Jerry, you are far faster at responding than I am! Looks like I repeated most of what you said :-)
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Actually, my output voltage was 12.012 volts. I used 1% surface mount resistors and checked that they were, indeed, within a few ohms. That made me wonder about the LP2950-5 regulator that I used for reference; So I pulled out one of my boards and checked. Yep! 5.0047 volts. So close 'nuff. Not to worry. It's a curiosity sometimes when there are small discrepancies betwixt design and prototype results. Perhaps someday I will investigate further.
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Oh, by the way...I did try zener reference for the circuit. All of them proved quite sloppy without buffering. Usually .2 to .3 volt change from 12 volt to 20 volt input. That and the higher current overhead led me to my old favorite regulator.
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Thanks for posting the schematic!
73, Don
Don, ND6T