In the url below is a jpeg of a vehicular power supply, and I'm wanting to
determine if the zener in the lower left hand corner of the schematic is of
the appropriate voltage rating.  Its a 12V zener, and I'm thinking since
this circuit is used in a car the supply voltage would be around 13.8V.  I'm
thinking an 18V zener would be more appropriate, unless I'm missing
something here... I'm guessing it may have something to do with hooking up
jumper cables to a battery in reverse, and this branch would prevent the
power supply from frying itself.  What I'm really saying is I really don't
understand what the two diodes do.  If someone could enlighten me I'd
appreciate it.

http://www.airkitllc.com/pwr.jpg

Regards,
Bob
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