user8172: 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!

user7991: 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.

user8075: 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.

user40167: 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. 

8172: <p>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.</p>

<p>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?</p>

<p>Thanks a lot!</p>
 7991: <p>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.</p>

<p>A typical wall wart will have a free floating output.</p>
 8075: <p>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.</p>
 40167: <p>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. </p>
