>>If you put a 4.7V zener, anode to your I/O pin and cathode to
>>Vdd, if will conduct when the I/O pin is below -4.7V respect
>>to Vdd, i.e. when it's below ~0.2V respect to Vss.
>>
>>It works.. I checked it.. but I wish I had an oscilloscope to
>>check the transients as well. For this application though (low
>>frequency) the rise/fall time is so slow that it's not going
>>to be a problem.. with (and probably also without) a cap. But
>>I like the idea of the double 4V7 zeners (one to Vss and one
>>to Vdd) and was wondering how they behave with transients, at
>>high frequences.
>
>Beware when using zeners to clamp low level signals!  Low voltage zeners
>have a very soft "knee", and the zeners rated voltage is defined at an
>appreciable current (usualy around 5-20mA).  At low current levels the zener
>voltage can be significantly lower! e.g at 1mA a typical 4.7v zener may only
>have a potential of (say) 3.7v across it.

This means that (given the other problems that you outlined fixed)
zeners should work ok against overshoot?

Greets,
Andrea


>
>Regards
>
>Mike

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