillations. Also, being near the coil while operating will not effect how much feedback it gets, and the freq seems to be more steady and it seemed to vary the freq better as load was added, such as an arc or something grounded being in close proximity.

To use a CT, just get a good high permeability toroid, and wind about 30-60 turns for the secondary, and 1-3 for the primary (at least this is what i used). One side of the secondary is grounded to the circuits common ground, and the other side has a 10kohm resistor and .1uf film cap in series, to the 74hc14 with the clamping diodes. You should also have a shunt resistor in parallel with the secondary to avoid getting too high feedback voltage, i guess something like 10-100 ohms will work. At first i did not use a resistor, and it worked great but the 10kohm resistor fried.

Good luck, man.

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