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My Pi is destined for the skies, so it will receive power at roundabout 5.3V via the BEC (battery eliminator circuit) which will be capable of delivering up to 10A (heavy duty BEC it is), so sourcing the actual power will not be a problem.

Also not a problem is splicing a Micro USB male cable for delivering power the normal way.

However, my Wifi adapter, a TP-LINK TL-WN722N may not be fully powered due to various factors if the Micro USB plug is the only source. The selling point is "High Gain"! I may need it, there might be some distance to cover, after all.

So I wonder what the best approach is? Should I send power to the Pi simultaneously via the 5V pin on the GPIO header and the Micro USB cable? Should I build a modified USB A male plug (or splice the connection in such a way) that will pass the data pins through to the Pi but divert the Wifi module's 5V USB power straight to the same source of 5V power as the USB cable? Should I make all three connections?

My hope is that the 5V GPIO pin should be directly wired up to the USB 5V supply that goes to the WiFi adapter.

So the question then would be whether it is the case that as long as I also have the Micro USB plug also providing the power, (and not with the GPIO being the only source of power) that I should no longer be at risk of frying (or distressing) anything.

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"Should I send power to the Pi simultaneously via the 5V pin on the GPIO header and the Micro USB cable?"

No. If you are supplying via the 5V pin on the GPIO header there is no need for any other connection.

"Should I build a modified USB A male plug (or splice the connection in such a way) that will pass the data pins through to the Pi but divert the Wifi module's 5V USB power straight to the same source of 5V power as the USB cable?"

This would work - make sure you connect the ground to both the power and data.

If you are going to do the 1st, you might as well just bypass the polyfuse on the Pi and use the Micro USB cable.

NOTE1 5.3v exceeds the specification for USB (and the Pi).

NOTE2 the Micro USB feeds all the 5v via the polyfuse. Bypassing this effectively prevents the Transient Voltage Suppression Diode from functioning.

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If you Pi is a Model A or Model B Revision 2, then you should be fine providing power only to the +5V GPIO. If your Pi is a Model B Revision 1, then you'll need to either short out the 100 mA polyfuse, or use a hacked USB cable as you suggest.

If you bypass the Micro USB connector (by powering over the GPIO) you increase the risk of frying regardless because you no longer have the fuse or voltage stabilization that the Pi provides by default. If your power source is pretty stable though, it's probably safe enough.

  • The general consensus is that I'll want a powered USB hub to attach the TL-WN722N with, since it will draw more than 300mA. That adds more bulk than I need and I only need to power this one device. What would be the most simple circuit that serves the role of a powered USB hub? Connect the ground but serve 5V to the adapter independently? – Steven Lu Apr 21 '14 at 6:30
  • Oops, Gerben's comment's answer hit the nail on the head. I'm doing that. Oh and I noticed Milliways' answer also mentions this. – Steven Lu Apr 21 '14 at 6:32

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