Measuring 120 V DC voltage

So I have a DC power supply which has a voltage of about 120 volts.

I want to measure this voltage with the Arduino. One idea, was to connect a 10 Mohm and 100 kOhm resistor in series across the DC voltage, where the voltage on the 100k resistor should be 1/100th of the total voltage, i.e. about 1.2 volts, and measure this with the analog input of the Arduino.

The current flowing through this would be about 12 microamps. If you had a current much larger than this, you would be running into the power dissipation limit of the resistor.

For this to work, I would have to have a common negative for the 120 volt DC supply and the 7 volt DC supply that the Arduino uses, which are both powered by the same mains AC supply. Apart from the obvious short circuit hazards of this, would it actually work ?

Are there any other practicable methods of measuring the DC voltage, which don't involve a common ground for the power supplies ?

would it actually work

Sort of. 100K is a bit high if you go switching other analogue inputs you will probably have to resort to the read twice trick.

Are there any other practicable methods of measuring the DC voltage, which don't involve a common ground for the power supplies ?

Yes. You can make a voltage controlled oscillator on the HT side and use an opto coupler to get it to the Arduino where you measure the frequency and hence the voltage.

michinyon:
Are there any other practicable methods of measuring the DC voltage, which don’t involve a common ground for the power supplies ?

Well I have a multimeter that has an opto-isolated USB serial socket on the back - that
sort of device could be directly used (apart from the slight issue that the multimeter
is battery powered).

In fact the chips used are the FS9922-DMM4 (multimeter on a chip, outputs
serial), CP2102 single-chip USB<->UART bridge, plus an opto coupler. Given the
datasheet for the FS9922 is quite detailed it would be possible to roll-your-own
isolated USB voltmeter (if you can handle 100 pin surface mount chips!).

Simpler though is to use a small footprint Arduino such as the Pro Mini as the
voltmeter sensor, output the value as serial to an opto coupler, read with another
Arduino - not the same precision as a 3.5 digit multimeter though.

Yet another technique is a differential amplifier with a gain below 1 - effectively
two voltage dividers from each side of the high voltage supply, take the difference.