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Help with base current feedback

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LAN_403

Zum Beispiel

Sun Jul 22 2007, 02:55PM

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

I build a SSTC, pretty much a clone of Steve Ward's SSTC 1 (halfbridge, half wave rectified mains input, start-up oscillator). Now that the coil is working well, I'd like to box it up (25cm x 25cm "table"). Only problem is that the coil doesn't like it if the antenna is too close to the resonator. The sound of the discharge gets harsh and the antenna is covered in corona, which I suppose is a bad thing

.

Seeing how the antenna feedback won't work, I decided to use a current transformer on the secondary base to get the feedback. The ground wire is passed once trough the CT and the CT's secondary is hooked up where the antenna used to be (like on Steve's DRSSTC 1) Only problem: The coil won't oscillate.

I've tried with two different commercial CT's and I've switched the feedback leads around about a 100 times, but nothing seems to work

. Both CT's give out enough signal to light a blue led (even with no break out on the coil) , so that's not the problem (right?).

Now I'm starting to wonder if it's even possible to use secondary base feedback on coils with half wave rectified input? Or is the start up oscillator interfering with it? Or are the CT's the wrong type? I have no info on them, but seeing how they give a strong signal... I don't know.

I thought about running the coil open loop, but I'd rather keep my MOSFETs in the "unexploded" state

If anyone has actually build a working SSTC with this type of feedback, any help would be appreciated.

CT2

Sun Jul 22 2007, 04:30PM

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

I had the exact same problem as you, I tried to figure it out but didn't come up with anything. Other people have gotten it to work though Link, so maybe they can let us know how!

Tom540

Sun Jul 22 2007, 08:12PM

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

Hmm, How many turns on the CT do you have? I haven't used the circuit you got there, so I'm not really sure what the problem could be. I think maybe your start up oscillator is being shorted to ground by the CT. I don't think you really need that start up oscillator or that MAX913 chip. If you want you could try this circuit. This is what I always use.

]1185135114_487_FT28811_sstc_logic.pdf[/file]

colin heath

Sun Jul 22 2007, 08:16PM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
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Posts: 162

Hi There,
As witnesses at the uk teslathon some years back will tell you, i spent a whole day non stop trying to solve this very problem. I tried the same with switching leads. it turned out i hadn't put a series resistor from the CT lead to the input of the sn74hc14. The power being enough to light the led is the problem. The signal is so strong it swamps the whole circuit with the feedback signal which also drowns out your interrupter signal.
Pop a 1K resistor in series with the output of current transformer.

Make sure though you put this as close to the board as possible other wise the line becomes high impedance leading to problems with signals.

Hope this help as i seriously beat my head against the wall with this one

cheers

Colin

Zum Beispiel

Sun Jul 22 2007, 09:50PM

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

Tom540 wrote ...

Hmm, How many turns on the CT do you have? I haven't used the circuit you got there, so I'm not really sure what the problem could be. I think maybe your start up oscillator is being shorted to ground by the CT. I don't think you really need that start up oscillator or that MAX913 chip.

I don't have a MAX913 in there, as I too read that it is not needed. Other than that the coil is pretty much the same. Even the resonator is roughly the same size.

I have a 100n cap and a 10k resistor in series with the CT's output and the start up osc is fed straight to the 74hc14's input (through a 100k resistor), so It shouldn't short out the start up oscillator, but I'll check that tomorrow.

I'm also starting to suspect that the CT's are not suited for this kind of duty. I've had them for quite a while, but I don't have any data on them. I'll wind my own transformer, and see if that makes a difference (I just don't like the idea of pulling hair thin wire a 100 times through the core).

Mike

Mon Jul 23 2007, 12:38AM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

Heres how i make my CT's. Take a ferrite core like you would use to wind a Gate drive transformer. wrap it tightly in 1 layer electrical tape and then use about 26-28 gauge magnet wire and wrap 50 turns. Then use some 20 gauge insulated wire and wrap 3-4 turns over top of that. Works great.

ragnar

Mon Jul 23 2007, 12:49AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Pulling a hair thin wire 100 times through a donut core would make a good CT for this. I've tried =)

Make sure you shunt the CT with a 100-or-so-ohm resistor, and that it's diode clamped to both rails, otherwise you'll cook something.

Tom540

Mon Jul 23 2007, 01:45AM

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

Sometimes, when I'm feeling lazy, (99% of the time) I wind a section of twisted pair LAN cable wire around a donut core around 30 times then I series them. Works just fine and I don't get all irritated winding 100 turns. I just have a .01Uf cap in series with the output along with the two diodes but I never bother with a resistor or startup oscillator.

Zum Beispiel

Mon Jul 23 2007, 11:12AM

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

Woot! It lives!

The problem was with the CT shorting the startup signal out.

Changed the cap for 10n to show a higher impedance to the startup signal and put a 10k resistor in series with the CT. It seems to work perfectly with half wave rectified input.

Thanks to everyone for your help!

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