Phoenixfury26
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Bouncing an idea.(wire power through spare ethernet wires)

Thu Jan 24, 2019 12:36 am

Someone, please correct me if I am wrong, has anyone considered running jumpers from 2 spare pins on the Eth port to the underside connectors of the power connector? I am struggling to see why this would not work but I do not have the spare money to risk burning out one of my pi's.

My thinking it, I could spice the other end of the eth cable to make sure I was running the 5v up the right lines and then pull the power through to the USB connectors.

I feel I might be missing something that is connected to the 2 spare pairs of eth cables but given the pi does not have Gbit I would be surprised if there is anything connected.

Any advice or education would be greatly appreciated.

Thanks in advance,
Chris.

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Re: Bouncing an idea.(wire power through spare ethernet wires)

Thu Jan 24, 2019 1:50 am

That's called "Power over Ethernet" (PoE). It normally uses a lot more than 5v. It requires an "injector" as part of the Ethernet switch and suitable hardware at the device end.

Now for the good news... The Pi3B+ has a special 4-pin header that, combined with a special HAT, supports PoE. So just get a Pi3B+ and a PoE HAT and Robert is one of your parents brother.

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Re: Bouncing an idea.(wire power through spare ethernet wires)

Thu Jan 24, 2019 3:20 am

There are plenty of 'off the shelf'' PoE injectors/ splitters out there. If you have a 3B+ then make sure you get one that will work with Gb Ethernet.

If you want to DIY then you will have to make a voltage injector at the hub/switch/router end. Then split off the power wires BEFORE the RPi, add an adapter to convert to 5V, then feed 5V into the RPi micro USB power port. I don't think you can DIY a Gb Ethernet PoE however.... So if RPi 3B+ you will have to use 10/100 Ethernet.
Unless specified otherwise my response is based on the latest and fully updated RPiOS Buster w/ Desktop OS.

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Re: Bouncing an idea.(wire power through spare ethernet wires)

Thu Jan 24, 2019 7:22 am

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Ernst
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Re: Bouncing an idea.(wire power through spare ethernet wires)

Thu Jan 24, 2019 7:31 am

Phoenixfury26 wrote:
Thu Jan 24, 2019 12:36 am
Someone, please correct me if I am wrong, has anyone considered running jumpers from 2 spare pins on the Eth port to the underside connectors of the power connector? I am struggling to see why this would not work but I do not have the spare money to risk burning out one of my pi's.

My thinking it, I could spice the other end of the eth cable to make sure I was running the 5v up the right lines and then pull the power through to the USB connectors.

I feel I might be missing something that is connected to the 2 spare pairs of eth cables but given the pi does not have Gbit I would be surprised if there is anything connected.

Any advice or education would be greatly appreciated.

Thanks in advance,
Chris.
Note: What the previous posts do not mention is that POE uses a higher voltage (upto 52V) to overcome losses in the cable and uses special circuitry to "transmit" the power. https://en.wikipedia.org/wiki/Power_over_Ethernet

What you mention does exist already as "passive PoE Injector and Splitter kits" are available to provide power to telephones, security cameras, etc. https://www.amazon.com/BeElion-Passive- ... r+ethernet
The problem with this is that there will be a large voltage drop over the length of the cable and the 5V input does not arrive at the output, a higher voltage is needed with some kind of regulator at the end to ensure that the Pi gets the 5V needed and does not get fried by the higher voltage.
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Phoenixfury26
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Re: Bouncing an idea.(wire power through spare ethernet wires)

Thu Jan 24, 2019 10:04 am

Thank you all for the information.

The PoE Phat is what gave me the idea but it is quite a large block on top and I have my pi's stacked on top of each other so this would double the stack size, plus at £24 each I can't really justify picking up 5 of them.

As for the voltage drop, I was thinking the cable length would be less the 12". If I supplied maybe 6v up the cat5 cable and then checked the voltage at the pi end could I add a resistor to the jumper from eth port to power or as Ernst said, a small regulator circuit?

The thing I would like to know is, power supply aside, is there something in the pi's hardware that would prevent this?

Again, thank you in advance,
Chris.

mfa298
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Re: Bouncing an idea.(wire power through spare ethernet wires)

Thu Jan 24, 2019 10:15 am

Phoenixfury26 wrote:
Thu Jan 24, 2019 12:36 am
Someone, please correct me if I am wrong, has anyone considered running jumpers from 2 spare pins on the Eth port to the underside connectors of the power connector? I am struggling to see why this would not work but I do not have the spare money to risk burning out one of my pi's.
The pins coming out of the Ethernet Jack don't map directly to the wires in the Ethernet cable. There's a bunch of transformers (magnetics) inside the Jack so what you see on the output of the jack is the outputs of those transformers. On a jack designed for PoE you either have extra pins or separate magnetics.

As Ernst hinted at, the wires in a network cable (cat5 or cat6) are relatively thin so aren't ideal for higher currents. If the cable was only a couple of meters you might be fine, if you're looking at something longer (10+m) then you'll likely have issues with voltage drop (which will vary depending on current drawn so you can't always just bump the voltage a bit). In did look at a project where this was an idea but after a bit of maths I decided it probably wasn't that viable (especially as the aim was to run several pis from the same power supply on differing lengths of cable)

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Imperf3kt
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Re: Bouncing an idea.(wire power through spare ethernet wires)

Thu Jan 24, 2019 8:39 pm

Cat-5 cable uses 26AWG wiring.
According to a voltage drop calculator, if you out 6volts into one end of a 12 inch cable, and you use about 1A, you will see 5.92v at the other end. Your Pi is toast.

If you absolutely max out the Pi and use multiple high drain USB devices so the current draw is 2.5A, the Pi will get 5.8v - your Pi is toast.

This is assuming you use one wire for vcc and one for ground return. If you twist more wires together, you'll get less voltage drop, approximately half if you twist two together.


But these numbers are based on copper, which I doubt ethernet cable is made of.
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JohnsUPS
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Re: Bouncing an idea.(wire power through spare ethernet wires)

Fri Jan 25, 2019 2:52 am

If the cable lengths are going to be rather short (OP mentioned ~12" or so), then voltage drop won't be a problem.

The Pi will work just fine on 10/100 Ethernet, which uses wires 1, 2, 3 & 6 for transmit & receive.

If you want to wire your own "poor man's POE" as I like to call it, just use the remaining wires, splitting them out on either end. When crimping on the ends, only insert the four wires (1, 2, 3 & 6) into the RJ45 plug before crimping (on either end).
I have seen commercially made (cheap, of course) patch cables that only have four wires in them.
If you want to incorporate a bit of voltage drop insurance, you can always put a voltage regulator at the end by the Pi. Just make sure that the regulator can handle the expected current draw, and that the voltage drop across the wire won't droop below the regulator's minimum operating input voltage at the maximum expected current.

If you have a POE switch, then the best option is either a POE HAT or a POE splitter that drops the voltage down to 5 volts. For regular POE, a 5VDC splitter should be able to provide about 2.4 amps (~12watts) at the far end. If everything is 802.3af or 802.3at compliant, then you should be able to run as far as 100m. You could also use a POE injector on one end (in lieu of a POE switch), and a POE splitter on the other. The overall goal is to minimize voltage drop across the Ethernet cable (which is specified to be able to work up to 100m) by raising the voltage prior to impressing it on the line.

Also keep in mind that you can't just plug a RJ45 plug into a POE switch and expect to measure 48V anywhere. A POE switch won't apply power to the wiring until a test is performed that validates that the end device is POE capable.

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