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Peteski... BUY one??? Are you mad? When I've got parts and junk lying around? Seriously, though, I already had this thing built, with nice meters and switches on it, and the only thing I never liked about it was the poor regulation. Once I realized I could use a center tap transformer (which I also had lying around) and a negative voltage reference, to get it very precise, I wanted to figure it out for myself.In case anybody cares, here's a typical example of what the circuit looks like. Typically, the data sheets will show a precision reference zener diode in there, instead of the two standard diodes I used. Note, this is NOT the same as putting two diodes inline wtih the output. The lower voltage connection (obscured by the railwire source label) is -30v in my case, but that is purely arbitrary. The key is just to get the lower end of the two diodes at about -1.4v (voltage below ground), and the 2.7k resistor is just a convenient value to draw enough current through the diodes so they turn on and drop "about" 0.7v. The trick to this is that the path through the diodes and the 2.7k resistor is very stable. The current doesn't vary with the load or output voltage of the regulator, so the two diodes drop a very constant amount in this case. A precision zener reference diode would be nicer, but these are pretty darn good.This circuit gives you "close to zero" output at the lower end, a few millivolts. And you can tweak the R2 and R4 if you are really determined to get a perfect zero output. The two C1's are mislabeled. Should be C1 and C2. 0.1 and 1.0 uF are typical values to prevent oscillation in the regulator.V+ needs to be about 3v higher than the highest output you want, so for a 12v max output, V+ needs to be at least 15.
