Bala.Chandar wrote:
> The Neutral terminal of the Mains is connected to a PIC
> input through a 4.7M resistor for detecting zero crossings.

You havn't thought about using a "AC-opto coupler" ?
These are specificaly built to handle this task.

See as an example the device SHF628A from Vishay :

http://www.vishay.com/optocouplers/list/product-83722/


You'll get much better mains <-> PIC isolation, and you can
more easily add a smothing R/C net between the optocoupler
and the input pin of the PIC (to filter out "spikes" in the mains).

Jan-Erik.

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