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Re: Eli, can't find your post, what were those small cap values?

Posted by Eli Duttman (A) on May 1, 2007 at 19:37:00

In Reply to: Re: Eli, can't find your post, what were those small cap values? posted by Tre' on May 1, 2007 at 19:00:11:

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.

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