A circuit to reduce DC voltage?

I have many old 12v DC power supplies and sometimes might need one of a lower voltage. Is there a simple circuit to reduce the voltage?

Answers:
Find out how much current and voltage the application takes. Get a resistor about = V/I. Buy a zener diode.
put the resister in series with the supply, the zener to the end of the resistor and ground the other end.

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