I've been tasked with switching a number of 115 VAC, 60 Hz supplies to various pieces of equipment, and monitoring these output supplies on a PIC18. I'm confortable with the PIC and ADC side, but where I'm struggling is the analog signal conditioning of the 115 V.

Playing around with a simple circuit simulator I've come up with the following which looks to be doing the right thing, but trying to interpret transformer datasheets is proving challenging.

I don't need very much power, I just want to isolate, scale, add a DC offset, and a little bit of low pass filtering. I've allowed some overhead for up to 200 V peak (~140 VAC RMS). The aim is to get an output between 0 V - 4 V, or something close to that.

Questions I have:

1. Am I approaching this the right way? Is there something fundamental I'm missing that could cause problems?
2. Any suggestions for suitable transformers for this application, and what would be the impact to my proposed circuit? My simulated circuit has a 1:1 coil ratio and a primary inductance of 100 H (which seems high!). Most transformers I've looked at don't seem to have this specified though. It's all very confusing!

I don't have any specific accuracy requirements, but I'd hope to be able to be within a few (2-3V) volts. Isolation wise this will be within an enclosure that's then fitted within a larger sealed rack, but I'm more worried about protecting the PIC microcontroller, obviously I can't just connect neutral to the 0V.

• "Whatever you can get without over-engineering a solution" means "put effort into designing it then I'll shoot it down if I don't like it". Go to whoever told you that and give them examples and see how they react -- i.e., tell them you're just going to sense the presence of any AC over 50V or so, without knowing what it is other than that. If they like it -- great. If they don't, tell them you're going to get a reading that's accurate to $\pm 20V$ -- check for their reaction. Keep that up until you have a specification in engineering units that you can write down. Then go design. Commented Jan 28 at 16:50