wrote in message
ups.com...
Most linear supplis have about +20V before the "pass element",
transistors.

And if a transistor shorts C to E, then ~+20V will appear at the
+12V output. A good OVP/crowbar is a must have!

But if an otherwise good supply lacks an OVP I would add my own.
The first link I gave has a diagram for a very good, in fact better
then
any I have ever seen built iinto any PS. The Areotech diagram specs 1%
resisitors. I used 5% with no decrease in functionality. Given the
orignal
use was to be in aircraft, the slight performance edge offered by 1%
ressitors was well worth it, that is a "life critical" mission. While I
value
my radios, I am more then willking to settle for a little less absolute
performance. I built the first one with teh specified 1% resisitors,
I built another unit with 5% resisitors, and as far as I can measure,
they both trip in about ~50mS. That is 0.05S. Thats for a jump of
13.69V
to +20V.

I use 13.69V because that is the most commonly suggested voltage to
keep lead acid gell cells "float charged". I added a 120V AC relay to
lift,
open, the + output from the PS so during power failures, my radios
keep running.

And as a rule, the larger a supply, the cooler it runs, and cool
equates
to LONG life.

Terry


Ok I see. I'm good with my hands, can build a computer with ease, have done
lots of soldering, but when it comes to mission critical items like a PSU
that could end the life of one of my radios or cause a fire, I don't trust
in myself enough to do it. But, I learn very quickly. If I were to be there
with you, and watch you do it once, I could repeat it.

Funny how all these wall worts we use for our radios have none of these
features or protection. Or even are regulated. But, it's very important to
know. Using a simple Radio Shack regulated PSU got rid of the hum that was
driving me batty on my HF-150.

BTW, linear = regulated correct?

Lucky