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Because I live in a bus, I use paid-for hotspots to connect to the internet. Sometimes I'm quite far from a hotspot, so I was thinking to use a raspberry-pi with both an ethernet connection and a wifi, attached to a long ethernet cable as my router. Now my questions are about the powersupply and possible interference with the ethernet cable. So, can I use a Cat5e and use the blue or brown wires to provide power over the same cable ? (Which I would then have to separate from the rest before they go in the RJ-45 connector, because I don't suppose the raspberry would support PoE)

Or would it better to be using a seperate ethernet cable for the power supply ?

How much Power would I need to provide at the source in order to still have 5 Volts over i.e. a 30 m ethernet cable ?

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  • \$\begingroup\$ PoE devices use a buck converter at the receiving end, so if you are using PoE you shouldn't worry about voltage drop as long as you stay within spec. That said, I think it would be just easier to provide a separate cable with say, 12V or 24V (not necessarily ethernet) and a cheap buck converter module with the RPi. \$\endgroup\$ – Wesley Lee Aug 18 '17 at 18:27
  • \$\begingroup\$ Wouldn't it be easier to use a cell connection, since you're already paying for the hotpots? \$\endgroup\$ – Bort Aug 18 '17 at 18:28
  • \$\begingroup\$ You can use PoE with your pi - just use a PoE splitter/receiver device at that end to separate the Ethernet from the power so you can feed the Ethernet to the pi and power to a 5V buck converter PSU for the pi. \$\endgroup\$ – brhans Aug 18 '17 at 18:36
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    \$\begingroup\$ First you should consider buying a ap or router with ap mode before making one out of a rpi. \$\endgroup\$ – user3528438 Aug 18 '17 at 19:19
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Yes, you can (ab)use the unused pair of wires in cat5e to provide power. A DC power supply would not interfere much with an Ethernet connection. You can buy passive POE injectors that take care of the wiring for you. Or an active POE to 5V adapter if you have real POE available.

But you need to calculate the voltage drop. Cat5e is 22 to 26 AWG wire, and a 30 meter one way run will have a significant voltage drop. Using a calculator and 24 awg at 180 feet round trip. A 1A load at 6V source will mean a 4.5V drop at the device end. Not very good. You would need a higher source voltage, and due to the varying load, a voltage regulator at the device end. 12V with a 12V to 5V car usb adapter would work lovely.

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  • \$\begingroup\$ No-one would realistically design a solution that had 4.5 V of droop on the wires. The voltage drop is of course defined by the current flowing and during boot of a remote MCU the voltage may vary quite markedly. There's a reason the 802.3af or at standard uses higher voltages ...and voltage droop is one of them. \$\endgroup\$ – Jack Creasey Aug 18 '17 at 22:18
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You can certainly power a 'Pi over POE without any impact at all on the Ethernet signals.

You can roll your own based on a couple or POE injectors, which I don't advise or use something more professional that actually meets POE standards. There are multiple POE 'Hats' for this purpose, her is one which has onboard brownout detection for consistent resets. This unit is designed for 48 V source supply.
The higher the POE voltage, the lower the cable current, so I'd recommend at least 24 V at the source end of any roll your own.

Units such as the Pi POE Hat solve a slew of problems you might potentially have with hangs and resets on a roll your own configuration. If anything goes wrong at the end of 30 m of cable, manual resets are not what you want.

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  • \$\begingroup\$ Thank you, that's a very valuable comment. I will probably choose for your solution in practice, but I was also happy to learn about the voltage-drop-explanation from passerby :-) \$\endgroup\$ – oneindelijk Aug 18 '17 at 19:33
  • \$\begingroup\$ You don't advise them why? \$\endgroup\$ – Passerby Aug 18 '17 at 21:25
  • \$\begingroup\$ I don't advise roll your own POE (typically done with lower voltage power supplies)as it's easy to end up with too much current in the wires, and typically users tend to use lower quality power supplies instead of units designed with the correct isolation. A properly designed POE endpoint is both intelligent, which improves reliability; and features excellent isolation from the source. \$\endgroup\$ – Jack Creasey Aug 18 '17 at 22:08

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