Until one of your zener diodes short out (a zener will eventually short
out due to inrush current issues is my guess) and you then punch around
19V to the circuit that wants 12....not a good design as a result, more
of a hack.

John :-#(#



Yebbut, the zeners are not dropping the supply to 12v. They are *reducing*
the input supply to a full blown 12 v regulator sub-circuit that the OP
already has. Further, that regulator is expecting smooth DC in, so probably
only has a few tens of uF hung across its input, for decoupling purposes, so
there will not be any serious inrush current through the zeners. Given that
fact, there is no reason why the zeners should ever fail, particularly as
they are high wattage stud types anyway. The technique of placing zeners in
series with voltages that you want to reduce by a steady and relatively
current-independent amount, is well known, and not uncommon in commercial
designs.

Arfa