Shunt Voltage Regulator

10/16/2006 Shunt Voltage Regulator
I am planning to run a DC supply line around the layout and in an attempt to prevent accidental over-voltages I would like to insert a Zener diode across the supply like this...

The return (negative DC) will be via the common ground of the layout. i.e. it is shared with 16VAC and Digital ground.

D1 is a regular diode just as a precaution to prevent any accidental reverse current from going anywhere. Do I need that ? Should it be where I show it or on the consumer side...?

D2 - do I have this pointing the right way? I understand it has to be reversed but I don't know if Zeners are already reversed symbolically.
Assuming I use a 12V supply, the Zener voltage would be slightly above 12V

In order to calculate the rating of D2 - I guess I would need to know how much current may be present but this is an unknown, but typical current that may come into contact with the DC circuit might be 16VAC or the digital signal or perhaps rectified 16VAC - 22V, or good old 110VAC...

Since my 16VAC supplies are 100W each with fast acting 3A fuses, I suppose 50W could get introduced by accident. I suspect a 50W Zener is a little crazy... (if not impossible)

Suggestions and corrections will be greatly appreciated....

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