Rule of Thumb For Zener Volatage?

Hi All,

I was wondering if there is a rule of thumb for choosing a zener given a specific rail voltage. For example I have been using 6.8V tranzorbs in a 5V system. I'm about to design a board to use a 3.3V rail. It wont be consuming much power. Is there a rule of thumb for choosing the breakdown voltage?

-mike

I would choose one slightly above the nominal voltage such as 3.6V or 3.9V for a 3.3V rail.

Why are you putting zeners on your power rail?

Ok thanks. That's what I've done in the past but just wasn't sure if there was a more theoretical way of doing it.

This is an old design which I'm mod'ing to use a much cheaper micro but this micro needs 3.3V not 5V. The original designer had a 6.2V zener across the output of the regulator. The only thing I could of for a reason to put the zener there is for extra regulation of the voltage. But I think I know what you're getting at. For the most part that part is useless.....