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It's time for an SSTC

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Dr. Dark Current

Mon Oct 01 2007, 02:12PM

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

Ok, I think I'll go with secondary base feedback instead of antenna, because I feel it's a bit "safer" than the antenna especially when drawing arcs. I understand this is done with a current transformer, how should be the turns ratio and number of turns (pri/sec) on it? Can I connect it's output directly where the antenna was?

CT2

Mon Oct 01 2007, 03:28PM

Registered Member #180 Joined: Thu Feb 16 2006, 02:12AM
Location: Ontario, Canada
Posts: 187

For the CT you can use the same core as you GDT, an it needs about 100:1 ratio. You can hook it directly where the antenna is (the other side goes to ground), but you need to add a resistor and/or blocking cap with it or else it may short your start up signal (there is a thread on this somewhere).

Dr. Dark Current

Mon Oct 01 2007, 05:20PM

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

CT2 wrote ...

For the CT you can use the same core as you GDT, an it needs about 100:1 ratio. You can hook it directly where the antenna is (the other side goes to ground), but you need to add a resistor and/or blocking cap with it or else it may short your start up signal (there is a thread on this somewhere).

I think i found the thread

but what I'm still not sure about, the 100:1 ratio means I connect the 1 turn winding to the secondary base and the 100 turn winding to the input of the 74hc14 in series with a .01UF cap, or the other way round?

Zum Beispiel

Mon Oct 01 2007, 06:10PM

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

Yes, exactly. You put the secondary ground wire through the CT once and connect the 100 turn winding to the 74hc14. Remember to use clamping diodes or you'll fry the sensitive 5V logic chip.

If you use secondary base feedback, you must make absolutely sure that you have the ground connected before you run the coil. With antenna feedback it won't explode immediately, but with CT no ground = no feedback = BOOM!.

I once managed to blow my bridge like this. The thin ground wire got cut inside the case, the bridge was left running on start-up osc (160kHz, secondary resonant at 260kHz). Crank the variac to around 50% and Poof!

Dr. Dark Current

Tue Oct 02 2007, 03:30PM

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

I'm designing a mains-synchronised interrupter (I can never give up that characteristic mains buzz

) using one of the unused 74hc14 inverter gates. However I have no idea how big on/off ratio (min/max) usually the interrupter onthese coils runs, can someone advice? Also isn't 100hz break rate too low for a SSTC?

The samples should come in a few days (according to the ship date I received on order), so this is when the fun starts

Dr. Dark Current

Wed Oct 03 2007, 05:07PM

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

Please excuse my stupid questions but what's the right way to wire up the CT? In the first diagram the cap could see high voltages across it and maybe detune the coil a bit (though I'm not sure about these) and in the other diagram I'm afraid the cap wouldn't do anything at all.

wrote ...

If you use secondary base feedback, you must make absolutely sure that you have the ground connected before you run the coil. With antenna feedback it won't explode immediately, but with CT no ground = no feedback = BOOM!.

By "no ground" you also mean a loose connection (gap in the connection)? I have the secondary loosely connected to central heating radiator, there could be some minimal gap.

Edit- of course I meant CT not GDT

uzzors2k

Wed Oct 03 2007, 05:58PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

The cap acts as a DC block, so the duty cycle stays even. In the second configuration it will never see more than 5V. I would burden the CT and add a resistor in series with it. Colin Heath had lots of trouble with buffer chips and strong CT feedback signals. 1k is about right I think, and 10 ohms for the burden. All of this is in the thread you found though.

As for grounding I think you'll be fine, as long as the CT is hooked up it should oscillate. I got a variation of the Skori coil working today with just a lousy ground connection to an ATX PSU. It's my first SSTC.

Playing with flybacks can only go so far, the next step up would be a CCPS, which I don't have the ferrite for.

Dr. Dark Current

Fri Oct 05 2007, 09:08AM

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

Samples came today, TI ships samples really fast! Let's start working on the coil

Update: well, I think it lives, but I have no way to confirm that

I was getting some 1mm sparks to ground with a 25W bulb in series with the mains input, when my mom shouted from across the house that there's some interference on the TV. This was from like 15m away! I'm a little worried trying it with higher power, it might also fry some of the expensive electronic stuff stuff in my room

Update2- I'm a little confused now, there are 4 combinations how to connect the feedback path (you can reverse the CT and GDT) but the strange thing is that I'm getting some output from the coil on 3 of these 4 combinations, just one gives a little better output. Should I use that? Also the discharge makes some sputtering irregular noise, like it does not start every time.

Update3, lots of questions

I now got it to make a softly buzzing ~1/2" discharge, but the problem is that my fets (for now just weak irf740's) are getting quite warm, but the low side one gets much hotter than the other one. Could this be because the are different batch or could there be a problem with my gate drive?

Last update for now: The hotter fet stopped working. Its gate is near short but d-s is open. I think it was defective

Dr. Dark Current

Sat Oct 06 2007, 09:18AM

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

here's the design of my mains syncronised interrupter, if anyone is interested. It makes use of the remaining inverter gates in th 74hc14 so no additional ic requied. The on time can be smoothly varied from 0 to some 90%. And yes it's a bit diode rich but it works great

The 5v line should be loaded with a resistor (1k is ok) so it's voltage doesn't rise as a result of the additional voltages being clamped to it.

Dr. Dark Current

Mon Oct 08 2007, 02:29PM

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

I'm getting some weird stability problems with the SSTC.
This is a very low power test. The problem is, the coil fails to start oscillating on some "bursts" especially when I put my hand close to the CT/control electronics. This is NOT a problem of the interrupter, I've looked at this on a scope. The problem seems to go away a bit when I un-ground the control electronics. Whem looking at the gate waveform it seems that in these "misfires" the frequency goes to 1/2 or 1/3 the resonant freq. but I'm not sure.

I don't know, maybe this will go away with higher powers?

Can anyone advise? I'm attaching a video of the problem.

]1191853682_152_FT31895_pict0186dsf.avi[/file]

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