Using Unpowered USB

Question

Does the Raspberry Pi's USB need the 5V/GND wires in order to work with peripherals that are already connected to the same 5V/GND rails as the Pi?

I have an application where a RPi is connected to an Arduino Uno via a USB cable. The Arduino acts as a "power manager" and when signalled, disconnects power to the RPi in order to conserve power (with the RPi disconnected, my power usage drops from 600mA to 100mA).

Currently, there are two sets of wires going to my RPi. The main power line going to the RPi's power jack, and the USB cable going to the Arduino.

To simplify wiring, I'd like to remove the 5V wire from the USB cable so I only need to use one transistor to "turn off" the main power line. I found that if I leave the USB cable in place, it draws around 200mA of current, which isn't enough to power up the RPi, but still enough to be a huge waste, so I either need to remove the wire entirely or use a second transistor to programmatically disconnect it.

Will the RPi still be able to communicate with the Arduino over USB without the 5V/GND wires, since they both have a common ground?

• goldilocks: Why not? Seems to me you might as well try it, probably with the ground wire still in first just in case there is some kind of level check for safety on that.

Answer 1

The Pi shouldn't draw power from USB (at least the newer Pi B+ or Pi2) (unless the current is going to other USB peripherals).

It is quite safe to cut the 5V line in the USB cable, but leave the GND connected to both ends.