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I'm making a powered daughter board to mount my B+ Pi on. Can I power the Pi entirely through the 5v pins on the GPIO header?

More importantly, can I also supply reasonable currents to USB devices through these pins?

Am I right in thinking this has changed between the B and B+?

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Yes, you can power the B and the B+ through the 5V and ground pins on the expansion header.

The difference between the B and B+ in this area is that on the B+ you can no longer backpower the Pi through the USB ports.

EDITED TO ADD

You can actually backpower the A+/B+/Pi2 via the USB ports, albeit in a pretty useless way. Once the Pi has been booted via power from the microUSB or expansion header it is then possible to backpower from the USB sockets.

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It has always been possible to power a B through the GPIO. An unstated requirement is to ensure the supply is well regulated and not activate the overvoltage/transient protection diode. The recommended practice for HAT is to use an isolation circuit (similar to that on the B+). On the B+ USB can use 600mA/1200MA (depending on settings).

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