> -----Original Message-----
> From: Graeme Zimmer [SMTP:gzimmer@OPTUSHOME.COM.AU]
> Sent: Monday, February 25, 2002 2:51 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [PIC] kill a PIC
>
> > 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).
>
> 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.
>
> 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 !!)
>
> 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 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.
>
> Just pointing out that the App note isn't crazy when you actually do the
> sums...
>
This topic has been discussed numerous times on the list.  IIRC, the general
feeling was that injecting currents into the protection diodes caused
unpredictable behaviour and was best avoided.  The magnitude of current was
well below that which could cause any kind of SCR latchup as well.

I'll have a scan through the archives if I get 5 minutes.

Mike

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