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Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
jmartis, no difference. But I checked something and I was surprised. I did unconnect BD139's collector from +. Nothing changed. Plugged again. Again, nothing changed. I thought transistors were soldered in wrong way, but no luck, I've checked pinouts from datasheet few times and it seems to be okay. What it does mean?
Anyway, I tried with iron powdered core that was laying around. The results were a lot better. Just see pics. The copper wire is primary, green wire a secondary.
The GDT core itself: Primary with no load: Secondary without load: Secondary with load (IRFP + 10ohm resistor):
Very similiar, the only difference is overshooting which was biggest at secondary side with load. What do you think?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Tonic wrote ...
jmartis, no difference.
If the 555 puts out this crappy wave there must be something wrong with it, or its overloaded- it doesn't like overloading when its connected up like in your schematic (feedback from output).
Tonic wrote ...
But I checked something and I was surprised. I did unconnect BD139's collector from +. Nothing changed. Plugged again. Again, nothing changed. I thought transistors were soldered in wrong way, but no luck, I've checked pinouts from datasheet few times and it seems to be okay. What it does mean?
It means that the 555 was driving the GDT through the upper transistor's base junction
Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
jmartis wrote ...
If the 555 puts out this crappy wave there must be something wrong with it, or its overloaded- it doesn't like overloading when its connected up like in your schematic (feedback from output).
My error, I meant that NE555's output constantly gives square wave. The problem lies most likely with GDT and probably with BD13X's.
wrote ...
It means that the 555 was driving the GDT through the upper transistor's base junction
More details, please :) It's normal? Whan can I do? Any methods to check?
By the way, I've tested iron powdered core at 570 kHz frequency. After reading people saying that iron ones are a no-no for high frequency, I was surprised that waveform improved instead of being ruined.
Primary without load: Secondary without load: Secondary with load:
I didn't notice any core heating.
I have the feeling that my problems with ferrite GDT cores is a matter of turns.. guys, say, is it possible that I might need tens of turns to get square wave? From what I saw, I thought that at least 10 turns is enough.
Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
Then, since collector doesn't change nothing, it means that C-E is closed and only NE555's current flows to GDT? If yes, why BD139 doesn't work as it should?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Tonic wrote ...
Then, since collector doesn't change nothing, it means that C-E is closed and only NE555's current flows to GDT? If yes, why BD139 doesn't work as it should?
I think it works as it should, just when you disconnect the BD from supply, the 555 takes over (through the B-E diode). There will be some drop of output voltage, but it can be very small if the output current isn't overly large.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
It worked well for me right from the start, but it wasn't the first time I played with GDTs. I got 1 µs rise-times when running two IRFP450s, so at 500kHz it was mostly sinewave drive. At lower frequencies the waveform was more acceptable. You seem to have it working now, your waveforms look good with the powdered iron core. The ringing at 500kHz is probably just due to leakage inductance. By using a trifilar winding style instead of delta you can minimize leakage massively.
If you are still having problems with the circuit check that the output from the BD1XX totem pole is good before running a GDT. That way you can tell whether the drive circuitry needs fixing or the GDT. But with the BD1XX collectors connected directly to ground and +12V it should work perfectly.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Tonic wrote ...
Don't you think that after disconnecting from +30V a voltage drop should be bigger? Because I don't see the difference when I plug to +30V, nor +12V.
hey, of course it will not go upto 30V if youre driving it with just 12. Maybe you should learn some basics on transistors and you'll se why is this, the transistors work as "emitter followers" so when the input is high, the output of the totem pole will never go above the input (actually it will be few 100s of mV lower) and vice versa.
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NE555 interrupter and GDT problems
