Well what an idiot !

[5th element]

I suppose you could say an over voltage spike will always destroy something. You could place a clamp there but then a spike big enough will fry the clamp. Where do you draw the line?

 

[nigelwright7557]

Nigel, perhaps you are being a bit defensive as there may be a problem with your circuit because the top part is not regulated. You unwisely depend on a wall-wart SMPS never offering an unregulated voltage spike upon power up. When such a spike occurs it will only appear on your top supply and the overvoltage would kill the 3v3 reg as your Customer observed.

The customer didnt observe a blown regulator.
They found found 12 volts on the microcontroller etc.
Given that zero volts reference he used was -12 volts then he would read 12 volts after the reg.
So the reg was fine and the circuit still works, He just unplugged USB cable and put it back in and it works. So probably just a USB glitch..
He was just a bit pissed with me because he thought I was putting 24 volts into the reg that has max input of 16 volts.

The power supply is a SMPS regulated 24 volts supply. I have decoupled the rails well at pcb end so glitches shouldnt be a problem.