simple overvoltage protection circuit using only a zener #Z1 to set the threshold. two pnps #Q1 and #Q2 form a schmitt trigger with #R2, #R3, #R4 and #R5, with some positive feedback through #R3 giving a volt or two of histeresis. The schmitt trigger drives the gate of a large power pmos #Q3, normally held low by #R1, which is in series with the positive supply and the load, isolating the load when #Vin goes above the trip point set by the zener by pulling #Q3's gate high. protects a low voltage load from excessive supply voltages up to the pmos's voltage rating.
so long as maximum ratings are respected, this circuit should be pretty versatile. the histeresis is critical as it prevents a slowly varying supply voltage from partially turning on the mosfet and burning it up by leaving it in the ohmic region of its characteristic curve. try changing the period of Vin from 20ms to 1ms or 200ms, you'll see that the turn-on and turn-off of the load slopes are constant regardless of the rate of rise or fall of #Vin.
