The Raspberry Pi is often not able to supply enough power to USB peripherals through it’s own power supply and a USB hub is recommended to ensure power drops don’t cause problems in the processor.
This means you will need two plug packs. One to power the Pi and one to Power the hub. The setup is illustrated in the diagram to the left. Red lines represent the power lines, the blue line the USB type B connector which is expanding the USB ports available to the Pi.
For such a small form factor device it isn’t elegant to require so many big power connectors for a project.
One way to eliminate the multiple plug packs is to power the Pi from the USB hub itself – as illustrated in the diagram to the right.
This isn’t straight forward however as the USB hub needs some design features so as to make this a safe operation for the Pi which could otherwise be damaged.
This eLinux.org page is dedicated to Raspberry Pi hubs and describes how to test your USB hub for this feature and so I set about testing my $17 MBeat USB-M7HUB 7 port hub. I had already been using this hub to power my Pi as it was doing better than a cheap 5V wall plug that let the Pi crash under load. In the same conditions, the MBeat hub kept the Pi running fine. I wanted to see if I could use it to power the Pi and use it as a hub for the Pi at the same time.
First, I acquired some 10W resistors for the test. The eLinux page specifies 50 and 5 ohm resistors, I figured these would be near enough.
Next bit of equipment I didn’t have but managed to source was a USB Tester which conveniently exposes test points in the circuit for connecting a multimeter to measure current and voltage.
With all equipment sourced I was able to carry out the eLinux tests:
Without load, the voltage from the hub supply gave 5.2V. With the 5.6 ohm load resistor in place, the voltage across the load measured 4.8V. This just met the requirement of 4.75V as specified by eLinux.org. As a side note that resistor got uncomfortably warm to the touch after being in circuit.
For more detail: MBeat USB-M7HUB 7 port hub Schematic