RadioBanter - View Single Post - 13.8V high current power supply

May 8th 06, 08:56 PM posted to rec.radio.amateur.homebrew

wrote:

Hi all and thanks again for all the responses.

Well I had another 10 minutes to spare again today so took a different
secondary tapping to try to get more voltage headroom. Following that
tweak, I now have 25.5VDC available across the large electrolytics.
This is obviously plenty enough for 13.8VDC regulated; but is it now
*too* much to drop efficiently?
Thanks,
p.

I find that about 19 to 20 Volts off-load is about ideal for 13.8 Volts
output - if your unregulated voltage is much higher than that, you make
a /lot/ of heat! If you're using a suitably rated transformer (so that the
secondary voltage doesn't dip too much under load), 19 Volts gives ample
headroom.

A brief word of advice - if you're going to use a 723 type regulator
(usually a good choice), be careful to RF-bypass the op-amp (inside the
723) and provide an over-voltage trip of some kind. It is sometimes
possible for the 723 to suffer from "RF brain damage" and completely lose
the plot if you get enough RF in the wrong places! The best over-voltage
trip is really brutal - a high current thyristor to ground after a fuse in
the unregulated DC rail driven by a simple transistor and zener comparator
- if you get it right, you can get a large "thump" out of the blowing fuse!

Bob

--
Everything gets easier with practice, except getting up in the morning!