Indeed, there it is :smiley_cat:
This is a longer thread : http://electronics.stackexchange.com/questions/65576/arduino-uno-r3-directly-supply-regulated-5v-to-5v-pin
Here is it once more at #5 : http://www.ruggedcircuits.com/10-ways-to-destroy-an-arduino
Some say very loud that the Arduino board or the USB components in the computer might get damaged. Others are more reasonably and say it can be done if you know the risks.
I don't see a problem. I do it all time 8)
I do have current flowing into my computer. Yes, it happens. Will it damage my computer ? No. Will it always work for every computer ... well.. no guarantees ::)
This is the reference page for Arduino Uno : http://arduino.cc/en/main/arduinoBoardUno
That page has a pdf-file for the schematic.
The 5V voltage regulator on the Arduino Uno board is only to make 5V from the power jack and VIN inputs. It is not used when the Arduino is powered with USB.
These are two seperate things : Connecting it to the computer and the protection diode.
Sorry for the confusion that I caused.
1 ) Arduino powered with 5V and connecting the USB to the computer. Current can flow into the computer.
2 ) The protection diode. That is when the Arduino board is not powered an all capacitors are empty. A big blast of reverse current through the voltage regulator could damage it. A protection diode from 5V to VIN prevents that.
What should you do ?
For home usage with your own computer, then I don't see a problem.
But out there in the dangerous world, where anyone should be able to connect to the Arduino, then it would not be nice when current is pushed into the computer.
If you use the USB connecter ot power the Arduino board with 5V, the problem is gone. But the Arduino will get temporarily no power when someone switches that cable with a USB cable to the computer.
It might be possible to change the hardware of the Uno board, but that has other consequences.