piclist 2002\09\22\014600a >
Thread: optically isolated mains voltage acquisition
www.piclist.com/techref/power.htm?key=voltage
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face BY : Russell McMahon email (remove spam text)



> I'm trying to make a circuit to do mains voltage acquisition using the
> ADC of a PIC and a pair of optos, but I'm having a hard time trying to
> make it work correctly. The circuit is:


The "trick" in transferring analog signals via optos is either to linearise
the opto OR know its response curve OR (by far the easiest) use specially
designed optos which have a single transmitter and two matched receivers
which are both illuminated by the same transmit LED. One receiver is placed
in a  feedback loop on the sending side and a matching circuit is
used by the other receiver on the receiving side. The "receiver-receiver"
tracks the behaviour of the "transmit-receiver" and thereby replicates the
input condition.

To save explaining this and all its variants at length I'll cut to the chase
and suggest you look at the Siemens / Infineon IL300 as one example of a
purpose built part. Infineon's app note 50 is a 17 page PDF that shows a
number of ways of using the IL300. (I can send a copy offlist if you can't
find it). While it is normal to use an opamp in the input circuit their
simplest circuit requires two transistors on the input side and a single
resistor on the output.



       Russell McMahon

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