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Registered Member #2004
Joined: Sat Feb 28 2009, 11:43PM
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Posts: 39
Hi all. I'm having some unusual issues with a flyback I wound recently. It was designed for charging capacitors to a max of around 400v @ 18v input.
Here's the problem: It works fine from 8-11 volts, but at around 13+ I can't get any oscillation when I turn it on (just clicks slightly and heats up the transistor). However, if I tap one input lead against the battery terminal very rapidly, it usually kicks on and runs great until disconnected.
Specs: Basic super-simple driver circuit. 33 turn prim 750 turn sec 13 turn feedback default tv transistor (D1556) half-wave rectification w/ single diode Core haves are physically touching. Pictures available at request
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
try better transistor, the high voltage transistors from TV's ("HOTs") usually fail in this circuit, because they have very low gain and are slow. The infamous 2n3055 is better.
Edit: Did you try reversing the rectifying diode? You need to catch the flyback pulse, not the forward one.
Also, you absolutely should put a small gap in the core, a bit of paper it a good start.
Registered Member #2004
Joined: Sat Feb 28 2009, 11:43PM
Location:
Posts: 39
Thanks for the advice. The transistor barely gets warm even at 13 volts, so I'm not sure it's the problem, though I will try a better one eventually.
I think I figured out what's going, but I'm not sure how to correct it:
Apparently when I'm tapping the battery terminal, I'm causing the flyback to act as a pulse tranformer, and putting a small amount of voltage into the capacitor. The flyback will not start until there there a certain level of initial charge. But why? Could it be related to the resistance of the cap as the voltage increases?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Fnord wrote ...
Thanks for the advice. The transistor barely gets warm even at 13 volts, so I'm not sure it's the problem, though I will try a better one eventually.
I think I figured out what's going, but I'm not sure how to correct it:
Apparently when I'm tapping the battery terminal, I'm causing the flyback to act as a pulse tranformer, and putting a small amount of voltage into the capacitor. The flyback will not start until there there a certain level of initial charge. But why? Could it be related to the resistance of the cap as the voltage increases?
The circuit does not have enough feedback to oscillate, this is probably caused by the transistor as I said earlier. Try decreasing the values of both resistors and increasing the feedback turns to 3 or 4.
However a new transistor (2N3055 or equiv.) will surely get it oscillating better.
Registered Member #2004
Joined: Sat Feb 28 2009, 11:43PM
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Posts: 39
Double post(sorry).
I just found out this thing is producing WAAAY more than the intended 400V... I set up a small 1mm spark gap and it jumped it easily. I don't want to see how much voltage I'm getting by drawing a longer arc because I don't have any oil in my secondary, just plain magnet wire insulation.
Is adding a ton of primary turns likely to drop the voltage to acceptable levels? As soon as I think I understand these things they do something completely unexpected. I haven't had any issues charge 300-400v caps so far, but is it really safe to put 1k+ into a 400v rated cap? I wouldn't logically think so...
Registered Member #1917
Joined: Fri Jan 09 2009, 02:38AM
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Posts: 62
The MOT based charger for my ETG could put out about 2.5kVDC, and I was only charging the caps to 1.6kV. It didn't cause any damage, because it was shut off before the caps became overcharged. As long as it doesn't supply enough power to charge the caps REALLY quickly (i.e., hundreds of watts), it shouldn't be too dangerous. The ability to arc 1mm might have something to do with the very high frequency (better at breaking out than constant voltage or 60Hz AC current), or it could be putting out more voltage than you it should.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Fnord wrote ...
Edit: From the HV wiki...
Although popular this circuit is somewhat unstable. The 2n3055 tends to die easily, especially when driven over 12V
This worries me a little, though it is referring to flybacks designed for HV output, which mine is not. Will it still work (at ~18v)?
I don't know, if you adjust the primary turns, it should work OK.
Fnord wrote ...
I just found out this thing is producing WAAAY more than the intended 400V... I set up a small 1mm spark gap and it jumped it easily. I don't want to see how much voltage I'm getting by drawing a longer arc because I don't have any oil in my secondary, just plain magnet wire insulation.
Well, if you calculated the output voltage as supply voltage times turns ratio, it does not work like this, the circuit creates voltage spikes on the primary of many times supply voltage.
Better than increasing primary turns (which decreases power), remove some secondary turns. But you should have ~10 primary turns with 18V anyways, otherwise the transistor indeed gets very hot.
Registered Member #2004
Joined: Sat Feb 28 2009, 11:43PM
Location:
Posts: 39
wrote ... Well, if you calculated the output voltage as supply voltage times turns ratio, it does not work like this, the circuit creates voltage spikes on the primary of many times supply voltage.
Yeah, I found that out after I wound it:) But I did do a test yesterday with 160 primary turns and it still arced a 1mm gap, so I'm not going to be using that option (...this also could be because of what DYI suggested though).
I'll just wind a new set of secondaries and primaries. I made them interchangeable and it seems a waste to cut up a neatly done 750 turns.
Is there any way to calculate the voltage spike? That would be very useful.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Fnord wrote ...
wrote ... Well, if you calculated the output voltage as supply voltage times turns ratio, it does not work like this, the circuit creates voltage spikes on the primary of many times supply voltage.
Yeah, I found that out after I wound it:) But I did do a test yesterday with 160 primary turns and it still arced a 1mm gap, so I'm not going to be using that option (...this also could be because of what DYI suggested though).
I'll just wind a new set of secondaries and primaries. I made them interchangeable and it seems a waste to cut up a neatly done 750 turns.
Is there any way to calculate the voltage spike? That would be very useful.
Calculate - no, but can be guessed and measured. With 18 volts, i guess the spike to be 70-100V. If you don't need a lot of power, make a lot of turns on the primary (20?), this will keep the transistor cool, you still don't need to make the sec. that big.
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Flyback trouble
