Here is the image of Ethernet shield near RJ45 socket:

enter image description here

These small boxes are probably SMD capacitors which may mean, that internal board is glavanically disconnected from the UTP line.

Is this true?

If this is true, then what will happen, if I pass 5V DC current just into my UTP? Won't I be able to wire appropriate conductors just into Arduino power input? Won't I be able to power Arduino this way?

The question is about not using premade PoE splitter. I have an injector, which injects 5V into line.


I found my injector put power to lines 4,5 and GND to 7,8. Can I take this power on Arduino side?

1 Answer 1


Not exactly. That RJ45 connector labeled HR911105A is internally connected like this: scheme

It's possible with PoE version (but I suppose not with 5V but more like 12V or so)

Or there are variants without PoE module connected but with support, that should be possible to use 5V if you wire it correctly)

Or you can use external PoE injector and extractor: injector/extractor

For 100Mb networks are two unused communication pairs used as power lines: enter image description here

  • What is the purpose of connecting 4,5 and 7,8 on your first schematic? Lines are unused, then why to connect them this way?
    – Dims
    Aug 25, 2016 at 17:26
  • HR911105A schematics just means I can't reach my power lines from board side, right?
    – Dims
    Aug 25, 2016 at 17:29
  • 1
    Yes, you can't, but if you use injector/extractor from the secon picture, it doesn't matter in this case. Just don't forget wires are not suited for much current. (that's why some injectors are using 24 or 48Volts)
    – KIIV
    Aug 25, 2016 at 17:32
  • You could replace the RJ45 socket with a different one (-;
    – Gerben
    Aug 25, 2016 at 18:09

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