Power supply filtering and bleeder resistor calcs

From: seware (seware_at_somewhere.com)
Date: 01/26/05 

Date: Wed, 26 Jan 2005 04:23:16 GMT

I am wanting to build a power supply and need a bit o' help. I have an
"Electronics for the hobbiest" type book that goes into fair detail on the
subject, but where it talks of filtering and bleeder resistors it speaks of
their use and suggests values for an example power supply but not how to
calculate the values for your own. Their example uses a transformer with a
12.6V 3A secondary wth a full-bridge rectifier. The bridge outputs have 3
1000uF 35V caps in parallel and then a 1K 1/2W resistor in parallel with the
caps for the bleeder. How do I calculate my needs for a transformer with a
15V 5A secondary? A worked example would be nice but I can plug in numbers
if someone will explain the reasoning. I understand that the size of the
bleeder depends on how fast I want to discharge the caps, but I don't know
what kind of time is reasonable. Should I reverse engineer the RC constant
in the example and work forward from that or are their some better rules to
follow? Thanks to all you professionals who sustain the questions of all of
us wannabees.

Steve