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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hello all, The power supply for my VTTC are 4 MOTs in parallel and 6 parallel MO caps for the doubler. The RF bypass cap is a series string of 7x 82nF 1600V caps and a 3A 1500V diode in parallel to each cap as the doubler diode. There is a ~3-4 mm safety gap across the MOTs.
When running the VTTC at full input voltage, I heard a loud bang and saw a flash and the arcs stopped. I think the flash came from the safety gap but I'm not 100 percent sure... It turned out that all of the doubler diodes were dead short. Well, I replaced each diode with a series string of two 3A 1300 diodes, so the total diode string rating was 18.2 kV. After some time of playing with the coil the same happened - a loud bang and flash, the MOTs and variac hummed loudly into the dead shorted diode string.....
I have absolutely no idea how the diodes die, just where could >18kV come from? Or do they die by some current spike? Why is the bang so loud, if it's really just the safety gap firing, it should be just half-discharging the RF bypass capacitance which is bit over 10nF...? Any ideas?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Well the resonant cap is 1.38 nF and RF bypass ca. 12 nF. I just can't see how the diodes could die by the RF, when they are blocked by the caps... Do you think ultrafast diodes could stand up better? I'm not sure about the choke, the power supply is already a bit too big...
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VTTC power supply mysterious failures
