Olin Lathrop, Embed Inc, 978-772-3129 wrote:
>
> > 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.


It is at the PIC. One resistor does not provide voltage
isolation. By the letter of the SPEC you are attaching
170vdc to the PIC pin. (Two resistors in a voltage divider
is a different matter) Let me explain..

You keep talking like this is a steady state 170vdc
with the clamp diode nicely biased on. The real world
doesn't work like that. What if the 170v comes from
a mechanical switch (very fast turn on times)??
Are you saying the little clamp diode is infinitely fast
turning on?? I work with TVs and i'm constantly ordering
special diodes, fast switching, higher voltages.
Getting a diode that is both fast AND suitable for
higher voltages means a specialty diode, usually expensive
and generally large physically. There is no way the
tiny ESD diodes etched onto the PIC wafer are suitable
for this.

If the voltage increases at a rate faster than the
turn-on time of the clamp diode, and it exceeds
the miserable 20v or whatever the diodes and wafer
start to break down at, you have failure of the PIC.
And mains AC has the worst spikes and transients
you could imagine, both negative and positive spikes
up to kv's with vertical slew rates, ringing and
the works.

So when you get high voltage transients the PIC
wafer and diodes are exposed to some real nasty stuff,
and that includes high voltages. If you had a decent
capacitor from the PIC input to gnd and one series
resistor this would be safer, but it is still a bad
design compared to R:RC or R:ZC.

Those little wires inside the PIC going to the wafer
and the wafer itself are rated for a set isolation
voltage. That voltage is 5v. Connecting a high voltage
directly to a PIC pin (series resistor or not) is
very out of spec. :o)
-Roman

PS. Even the very nastiest Asian switchmode suuplies
have inductive mains filter, a number of large filter
caps, and always a cap and resistive divider or zener
before the high voltage enters the first semi.
The product would not be reliable enough to sell
otherwise. I'm sure if Microchip had to manufacture
their circuit it would look VERY different.

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