I have a question regarding the use of an Arduino board to measure voltage of a DC-powersupply without sharing grounds. So there should be a galvanic separation between the circuitry of the Arduino and the 48VDC-powersupply. A friend of mine told me to use the LM321 (Voltage to frequency converter, use the generated pulse-train for Arduino to measure and calculate corresponding voltage) but it can't handle a voltage in excess of 40 VDC. The next problem is, that if the LM321 circuitry uses the same powersupply as the supply it needs to probe, the frequency-value doesn't change when the voltage drops. So how am I going to monitor the output voltage of a 48VDC powersupply with an Arduino, without sharing a common ground? Thanks for your help in advance!
Use a voltage regulator on the LM321 to run it at lower than the voltage you are trying to measure. Then use a potential divider to feed a fraction of the voltage to measure into the chip. Then couple the output of this chip through a transformer or opto isolator into the arduino.
When the precision is not so important you may simply use an opto-coupler. You have to calibrate it as it is not linear.
There are chips designed for dealing with floating power supplies that can provide ISOLATION.
Example:
DC or VARIABLE FREQUENCY VOLTAGE TRANSDUCER
The voltage transducer is pricy, since it has only industrial grade, no consumer grade voltage transducer.