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DRSSTC Feedback Problem (Help!)

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Daniel Kramnik

Mon May 23 2011, 06:58AM

Registered Member #3885 Joined: Sun May 15 2011, 12:47AM
Location: Newton, Massachusetts, United States
Posts: 94

Hello again,

I am currently building a small-sized DRSSTC for an AP physics project that is due this coming Wednesday. At this time, I have completed a prototype lashup on my test bench, but it doesn't work!

My driver is based on Steve Ward's DRSSTC 3 controller with primary feedback and OCD. I also used the same size secondary coil and tank capacitor that Steve used (I would love to experiment more with the component values, but given the time I have, I would rather stick to something that has been tried and tested before). Some notable differences between my coil and Steve's are that I used an 0.68uF bypass capacitor across the bridge instead of a 10uF capacitor, I substituted IXGH40N60C2 TO-247 IGBTs for the HGT1N40N60A4D SOT-227 IGBTs he used, and that I added 5 ohms of gate resistance. I do not believe that any of these changes will make any sort of difference at this stage.

When I first powered up the coil, I only managed to obtain small (several mm) sparks even at full power. Adjusting the feedback, I managed to get the coil to shoot out some sporadic SSTC-like streamers, only to have both of my IGBTs explode. the streamers were very hot and thick and did not seem to correspond to what one might expect from the interrupter setting I was using (High BPS, Low Pulse Width). when I replaced the transistors, the same thing happened and both of them died again. Finally, I replaced them one more time and now nothing happens when I power the coil up, except I can sometimes hear the interrupter signal emanating from the GDT (By this I mean that the sound is literally coming from the GDT).

I have tried reversing all sorts of connections (Feedback CT, GDT, Primary, etc.) and removing the OCD, but to no avail - the coil just doesn't work. I know that the interrupter is working properly because it works with my other coils and I even tried replacing the GDT, but again with no success. I have also tried replacing all the ICs and even switching to secondary base feedback.

I know that this coil DOES work because I could get it to spark before in the same configuration. I suspect that the trouble is coming from the feedback; it's just not oscillating. Unfortunately, I do not have a scope at home to test anything with. Does anyone have any ideas as to what may be preventing the coil from oscillating? could it be connected to whatever limited its performance in the first place?

Here are some pictures of the GDT, the feedback CT, and the H-Bridge (Note the different GDT in this picture, I tried to use it instead, but it didn't work):

Thank you!

MRacerxdl

Mon May 23 2011, 03:53PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

That big core with many turns is the CT? You know that you have made about three turns with the thick wire, dont you?

I never built this steve ward circuit, so I dont know much about it. If you dont have a scope, you can make a few tests like I done in the past (when I has no scope):

1) Pick your driver, and put a speaker on the place of the GDT primary. With the interruptor on, you should get the Interrupter sound at the speaker.

2) Measure the current draw from the bridge power supply, see if there is nothing wrong with it. If you are using 4 IGBT's, see if you make the correct wirings from the two sides of the bridge. They need to be at cross, not parallel, so the igbt's will conduct in cross (Upper from the right, with bellow from the left). Making the same wirings on the two sides, will make the two of top or two of below switch together, that will not make anything, not even a noise.

Good luck!

ScotchTapeLord

Mon May 23 2011, 07:04PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

MRacerxdl wrote ...

That big core with many turns is the CT? You know that you have made about three turns with the thick wire, dont you?

That would appear to be two turns. There have been discussions on windings and I have been told that what matters is how many times the conductor passes through the transformer core.

MRacerxdl

Mon May 23 2011, 07:51PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

ScotchTapeLord wrote ...

MRacerxdl wrote ...

That big core with many turns is the CT? You know that you have made about three turns with the thick wire, dont you?

That would appear to be two turns. There have been discussions on windings and I have been told that what matters is how many times the conductor passes through the transformer core.

Ahh, okay. I has a similarity feedback problem a long time ago, and was because low CT ratio (if you made two turns in the core, it will be the half ratio).

doctor electrons

Mon May 23 2011, 10:35PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

ScotchTapeLord wrote ...

MRacerxdl wrote ...

That big core with many turns is the CT? You know that you have made about three turns with the thick wire, dont you?

That would appear to be two turns. There have been discussions on windings and I have been told that what matters is how many times the conductor passes through the transformer core.

In the middle photo there is actually only one complete turn. The other ones just exit the core and do not form
complete turns. Maybe that is your issue, the ratio is wayyy off.

Ash Small

Mon May 23 2011, 11:14PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

doctor electrons wrote ...

.
In the middle photo there is actually only one complete turn. The other ones just exit the core and do not form
complete turns. Maybe that is your issue, the ratio is wayyy off.

It would appear to me that there are actually two incomplete turns. ie the wire passes through the core twice.

If it were to pass through the core once, but also wrap around the outside, that would produce one complete turn.

Maybe this amounts to the same thing, but I'd argue that you want one pass through the core, but both ends to wrap around the outside. That is how I'd define 'one complete turn'.

(I spent a winter ten years ago working in a coil winding factory, and that is what the workers were instructed.)

EDIT: I'm not saying Dr Electrons is wrong. Just passing on what I learned when working in a coil winding factory as 'correct procedure'. they both look equivalent, but one has two passes through the core even if they are incomplete.

Daniel Kramnik

Tue May 24 2011, 02:08AM

Registered Member #3885 Joined: Sun May 15 2011, 12:47AM
Location: Newton, Massachusetts, United States
Posts: 94

The schematic specifies a 1:200 turn ratio - I wound 200 turns of 26 AWG wire for the secondary side and I have tried using various numbers of turns on the primary with no success. Are there any non CT related problems that could be causing this? Also, if the turns ratio on the CT does happen to be off, will this completely prevent the coil from oscillating?

doctor electrons

Tue May 24 2011, 03:20AM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

If the ratio you are looking for is 1:200, then that would appear to be what you have. A complete
circle defines a turn. In the photo you have 1. (Technically, anything over 300 degrees would be a turn,
in electrical engineering terms that is, and only for a toroidial core.) I would suggest changing the wire gage
for the feedback winding. Go a tad smaller and see what happens. Also, the more evenly spaced the windings are,
for both, the better your results will be. From the photo, it looks like you already did a fine job of that.
You should look into the tuning. Several things can cause the coil to stop throwing streamers. If you retune
in its present location, you may be suprised! If it worked before, it is most likely a tuning issue.

Daniel Kramnik

Tue May 24 2011, 03:36AM

Registered Member #3885 Joined: Sun May 15 2011, 12:47AM
Location: Newton, Massachusetts, United States
Posts: 94

The problem isn't that the coil fails to throw streamers (although that was a problem before it stopped working), but that it fails to oscillate altogether. Can this be caused by a tuning problem? This is my first DRSSTC, so I'm not sure.

Since I don't want to unwind this particular core (I know that it worked before), I will need to use a new one, but I don't have the same size available - I only have 1" OD cores while this one was closer to 1.75". Steve's Schematic indicates that he used a 1.25" OD toroid, could part of my problem be that my toroid is too big? Also, if I do choose to wind a new CT, will the AL value make a big difference? I have several 1" OD ferrite cores with AL values of 4k or 10k - is the higher AL value better? (I read that the higher the AL, the better it is for GDTs here, but I'm not sure if this applies to CTs).

Thanks!

Ash Small

Tue May 24 2011, 08:23AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I assume you've checked already, but is the CT connected the right way round?

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