I want to power an Arduino Leonardo with an external power supply and simultanously communicate via USB. I have read some other threads on this topic, therefore, I know that if one supplies more than 7V over the power jack, the board will use the external power supply in this case.

My suspicion is rather concerning the ground connection. From the schematic, I can tell that the USB ground is connected via a tiny coil to the board ground, so with DC it is shorted. Wouldn't that cause problems if the computer's USB ground is at a different potential than the external power supply of the board, resulting in a voltage between the two grounds? What if this voltage is up to several Volts?

Thanks a lot!

  • 1
    USB is mains earth references (except for laptops). You other power supply is either also main-earth references, or floating. In either case no current will flow. That's the way I understand it. Either way, everybody does it this way, so it's not a problem..
    – Gerben
    Commented Mar 30, 2015 at 20:18

4 Answers 4


Why do you suspect a potential difference? Where is th 7V coming from? If the 7V power supply is free floating and used for nothing else, there is no source for potential difference.

A typical wall wart will have a free floating output.

  • I intend to use the power supply of another PC. Are these ground potentials always guaranteed to be connected to Earth?
    – Dawamaha
    Commented Mar 30, 2015 at 22:50
  • Connect the grounds of the two PC together, then they will be.
    – Wirewrap
    Commented Mar 31, 2015 at 21:19
  • @Dawamaha or, if you want to be 100% sure, use a multimeter...
    – frarugi87
    Commented May 30, 2017 at 15:03

When you connect the USB, you also connect the ground of the computer to he arduino board. And then when the external power supply is connected to the arduino board the negative pin or the ground pin coming from the external power supply is connected to the ground pin of the USB. Thus the external power supply and the PC share the same ground.


I am going to clear one thing, many students and hobbyists are still confused and think that their Arduino board will blow away if they use the External power supply at the same time when Arduino board is connected to the pc through USB cable. This statement is totally wrong, you can give power to the Arduino board by using both sources at the same time and Arduino board is intelligent device it will choose the best input power having more current as compared to the other one.

  • It's actually a choice of whether or not the voltage from the power jack exceeds 7.3V. This is because there is a MOSFET on the board that chooses USB input if Vin exceeds 6.6V (a voltage divider lets it compare the output of the 3.3V voltage regulator to that). Since Vin is on the other side of a protection diode which drops 0.7V, then you need to put 7.3V or more into the input jack for it to switch to using that.
    – Nick Gammon
    Commented Dec 3, 2017 at 22:37

To answer your question: I have never seen a conflict like that.

I power my Unos with CCTV-style power supplies (12V) when I am using the HC-11 Wireless module which requires 100 mA or more in order to communicate.

When powered on just USB they can't talk at all. I mean, not at all - like you simply can't make them work until you plug in the second power supply to get more current.

Everything in the board is grounded together, and that it 99% of the battle when using two power supplies. Whether I am programming from a Raspberry Pi or a laptop, it all works the same.

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