I am assuming here that you are working with an unisolated circuit and know what you are doing with
live circuitry.
Also please note that this is just one way of doing things and I am sure there are 101 'better' ways
of doing the job;

A pair of resistors will provide the attenuation you require. Choose the values to give you a full
range swing over the range you actually want to _measure_. Then deal with the over-range
possibility (probability) by clamping the output of your attenuator to your + and - power rails with
fast diodes.
You then presumably need to level shift the voltage so it sits in the 0 to 5 volt range. A standard
opamp based summing junction will do this for you.

I hope this helps.
Simon




I need a circuit that can attenuate (is that the word?) a 110vac to 140vac
(rms.) voltage down to a readable voltage for the ADC on a PIC (5volts and
under).  It has to be fairly accurate (within 1%), or be setup to be self
calibrating (preferable).  For example, if the AC RMS voltage is 135vac,
then Vpeak would be 190.947 volts, and it would have to be translated down
to 5VDC.

Also, I'm not sure where I should set the top end of it....  I'm assuming
most places in the US don't go much over 140VAC (RMS).  I'm wanting to use
the most steps (8 bit ADC) for the best accuracy, but don't want to have the
device not work if the user's nominal voltage is normally above my top end
settings.  Any suggestions, or comments, on where it should be set at?
Sorry, I can't be using a transformer for this, and I'd rather not have to
resort to trim pots in the design either if possible (manual setup, and they
drift over time).

Any help on this would be greatly appreciated...

Hopefully this message will FINALLY get sent to somewhere other than the OT
list.

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