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When measuring a voltage that is higher than the maximum rating, the voltage of interest is tied to ground with a voltage divider. When measuring a negative voltage, you instead have to tie the voltage of interest to the positive supply voltage (study the answers to this question). In theory you can use the internal pullup to spare one resistor, but I would ...

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As others have said, an Arduino pin can't handle 12V, and can't handle anywhere near 10A of current directly. A digital pin on an Arduino is limited to 20mA, or 1/500 as much current as you need, at 5V, which is less than half the voltage you need. If you try to connect 12V to a pin on an Arduino pin you will almost certainly destroy that pin, and may ...

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I guess that you are showing the voltages there directly calculated from analogRead() and that you connected the AC voltage directly to the analog input (and it's ground with the Arduino's ground). The reference for every communication/voltage measurement/digital input is ground. Naturally the Arduino can only measure voltages inside of it's own supplied ...

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