SSTC oscillating but no breakout
MrSnrub, Tue May 08 2007, 12:18PMHello all,
I recently built my first SSTC (not my first coil though, I built a spark gap one a few years ago), but have run into some problems along the way.
The circuit is almost identical to Steve Ward's Mini SSTC, except that I am using IRFP460s in the half-bridge and MAX4420/MAX4429 for the MOSFET drivers. The GDT is 16 turns of 22AWG magnet wire trifilar wound, the secondary is 4" x 10" of 28AWG, and I'm using a four-turn primary. No toroid, just a pointy self-tapping screw about an inch long as a breakout point.
The coil looks like it's oscillating: when i run the bridge off 30VAC via a variac and using a scope probe dangled next to the coil, I see a flat trace which changes to a sine wave at a few hundred kHz when I touch the antenna with a screwdriver to kick-start it (and it stays that way after I take the screwdriver away). So it looks like the GDT phasing is correct, but I'm getting no breakout.
Should probably also say that the front end all looks OK, I'm getting nice square waveforms out of the 74HC14 and at the driver chip outputs. I've got no isolation transformer, so I don't think I'm able to check the gate drive.
Should I expect breakout at this voltage? I'm tempted to crank up the variac a bit, but I thought I'd check to see if I'm missing anything obvious.
Thanks,
Stacy.
Re: SSTC oscillating but no breakout
Tom540, Tue May 08 2007, 04:48PM
Hmm, Half bridge with 30 volts ac. I wouldn't expect any breakout. Have you tried arcing it to ground? I didn't get breakout on my latest coil until about 30Vdc on a full bridge. So you only have half that one a half bridge. You should be able to get very small arcs to a screw driver or ground. Just try brining the voltage up a tad.
Tom540, Tue May 08 2007, 04:48PM
Hmm, Half bridge with 30 volts ac. I wouldn't expect any breakout. Have you tried arcing it to ground? I didn't get breakout on my latest coil until about 30Vdc on a full bridge. So you only have half that one a half bridge. You should be able to get very small arcs to a screw driver or ground. Just try brining the voltage up a tad.
Re: SSTC oscillating but no breakout
Steve Ward, Tue May 08 2007, 06:08PM
You do have a sharp point to allow for breakout, yes? It should extend about an inch off the surface of your topload. Also, SSTCs have more "difficulty" when operating with large toploads, so dont make it too big.
As long as its oscillating at the frequency you expected, it sounds like its safe to turn up the input voltage. If the feedback is the wrong phase, then you will oscillate at a very high frequency (usually 1MHz +). Make sure thats not the problem (but i think you would have noticed).
Steve Ward, Tue May 08 2007, 06:08PM
You do have a sharp point to allow for breakout, yes? It should extend about an inch off the surface of your topload. Also, SSTCs have more "difficulty" when operating with large toploads, so dont make it too big.
As long as its oscillating at the frequency you expected, it sounds like its safe to turn up the input voltage. If the feedback is the wrong phase, then you will oscillate at a very high frequency (usually 1MHz +). Make sure thats not the problem (but i think you would have noticed).
Re: SSTC oscillating but no breakout
MrSnrub, Wed May 09 2007, 12:00PM
Thanks for the advice. I tried drawing some arcs off the breakout point with 30VAC in to the bridge, and I did get some tiny arcs. So then I turned the variac up to 80VAC, where some corona started to form and I could draw arcs about an inch long.
Unfortunately, that's when the variac suddenly started humming like mad, and one of the bridge filter caps let out the magic smoke
So back to square one for now... it seems like a strange failure mode though, as the caps were rated at 400VDC at 100uF (I was using four of them in parallel).
MrSnrub, Wed May 09 2007, 12:00PM
Thanks for the advice. I tried drawing some arcs off the breakout point with 30VAC in to the bridge, and I did get some tiny arcs. So then I turned the variac up to 80VAC, where some corona started to form and I could draw arcs about an inch long.
Unfortunately, that's when the variac suddenly started humming like mad, and one of the bridge filter caps let out the magic smoke
So back to square one for now... it seems like a strange failure mode though, as the caps were rated at 400VDC at 100uF (I was using four of them in parallel).
Re: SSTC oscillating but no breakout
ragnar, Wed May 09 2007, 01:33PM
Bridge filter caps let out magic smoke?
Did you have a filter cap in backwards, which would have been sucking all your current, and effectively minimising the voltage your MOSFETs could switch?
ragnar, Wed May 09 2007, 01:33PM
Bridge filter caps let out magic smoke?
Did you have a filter cap in backwards, which would have been sucking all your current, and effectively minimising the voltage your MOSFETs could switch?
Re: SSTC oscillating but no breakout
Sulaiman, Wed May 09 2007, 06:19PM
Sounds like your coil MAY be operating at an unexpectedly high frequency
(due to reverse-polarity feedback as mentioned by Steve)
that would cause a large high frequency current in the bypass capacitor which could be the cause of failure
Try reversing the feedback polarity (carefully)
Sulaiman, Wed May 09 2007, 06:19PM
Sounds like your coil MAY be operating at an unexpectedly high frequency
(due to reverse-polarity feedback as mentioned by Steve)
that would cause a large high frequency current in the bypass capacitor which could be the cause of failure
Try reversing the feedback polarity (carefully)
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