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In Reply to: Eli, can't find your post, what were those small cap values? posted by Vinnie on May 1, 2007 at 16:26:56:
I would try a .5Uf to start with.Bring it up slow and see what happens.
Follow Ups:
The hard part will be finding one with a high enough voltage rating.
Thanks
Buy 4X Xicon 0.47 muF./630 WVDC MPP caps. and a pair of voltage equalizing resistors. I know you can figure it out. Mouser stocks the parts.Be sure to stay under 1 muF. net capacitance in the "cheater" position.
Eli D.
Thanks Eli, I will give that a try.
Eli, what happens after 1Uf? Would it be higher PIV on the diodes?Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Somewhere in the vicinity of 1 muF. behavior changes from primarily choke I/P to primarily cap. I/P."Cheater" caps. that push the DC rail voltage up to the AC RMS voltage are certainly OK. Regulation degrades beyond that point. How much degradation is too much is a case by case thing.
Also, think about the available B+ current. A "pure" choke I/P filter can deliver slightly more "juice" than the AC RMS value. Adding capacitance in front of the 1st inductor lowers the amount of B+ current that can be drawn. As the conduction angle in the rectifier winding get smaller (larger cap. = smaller angle) the DC draw that can be supported declines. It's a matter of I^2R heating in the winding. The heating is inversely proportional to the amount of time conduction occurs and varies directly as the SQUARE of the current drawn. The I^2 term is dominant. That's why approx. only 0.5X of the AC RMS current is available as DC when cap. I/P filtration is used. A choke I/P filter allows access to the full VA capability of a rectifier winding.
A choke I/P filter is current rich and voltage poor. A cap. I/P filter is voltage rich and current poor.
Eli D.
Thanks for the explanation. That's just what I am seeing running sims on the Duncan PSUD 2.Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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