wallwarts and regulation..

KA4HJH terrymbowman at rica.net
Fri Mar 6 19:57:18 CET 1998

Jay wrote:
>Don't everybody
>throw tomatoes at once.

Sorry I if sounded too critical. It certainly wasn't personal. Like I said,
one of these tricks could come in very handy someday. I totally agree with
your experiences with transformers: the less excess unregulated voltage the
better, especially if you don't have room for a big-enough heat sink or
you're using 78L/79L's. The trick is to get the optimal voltage with your
target load. I have a big bag full of wallwarts (always keep your eyes open
for them at a hamfest) and with a little experimentation I can usually find
one that's just right for the job (the fabled Cinderella wallwart).

A good example of where things go awry is if you're trying to run a 6V
device from a 12V car battery. No matter what you do you're wasting 6V x
Iload. I have a portable CD player that gets a heat-sinked LM7806 quite
warm. Of course, I suppose I could always build a switching supply, but
then I'd have more RFI in my audio system...so much for keeping it simple.

BTW, one of the neatest tricks I'ver ever seen is a bipolar supply made
with two stacked positive supplies run from an *untapped* transformer. The
trick is to half-wave rectify both sides of the transformer, instead of
making one side common. It saved my butt one night when I burned up the
only negative regulator I had left. Fortunately, I had an extra positive
one and I remembered where the schematic was. 20 minutes later I had a
functioning circuit. So it really does happen, folks.

"See, I told you I was fat--I mean right"--Rush Limbaugh

Terry Bowman, KA4HJH

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