Over-voltage protection

[richard]

I absolutely hate products that DIE, and you're left clueless what went wrong.

me too, that's why i use modern switch mode power modules with wide input supply ranges and generally fail safe outputs

[amgen]

A simpler circuit as 'crowbar' type circuit. Use 2 or 3 -5.6 volt zener diodes, if/when it trips, probably will short/ruin one z-diode. Has your .6 amp fuse and you can add one or more 1n4004 type diodes in series to raise the trip-out voltage by about .6 volt for each diode. Also add a, maybe a
1 or 10 microfarad cap across z-d's to absorb any high surge if over 5v