Power from 5V USB wall adapter

I want to power the UNO with the USB power adapter (the charger of a phone), the adapter delivers 5.0 volts and comes from the wall (in the photo a regulator is added). However, the legs of the USB adapter look the same, can I plug it to the wall indistinctly like I do when I charge the phone, or do I need to know which leg is + and which one is - ? (I don't have measurement equipment).

  • the circuitry of the adapter corrects the polarity to the USB cable for you.
    – dandavis
    Commented Feb 28, 2018 at 17:51
  • Does the phone charge OK whichever way you plug it in? If so, why wouldn't the Arduino?
    – Nick Gammon
    Commented Mar 1, 2018 at 7:19

2 Answers 2


Power (which comes from your wall outlet), is AC, which means alternating current. It changes 50 or 60 times per second from direction, so this is the reason it does not matter if you plug it reversed or not. Some countries have plugs which can be plugged in normally or 180 degrees rotated (but you cannot see what is 0 or 180 degrees since these are symmetrical).

The circuit inside the adapter makes sure the AC is converted to DC, where it matters what is Plus or Minus.


It does not matter which orientation you plug it into the mains plug.

The power supply produces the same 5V regardless, and the USB plug has a fixed/standard polarity on the pinout.

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