3 terminal regulator max input voltage with zener at adjust pin?
Quick question: If I stick a zener at the adjust pin of a 3 terminal linear voltage regulator, the allowable maximum input voltage will also increase by the breakdown voltage of the zener used... right?


Quick answer, yes. The regulator only sees voltages between its pins.

Beware fault conditions that could apply the full input voltage across the regulator. 

Before you shunt the zener with a capacitor to reduce noise, bear in mind that the requirement to charge the capacitor will cause the full input voltage to appear across the regulator if the input voltage is applied suddenly. As Spehro points out in comments, this applies to the output capacitor as well. Solutions include protecting the regulator with zeners across the pins, or ramping the input voltage up sufficiently slowly. Are either worth the trouble, compared to just using a suitable regulator?