I want to use my Arduino to measure the voltage of a (regulated) power supply to test the Arduino's accuracy (and my program). Can I simply connect the minus pin to an analog pin on the board and the plus to Arduino's GND? If so, would that impact the display and button I connected to the same GND pin? If it is of any relevance, the Arduino is powered via USB. Would that be safe? I'd love to try it myself, but as an electronics beginner I don't want to fry the chip or power supply.
Thanks a lot!