isolated mains power monitoring arduino

Posted on Feb 5, 2014

I`ve been looking around for a mains (220-240VAC) power monitoring circuit that I might be able to interface with an Arduino. There is of course the OpenEnergyMonitor solution, but they seem to use a transformer to isolate and measure the mains voltage. The transformer doesn`t exactly couple to the mains nicely: so the voltage on the secondary side is not a scaled

isolated mains power monitoring arduino
Click here to download the full size of the above Circuit.

voltage of the input side, the transformer itself will distort the voltage sine wave. Then over the weekend I came across Dave Berkeley`s Project Page, and he has a cool design for mains power usage monitor that does measure the line voltage directly. His Home Energy Monitor uses a dual channel ADC with opto-isolation, nice solution. There is loads of info over at his site, I`ve just changed a few little parts for my version. He uses a 10bit ADC [MPC2003] which I couldn`t get from RS, they were showing back order on the part, so I had a dig around and came up with a 12bit version of the same chip [MPC3202] just have to watch the conversion time, it`s not 200ksps but 100ksps, but I don`t see it being a problem (famous last words!). So started up KiCAD and got work, then on to RS to get the parts! If you like a PDF of the circuit diagram here you go: Mains Power Monitor Circuit Diagram (rev 1. 0) On Dave`s circuit he has a voltage divider that I couldn`t make sense of, it looks like he has 100K current limiting resisters on the two input lines and a divider made up from one of these and a 270R. The simulation on this only gave a 2. 08 -> 2. 92 volt swing on the ADC, given a 311V peek to peek AC input [which is 220VAC * sqt(2)]. I`ve used 1M0 on the current limiting and a divider of 1M0 and 10K, which give a little more input to the ADC.

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