Olin Lathrop wrote:

> guaranteed limit within which the current won't be exceeded.
>
> If this is indeed true (maybe it isn't), then I don't see the problem with
> connecting a PIC pin to 170V via 5Mohm.  That's only 34uA.  The voltage at
> the other end of the resistor is irrelevant (as long as the resistor can
> handle it) because that voltage never shows up at the PIC.
>
> I don't see Roman's point about not wanting to rely on the protection
> diodes, as long as the relevant parameters are specified and guaranteed by
> Microchip.


The problem is more real-world oriented than theory oriented,
like diodes and resistors have max voltage specs. Exposing a
small physical size resistor to a large voltage does cause
failures, there is significant current leakage, (can be well
over the current expected from it's value) there is
breakdown of the resistive material (especially carbon) and
even the paint breaks down. I replace these every day in
TVs and Monitors that use resistors exposed to more than
100v. Small resistors have a significant capacitive
reactance, you may be able to ignore this at 20v but not
on fast slewing spikes above 100v.

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. 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. What voltage isolation
is that silicon chip rated for?? I'll tell you, 5v.

The worst thing is that the 170v is not smoothed in any
way, mains spikes (which are often in the kV range) will
go straight through the bridge and the PIC will be
exposed to these scary peaks with only a leaky overspecced
5mohm resistor to protect it? Anyone think this is
smart design? :o)

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. In TV's there are many sections
where a higher voltage needs to be sensed by a chip,
this solution is reliable and cheap.

The 20mA absolute max spec for clamp diodes is typical
of modern logic chips. These are safety devices to
protect the input logic from infrequent FAULTS. They
are not a design spec to be run at 10mA continuous.
Or any other continuous "fault" current. Would you
connect the input of a 74xx series gate to 170v via
a resistor??

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.
-Roman

PS. Olin, do Microchip provide a continuous rating
for the clamp diodes?? Or just an absolute "this is
the worst abuse before it blows" current figure?? ;o)

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