Hi, I would like to monitor the KWhr useage on several of the 120VAC circuits in my home. Does anyone have any ideas on how to transform the line voltage and current signals down to the 5VDC that the MCU Analog to Digital converter can handle? I'm concerned about the safety of interfacing the low voltage and high (120/240VAC) signals. Would resistors and some sort of rectifier circuit work? Thanks for your input. Jim

Try a current transformer instead. It isolates the circuit from the power circuit. You will need to translate the signal into a usable DC signal but that is not too hard to do.

Use the output of the CT and rectify the signal, then across a known resistor somewhere around 1 to 10 ohms. The voltage across the resistor should be proportional to the current draw of what you are measuring. If you have an AC ammeter, you can calibrate your circuit to reflect what the meter reads.

Thanks Mongo. I could probably use a voltage transformer for the voltage as well. Any ideas as to where to get the CT?

You can make a current transformer pretty easily. A simple one is a torus core with a coil wrapped with about a hundred turns in just one area. Run the wire you want to measure through the torus and you have a CT. If you run multiple passes through the torus, you effectively double the sensitivity. Be careful though, as they can generate a pretty high voltage across an open circuit.

mongo-

'Current Transformer' - That is an interesting way to put it. Is there a reason to use the toroid, over simply wrapping the induction coil around the conductor being measured?

BM