[Milsurplus] FET Regulator: How It Do Dat?

[Hubert Miller]

I ain't so smart, but looks to me that the MOSFET the drain and gate voltages add to bias the gate in either
conduct more - conduct less condition. 
The 18 volt zener only conducts when S  is 18 volts lower than the drain. That's a form of current limit 
protection in case the output suffers some serious lowering of joltage, like a high current drain.  Looks like
to me. 
-H 

( *I read that ifn you spell ain't with an apostrophe init, you ain't all so dumb. )