Zener diode voltage for Mega GPIO protection

Got a quick question. I'm wanting to protect the GPIO pins of my new Mega, and am planning to build a GPIO protection board using zeners to bleed off any over-voltage. Digikey doesn't have any straight 5v thru-hole zeners; the closest they have is either 4v7 or 5v1. I'm guessing it would be better to use the 4v7 ones, but would that cause issues?

Thanks!

I'd been looking for "Absolute Maximum" voltages for the ATmega2560; on Atmel's product page it lists a "I/O supply class" voltage of "1.8 to 5.5" - would that be what I'm looking for? ( http://www.atmel.com/devices/atmega2560.aspx?tab=parameters )

I also found their datasheet (the link on the Arduino product page is dead btw.) It lists "Voltage on any pin except RESET" as -.5 to Vcc+0.5. I don't see a listing for Vcc specifically, just "maximum operating voltage" of 6v. So, could that mean that the GPIO pins could theoretically work with any voltage between -0.5 and +6.5v? Or that they could accept -0.5v to +6v?

Sorry for my ignorance, I'm just getting started with all of this, so a lot of the terminology is totally new for me...

When it says Vcc + 0.5V it means that whatever Vcc you have applied + 0.5V, so if the Vcc supply is 5.1 volts , the max to any input should not be over 5.6V. Or applied Vcc = 3.3V max input 3.8V.

Ok cool, thank you very much :)

Based on the Mega schematic, it looks like it uses a 117ST50T3G voltage regulator, which is supposed to produce a flat 5v output, which would mean that the IO could take up to 5v5. Since the zeners I'm looking at (NXP NZX5V1D) are 5v1 with a 2% tolerance, that means that they should theoretically allow through a maximum of 5v2, correct?

I'm just wanting to make sure that I'm properly protecting my board :)

Thanks again! :D

http://www.ruggedcircuits.com/10-ways-to-destroy-an-arduino/

May be of interest to you.

I came up with a "hardened Mega screw shield" design a while back, http://forum.arduino.cc/index.php?topic=185501.0 maybe give you some ideas.

chrismyers81: Digikey doesn't have any straight 5v thru-hole zeners; the closest they have is either 4v7 or 5v1. I'm guessing it would be better to use the 4v7 ones, but would that cause issues?

You want to use the higher value. Take a look at Figure 2 which shows how the diode will start conducting current (albeit small amounts) well below its rated voltage.

Awesome, thanks so very much everyone :) I've got a lot to learn, and I greatly appreciate all the advice, comments, and insights :) And, Crossroads, those shields you designed look awesome!

Thanks!