Richard,

Assuming there is a common ground, and input and output are the same
polarity, what you want is called a buck converter. To roll your own,
National and TI make chips. To buy a whole unit you need to go to a non
hobby distributor's web site like Newark, Allied, Avnet, or Arrow. Some
manufacturers of these things are Astec, Condor, Cosell, Elpac, and
Lambda(expensive).

Tam/WB2TT
"Richard Harrison" wrote in message
...
Off topic, but you can`t power an antenna without power to the
transmitter.

I`m looking for a d-c to d-c converter that takes 28 volts on the input
and supplies 12 volts on the output. Up to 8 amperes may regularly be
drawn from the supply. That`s about 100 watts.

I need it now. That`s why I`m imposing on this group. There is no more
knowledgeable group to ask that I know of. I don`t have time to build
anything, though I`ve built such inverters and converters.

A simple voltage divider or regulator is too lossy for high utilization.
I need lossless resistance.

Maybe there is an applicable aircraft converter that is surplus
somewhere, or maybe there is a stock automotive item as there are such
28-volt battery systems used in some big rigs. I`m in a hurry or I would
quietly do all the hunting myself and not bother anybody.

I would like rugged, reliable, efficient, replaceable, and cheap.

Thank you in advance for directions to the source of my d-c step-down
transformer.

Please excuse the misuse of this forum, but I believe it is my best
resource for most things technical and unusual.

Best regards, Richard Harrison, KB5WZI