AVR-Chat

> Background:
>
> I am considering driving LEDs directly from the ATMega16 I/O pin (current
> sink mode) to avoid having to use an additional driver chip, however, the
> LEDs are located as much as 800 feet from the ATMega16 on an outdoor
> buried
> CAT-5 cable (long story).
>
> The max current rating of the I/O port is 40 ma so I'm planning on putting
> a
> 125 ohm resistor between the I/O pin and the cable to limit fault current
> (wire shorted to Gnd) to 40 ma and protect the I/O pin.
>
> This means that, to deliver 10 ma @ 5v to the LED (assuming for the moment
> a
> 2v forward drop in the LED) I need a total R value of ~300 ohm.  The
> additional ~175 ohm will be placed at the LED location as part of surge
> protection at the LED (R + Transorb).
>
>
> Question:
>
> The ATMega16's datasheet shows that the I/O pins have diodes from the pin
> to
> Gnd and Vcc.  These would seem to protect the pin from surge voltages less
> than Gnd or greater than Vcc but I do not see any specification as to what
> the max current capability of these diodes is.
>
> Knowing this will tell me what over/under voltages I can stand on the
> outboard side of the 125 ohm resistor before having to consider placing
> additional external surge protection on the pin (Transorb).
>
>
Depending on where, in the world, this is being installed, it could last a
long time as you propose, or it could be toast by this time next year.
Here is the story -

I worked for a number of years for a company that makes video security
equipment. It became part of GE Security. There was a major installation
at Atlanta International Airport. Had long control runs and video coax in
plastic conduit buried to perimeter cameras. Almost every spring, they
would loose one or more cameras AND the video inputs to switchers would
get fried AND the RS485 control interfaces would get fried. It took
several years of sleuthing to figure out that near-by lightning strikes
resulted in a current sheet through the top several feet of the earth.
This would cause a very large voltage difference between the earth and the
wiring INSIDE the conduit (which was "referenced" to "ground" somewhere
outside of the current sheet). The voltage difference was so high that it
would arc through the conduit to both the RS485 control wiring and the
shield of the video coax. The resulting current spikes would then take out
video and controller hardware. Sometimes, it would take out the whole box
because it was current on the video ground shield that became current on
the equipment power system ground.

This is a tough one to protect against. I do NOT remember the details of
how we got the survival rate up (it never was totally immune). Simple
transorbs helped with the control lines but the video ground currents were
a different matter.

Jim,