wallwarts and regulation..

[KA4HJH]

Scott wrote:
>All in all, my impression is that there are better ways.  I know there
>are regulator ICs that are 'programmable' voltage wise and can be
>beefed up with a pass transistor for higher currents.  This will
>result in a higher component count, but also in a better regulator.
>I'd hate to introduce problems into a system using a questionable PS
>design that could be eliminated with an extra 2 hours of work and a
>few extra dollars worth of parts.

Amen to that sentiment. Now, I am certainly NOT an EE, as was said recently
in this forum (by somebody else but without the emphsis), but while this is
a classic example of a situation where there are all kinds of interesting
solutions worth chatting about, one of which may actually save or have
saved your butt in a pinch one day, THIS project cries out for a simple
LM317 and two resistors.

And for heaven's sake don't hook a pot up to an LM7805 and think you have
an adjustable regulator. It will regulate under load but not with respect
to the unregulated DC. I've tried it on the bench; it doesn't work.
According to the databooks I've seen, there has to be a current buffer
between the output feedback and the voltage divider, ie, an op-amp. The
LM317/319 don't need any extra components, so use them or the proper fixed
voltage regulator instead. KISS.

I mention the latter case because a certain well-known PS kit (which I
purchased years ago and will remain nameles--although I think it's still in
production) is designed just this way--7805/7905's turned into a variable
regulators with pots. I took it too work and everyone groaned. Like I said,
we tested it under real-world conditions and it didn't regulate with a
varying DC supply. The solution: I substituted 7815/7915's instead and it
worked fine. Rock steady.

"Sometimes the simplest solution is the best solution--unless you con't
figure out how to get rid of the body."

Terry Bowman, KA4HJH