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I've got a rudamentary range extender system set up using two raspberry pi 3s and an Ethernet cable. One of the pi's is in a hallway with a clear signal from the original WiFi network, and the Ethernet cable runs along the wall to the living room where the signal was weak to the second pi which is broadcasting a new WiFi network. Now here's where the question starts - I've got both boards running off of one power cable, as I have wires running from the 5v and ground gpio pins taped along the Ethernet cable and connected to the 5v and ground of the other pi (yes I know this isn't ideal, it's a calculated risk). Problem is, while this worked just fine in close range tests, it appears enough current is being lost through the longer power line for the second pi to not receive enough (red light is on when the network is not being used, but as soon as there starts to be a bit of a load the light goes off). I've thought of a couple solutions but want to make sure I'm not frying my Pi.

1 - as the pi 3 has two 5v pins, could I run a second power line to increase current carrying capacity? The question here would be does using the second 5v pin result in a 10v connection or would it just double the wire connecting the two 5v rails? If it just doubles the wire would I also have to use a second ground wire?

2 - what about the 3.3v pins? I know the 3.3v rail is also used to power things like the LAN, so if I ran the 3.3 to 3.3 would that just bridge the two 3.3v rails resulting in more power to the networking (as you recall the red light goes off when the WiFi is being used intensively) or would that result in an effective 8.3 volts and fry the board? If it would work, as with option 1, would I need a second ground wire?

3 (probably the most dangerous) - this one screams nope at me but is worth mentioning - I've got two power adapters that are both insufficient to power the Pi. If I connected one to each in addition to the gpio powerline, would bridging the two 5v result in sufficient power or just overload the Pi's?

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"current is being lost" indicates a poor understanding of circuit theory. Current flows through the whole circuit, it cannot be lost.

Your problem is high resistance in the cable i.e. voltage drop, and unfortunately there is no simple fix. You could use heavier gauge cables, but these would have to be impractically large (and expensive). It doesn't make any difference how many pins you use on the Pi.

Whatever you do DON"T apply power to 3.3V pins, the Foundation warns "Under no circumstances should a power source be connected to the 3.3V pins." See Raspberry Pi Power Limitations

Running 2 switch mode power supplies in parallel is not recommended, and the results unpredictable. You should independently power each Pi (ensuring the earths are connected), although if the only connection is through Ethernet this is unecessary.

  • Building off of the last paragraph, it's completely unclear why you don't just plug the second Pi into the wall. Even if you have to run an extension cord, that would be better than what you're doing. Tape that along the Ethernet cable instead. – Brick Oct 10 '17 at 3:52
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To answer your questions:

  1. Yes, you could use the 5V pins. It's not recommended since your are bypassing the polyfuse, but it's possible. Using both 5V pins will only increase the carrying capacity of the cable. They are on the same single 5v rail on the pi. To understand how to keep two pins at 5V and not create 10V, I suggest to try to learn some basic electronics, especially learn about series and parallel circuits.
  2. Do not use the 3.3V pins for anything other than powering peripherals.
  3. Buy a decent power supply and forget this. You will not overload the Pi's, but it's just not a fashionable way of powering anything. Dual power supplies are often used for redundancy, not for generating the needed capacity.

TLDR: Don't do it. Get a decent power supply.

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The idea sounds great ... (I tried to power my pi through a PC PSU) But its risky and a normal high quality psu is what you need ... just for you to know you can power the raspberry pi using its GPIO as you can see here : http://www.instructables.com/id/Make-an-Alternate-Raspberry-Pi-Power-cable/

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