I was wondering how you might protect your PIC inputs.
In the drawing, the normal range of input voltage for each input
is 0 to +5 volts, but there may be excursions up to 10 volts in
either direction.
The top PIC is shown getting a minus 10 volt
input applied to its series 1k resistor, and the current flows
into the PIC1 through its protective diode D2 (conventional current
flow). This seems to be ok as long as the current rating of D2 is
not exceeded. The current flow is shown as red arrowheads.
The bottom PIC however is getting a plus 10 volt input applied
to its series 1k resistor. The current flow is shown as violet
arrowheads and can be seen supplying part of the load current
for both pics as well as flowing back into the 78L05 regulator.
If the current is higher than the load current of the PICs it could
raise the +5v line to as high as +10 volts !!
I am wondering how other people protect againts this second
problem where the input goes a little higher than expected.
I've considered zeners and more load on the +5v supply.
The zeners might interfere with the signal measurement
and the extra load adds to the supply demand on the regulator.
Keep in mind however that if we put a zener on the input then
that means we have to put a zener on EVERY input, of which there
could be 8 or more with one PIC chip so that would mean 8 zeners
for one chip!
Also note that the max current requirement of the internal protection
diodes isnt that hard to meet in this application, so i was thinking maybe
one zener across the +5v line in case it tried to go too high. Anyone
ever try this?
