Have built a couple of devices for automotive use - they work on both 12 and 24V systems - but was wanting something to protect a possible voltage spike to prevent the poor little 78 sereis regulator from going over 40 volts. Initially I put a 25Vrms (31Vdc) MOV on thinking much over that and it will clamp - however after reading datasheets, this particular MOV doesn't really start to draw reasonable current till around 100Volts!!! - it's totally backwards to my initial understanding of an MOV - but there it is in black and white - I even put 45volts across it and it didn't really draw any discernable amount of current. If I use a lower voltage MOV - then they are rated below the voltage they will be running at - which might be a problem for them. I did think of using a zener - but not so sure how one of them will last with a but of a surge across it either.

Any protection suggestions would be greatly appreciated

Cheers