Comparing the Arduino Uno and the Nano ... It's important not to mix this up as the 5V pin is connected to a voltage regulator that can potentially be damaged if the polarities are reversed.

The 78XX series of regulators are very handy to use. If you need, say, a 5V regulator, you grab a 7805, add a capacitor for stability, and send in enough voltage for the regulator to work with.

Little Bird Electronics posted an article about using an analog voltage reference with Arduino ... regulator, can give you a reference voltage just above that upper limit; say 2.5V.