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Re: [TCML] Sstc problems

Your symptoms sound like you have damaged FETs in your H-bridge, I've
experienced the same exact issue in an SSTC where I only replaced the leg
that was obviously blown. An easy way to check if this is the case is to
disconnect the gates of the transistors that you did not replace (the ones
that are likely damaged), feed a signal generator into the feedback path to
generate the switching frequency while the only the driver and interrupter
are powered, and see if the gate signals return to normal on the
transistors that were replaced since the failure.

You could also scope the switching waveforms in the bridge, but you need a
way of isolating yourself from mains because your scope's ground clip will
otherwise present a short to hot. You could try running your coil's bus off
of a DC power supply that can be ground-referenced without trouble; you
don't want to crank it up to anywhere near full power while testing with a
scope anyway. Regardless, I don't think any of this should be necessary to
diagnose the cause of this particular failure.


On Mon, Jul 6, 2015 at 12:31 AM, Kerry Soileau <kerrys@xxxxxxxxxxxxxx>

> Hey guys I got an issue I'm hoping some of my fellow coilers can help me
> with. Last night I took my sstc coil over to a friends house for the fourth
> and was wowing everyone with it. Some drunk idiot decided he was gonna run
> up and touch the Sparks but managed to actually grab the breakout point and
> then immediately the lights went out. Tripped the breaker the coil was
> plugged into. There was no Pop or boom or smoke except the breaker in the
> panel tripping. Reset the breaker and turned it back on. Pop again. Decided
> to pack it up till I could dive in to when I got home today. So like a worm
> I plugged it in at home thinkin it would work but it didn't. Lol. Just
> popped the breaker. Took it apart piece be piece. Found two of four mosfets
> blown. Drivers where good as well as the 74hc14 inverter. All seemed to be
> outputting the normal square waves I saw on scope while building it and
> testing each section. Replaced the two mosfets since I didn't have four
> spares or I would of replac
>  ed them all. Re soldered it all up. Triple checked and fired it up....
> Bingo it worked again.......kinda. The Sparks before drunk asshole #1
> touched it were around 8-10". Now they are only around 2-3"..... Got the
> meter out and tested the voltage across the bridge rectifier. Ac side was
> 124vac. DC side was 100-113.....that's not right. I had tested the bridge
> as well with diode check and all was well but where did my 160-170vdc go..
> No matter it still was working. Started probing with the scope while the
> bus voltage and H bridge where disconnected and removed from the box.
> Noticed my GDT wave forms went from square waves with minimal rise and fall
> to just sharp spikes....  Rewound a new GDT of the same material as the
> first in the same fashion with same wire and turns. Tested it and the same
> out put as the first..... Now it seems there is another issue I've yet to
> track down. On pins 6/7 of ucc drivers the wave is nice and square. But
> when tested on any of the secondaries o
>  f GDT it's just sharp spikes. Not even close to being a square wave.
> IAlso replaced the .22uf cap with a new .68uf cap with no change. As well
> as the 74hc14 and ucc drivers with no change. Can't figure this one out or
> even why it happened. Any body got an idea what may have happened? Also I'm
> new to having a scope. I want to test the gate of the mosfets while the
> coil is turned on but I don't wanna blow my scope up. Is it possible to
> test it while running or without the bus voltage hooked up?
> Coil specs:
>  Primary - 14awg @3 turns
> Secondary - 4.5"D/ 10"H(windings) /30awg mag wire.
> H-Bridge - irfp260n *4
> Drivers -ucc37322/21
> Feedback-74hc14
> Enclosure is aluminum case.
> Interrupter is 555 timer.
> Design is based on Steve wards mini sstc with a H bridge instead of half
> bridge. And few minimal changes to interrupter values. I can make a you
> tube post of the scope shots of the GDT tomorrow.
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