Bob Ammerman wrote:
>
> Roman,
>
> Microchip have an application note (AN251? or AN521?) that talks about using
> just an R to high-voltage. In that note that state the diodes have a 500uA
> rating, which is presumably a continuous rating.
>


I'm familiar with the appnote and it makes me
shudder. Their attempt to prove that you only need
one PIC and very few extra parts to build the
circuit doesn't make it a good design. No
commercial products that I have repaired use
that system. Even the cheapest TVs use two
resistors AND a lower resistor (voltage
divider), so there is a finite max voltage
expected at the semi.

It's very common for one of the two pass resistors
to fail, generally arcing and taking the semi
with it. And the TV semi is usually a transistor
which is substantially more rugged than the
silicon inside a PIC. The better TVs use three
or four pass resistors, a zener and cap and sometimes
a lower resistor too. These pass resistors also
fail, but with less catastrophic effects on the
rest of the set.

We can all argue clamp diodes the same way we
argue commenting code, yes it probably will
work but is not the way a professional does
things. I won't expose my PIC pins or logic
chip gates to more than 5v. :o)
-Roman

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