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> From: Wayne Bennett <w.bennett@SCT.GU.EDU.AU>
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Optoisolators or MOV's
> Date: jeudi 29 mai 1997 23:47
>
> I'm monitoring five external lines for change in voltage level with the
74's
> ADC but have to consider the effects of induced voltage from lightning
> strikes.  I was thinking of using either MOV's or optoisolators but I'm
not
> very familar with either.
>
> A couple of points I need to consider is that I need precise mesurements
> from the ADC so devices that affect the reading are not desirable and the
> amount of induced voltage and current from a nearby lightning strike (in
the
> magnitude of thousands of volts).

There is two point of view :
1) No destruction
2) Always precise mesurements

I think MOV (or tranzorb or zener) are really good solution for protection
but really poor for precise mesurement because they only limit the voltage.
The solution for precise mesurement are filter because they limit the
variation of the voltage in the time. So I place always a tranzorb to
protect and limit the voltage and then a filter to have a precise
mesurement under induced voltage.

Optoisolators are really poor for induced voltage because of the internal
capacitor between the input and the output. They are only good for
isolation of low and middle frequence or low voltage but induced voltage
are some times 4000V with a rise time of 50nS.

Didier