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It's time for an SSTC

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Dr. Dark Current

Wed Oct 10 2007, 03:03PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

One of the fets just went dead for no apparent reason, it wasn't even warm. I have no idea what is going on here

BTW here is a pic of the present primary arrangement

Zum Beispiel

Wed Oct 10 2007, 05:12PM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

Rising your primary up and spreading it over a larger area will allow it to draw more power.

The IRFP250 is a 200V device, right? If you are running at 160V (and you mean 160VDC, not AC, right?) you may get spikes large enough to kill them.

Dr. Dark Current

Wed Oct 10 2007, 05:24PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Zb wrote ...

Rising your primary up and spreading it over a larger area will allow it to draw more power.

The IRFP250 is a 200V device, right? If you are running at 160V (and you mean 160VDC, not AC, right?) you may get spikes large enough to kill them.

I think the spikes couldn't have killed it, I have 2x 1.5uF PP decoupling caps soldered right to the fets. Yes it is 160V DC, but I think its actually a bit less since it's rectified from a centertap of a 150W mains xfmr.

I'll replace the dead fet, remake the primary and see what it does..

Ken M.

Wed Oct 10 2007, 09:52PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Secondary is 4x18, the primarys is 10 turns of 10AWG wrapped around a 1\2" thick rubber pipe couple which is wrapped around the secondary. I uploaded the vid to youtube, but it'll be a little while before its up.

Edit: The video finally got posted on youtube while I was at work, so heres the link.

If your wondering why it goes through a variety of frequencys its cuz my interuptor has 6 settings all with a pot, and I went throught the whole spectrum.

Dr. Dark Current

Thu Oct 11 2007, 06:25PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I've remade the primary and now with 6 turns I'm getting more power draw than with 4 previously

however I won't do a full power test until I find out why the coil starts mis-firing at increasing power level...

A strange thing is, I've tried zener-clamping the CT output and this reduced the output from the coil.. could this be due to the "slowness" of the zener diode?

EDIT- this is getting REALLY wierd! I connected a scope probe to the output of the interrupter to see if it wasn't the source of the mis-firing, and guess what! The mis-firing stopped

this reminds me of the double slit experiment

Tom540

Thu Oct 11 2007, 07:27PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

Maybe your CT needs more turns. I make mine by winding a twisted pair for 33 turns then series them for 66. Or when im not being lazy I wind 100 turns on them. Seems like more turns and then good clamping works better

Dr. Dark Current

Thu Oct 11 2007, 07:35PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Well I have the answer- it IS the interrupter. I've cranked it up a bit and it finally showed up also on the scope. I think there are some problems with the sync pulses from the xfmr possibly getting some noise from the output of the coil, it's time to rebuild it (the interrupter)I think...

update, I've fixed the interrupter and the problem still persisted.. so just out of curiosity I added a turn on the primary side of the CT, so the ratio is now 1:12.5 really low I know but it got rid of the problem

Dr. Dark Current

Fri Oct 12 2007, 01:51PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Yesterday the coil produced some 3-4" sparks with 6 turns on the primary and then it died again. Only one FET was dead (the low side one again) and the failure was silent. The fet was not shorted completely, in fact the coil could still make a little spark even with the dead FET (exactly the same failure as the last time).

Before I replace the fet and blow it again, does anybody have an idea wha could have killed it? It was perfectly cold when it failed, so the only two possibilites are too high peak current and voltage spikes, but I can't see how any of these could have been the reason for it's death...

Tom540

Fri Oct 12 2007, 03:21PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

I'm thinking you should add more turns to that primary. I don't know what your values are but 6 turns sounds like not enough. It probably isn't as crucial since your not running it in DR mode, but it might help.

Try raising your primary way up and spacing out the turns to get more coverage. I have an SSTC that has a 6 or so turn primary and I have it about halfway up the secondary and the windings are very far apart.

Dr. Dark Current

Fri Oct 12 2007, 04:10PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Tom540 wrote ...

I'm thinking you should add more turns to that primary. I don't know what your values are but 6 turns sounds like not enough. It probably isn't as crucial since your not running it in DR mode, but it might help.

Maybe I wasn't too exact, it's 6 turns for 160V supply voltage and I think that's enough. I remade the primary in a way that I've insulated the bottom 1/3 of the secondary and wound a primary on it, it's about on the bottom 1/10th of the coil.

What I don't get is why the fets die, this is beyond my understanding. However I need to fix this of course if I want to continue working on this coil.

Maybe I'll dig up my isolation transformer and see how the drain waveform looks like

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