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Is it safe to connect a USB input to an Arduino (Nano) at the same time as an external regulated 5V supply to its 5V pin is connected, assuming the supplies have common grounds ? Will (I hope) the external 5V be used so as to not draw too much current out of the USB supply ?

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From the Arduino Nano at Arduino.CC:

The Arduino Nano can be powered via the Mini-B USB connection, 6-20V unregulated external power supply (pin 30), or 5V regulated external power supply (pin 27). The power source is automatically selected to the highest voltage source.

Or, take if from the schematic:

Nano 3 Schematic

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OK, I see a section labelled +5V auto selector, with a Schotky diode between +5V and the VUSB rail, I don't understand enough about electronics to see what this does though. I get that if I supply 7V, say, to Vin then the Nano will use that as the 'highest voltage source'. I could perhaps use a variable voltage source fed from the batteries and set it to, say, 5.2V and force the Nano to use that? The reason for all this is that the Nano will be connected to a Raspberry Pi via USB for communicating with the ros system, and the Pi is unable to supply much from its USB ports, it cant run, for example, a WiFi dongle, mouse and keyboard at the same time.

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