Hardware Reference
In-Depth Information
I Still Want to Backpower the Pi!
OK, fine. There is a reasonably reliable way to do this. Some industrious
hackers in Australia had a custom USB 3.0 hub produced with the explicit
purpose of providing backpower for a Raspberry Pi.
Specifically, the interconnect port on their hub will send over the 1000 mA
(1 A) that the Raspberry Pi needs for normal operation. This will power the
Pi entirely off the connection to the USB hub (no separate power source is
necessary). You can check it out here:
hub/
The only downside to using this hub is that it has only a 2 A power supply,
and 1 A is going to the Raspberry Pi, leaving a little less than 1 A (some of
that needs to go to the hub itself) for the connected devices. That doesn't
leave a lot of room for too many high-power devices (.5 A at maximum five-
unit load).
Get Power Through GPIO Safely
Note that there is a big difference between using the Raspberry Pi GPIO pins to power
an attached device and pushing 5 V into the GPIO to power the Raspberry Pi. Lots of
the hacks in this topic need to draw some current from the GPIO pins, and this is safe
to do.
There are 3.3 V pins (P1-01 and P1-17), in addition to the 5 V pins (P1-02 and P1-04).
Maximum permitted current draw from the 3.3 V pins is 50 mA. Maximum permitted
current draw from the 5 V pins varies between the Raspberry Pi Model A and Model
B hardware. The value for the maximum permitted current draw from the 5 V pins is
calculated by starting with the USB input current (nominally 1 A), then subtracting
the current draw from the rest of the board.
On the Model A, the board has a current draw of 500 mA, so the max current draw off
the 5 V pin is 500 mA. On the Model B, because it has a higher current draw on the
board of 700 mA, the max current draw off the 5 V pin is 300 mA.
Remember, be very, very careful with those 5 V pins! If you short 5 V to any of the other
GPIO P1 pins, you're likely to fry the entire Raspberry Pi. While deep-fried raspberry
pie sounds like a delightful carnival snack, a fried Raspberry Pi circuit board is neither
tasty or desirable.
