# Zener diode protection question

I've got a board that I'd like to protect some 3V3 peripherals from accidently getting fried by 5V from some bonehead (might be me) misconfiguring jumpers. What's a good value for the current limiting resistor to put behind the zener to ground? 50 Ohms? I'm thinking 3V3 rail -> zener -> resistor -> gnd.

Check this link Zener Diode Calculator for the caluculator

Actually, I'd think you could get by with just using nominal 330 ohms series Rs in the
I/O lines. That will limit the max current into the pins to (5V - 3.3V)/330 = 5 mA,
which will probably not overwhelm the built-in clamping protection diodes on the I/O
lines.

What's a good value for the current limiting resistor to put behind the zener to ground?

Not sure that is the right place. The Zener goes from input to ground, the resistor goes from input to what you are connecting it to. A value of 330R is good as that limits the current to 10mA. The cases where you made the pin an output, set it high, and then had a switch short it out to ground.

also pay attention to zener diode temperature characteristics ]