> Theoretically the clamp diode is forward biased, so the
> voltage on its anode won't get more than 0.7v over the
> PIC 5v rail. But I cannot believe this part is a diode
> rated for 170v, and as a designer I don't use a low
> voltage diode for 170v, forward biased or not.

You keep talking about the 170V as being at the PIC.  It's at the other end
of a resistor whos job is to handle the high voltage.  What the PIC sees is
essentially a current source at a few tens of microamps.

> Think
> of where these diodes are and what they are attached to.
> You would be connecting 170v (+ spikes) directly to
> the silicon chip inside the PIC.

No, not unless something physically failed.

> A zener at this point will give absolute clamp especially
> with a capacitor across it to help decouple the spike
> energy. That system will take that punishment forever
> assuming the zener is run at low average disspation.
> If the zener fails they always go short, giving good
> protection to the PIC.

I guess what you are trying to say is that if something fails and you end up
with excessive current thru the 5Mohm resistor, the zener will protect other
parts from also failing.  This may be true, but not terribly important in
most applications.  If something fails, the unit will cease to operate as
expected.  Most likely the whole board will be replaced because it's a lot
cheaper than paying someone to chase down the problem and fix it.  I other
words, if anything fails the whole unit is probably headed for the trash
heap.  In that case, it makes no difference whether just the resistor
failed, or whether the PIC failed too.  In this scenario, getting the right
resistor that is less likely to fail is worth it, but the zener is just a
waste of money and board space.

> The spec is a 5v device, DON'T exceed 5v on any pin.
> Using a last resort safety component for continuous
> out of spec operation gets my goat.

As Bob pointed out, the specs are a bit contradictory.  As I said in my post
that you are replying to, this is what I would do *IF* the max current spec
interpretation is valid.  However, I believe it is for two reasons.  First,
it makes sense based on reasonable assumptions about the electronics.
Second, Microchip has apparently shown an example of this use in one of
their app notes.  (I haven't personally seen it, nor have I looked.  I am
basing this on what others have reported here.  I would certainly check it
out first hand before I did a design like that.)


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olin@embedinc.com, http://www.embedinc.com

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads