Help with base current feedback
Zum Beispiel, Sun Jul 22 2007, 02:55PMI 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.
Re: Help with base current feedback
CT2, Sun Jul 22 2007, 04:30PM
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!
CT2, Sun Jul 22 2007, 04:30PM
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!
Re: Help with base current feedback
Tom540, Sun Jul 22 2007, 08:12PM
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]
Tom540, Sun Jul 22 2007, 08:12PM
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]
Re: Help with base current feedback
colin heath, Sun Jul 22 2007, 08:16PM
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
colin heath, Sun Jul 22 2007, 08:16PM
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
Re: Help with base current feedback
Zum Beispiel, Sun Jul 22 2007, 09:50PM
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).
Zum Beispiel, Sun Jul 22 2007, 09:50PM
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. 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 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).
Re: Help with base current feedback
Mike, Mon Jul 23 2007, 12:38AM
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.
Mike, Mon Jul 23 2007, 12:38AM
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.
Re: Help with base current feedback
ragnar, Mon Jul 23 2007, 12:49AM
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.
ragnar, Mon Jul 23 2007, 12:49AM
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.
Re: Help with base current feedback
Tom540, Mon Jul 23 2007, 01:45AM
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.
Tom540, Mon Jul 23 2007, 01:45AM
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.
Re: Help with base current feedback
Zum Beispiel, Mon Jul 23 2007, 11:12AM
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!
Zum Beispiel, Mon Jul 23 2007, 11:12AM
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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