> > The internal diodes cannot protect from these spikes.
> > ......
> > That application note has always amased me because it is just pure bad
> > enginnering.
>
> Let's see, the PIC input diodes can take about 20mA (I'm guessing).

    20 ma is the correct number.

> With a 100K resistor, that's 2KV input
> With 1Meg  that's 20 KV .
>
> And BTW, that's continuous current.
> For narrow pulses, the PIC could handle proportionally more current.

    Not at all. That is instantaneous. The latch-up will occur very fast as
the semi conductor areas are very small. It is not triggered by temperature
rise, as fas as I know.

> My point is that the resistor will die long before the PIC !
> (a 1 Meg resistor with 20 KV across it is dissipating 400 Watts !!)

    The chip will enter scr latch up before the resistor even fells the
problem. The spikes will arc over the resistor and even if they dont they
will not harm the resistor because they are very fast.

> And of course you would have a small cap at the PIC to filter out the HF
> junk.
> This cap would  also greatly limit any spikes.

    The cap will not help much. It will not act fast enough !

> The weakest link would be the voltage rating of the resistor, you would
need
> a couple in series at least.
> And yes, I personally would put a MOV or something at the junction of the
> resistors.

    The MOV, a very fast one, would make the design more reliable. I would
use 2 resistors in series and the MOV in the junction of the resistors. That
should be enough to keep the high voltage out.

> Just pointing out that the App note isn't crazy when you actually do the
> sums...

    Sorry but I would never use that without the MOV's at a product of mine.

Best regards,
Alexandre Guimaraes

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu