PoE for $5 ?

8 posts
by jbeale » Thu Jul 19, 2012 4:27 am
I'm not a power-over-ethernet expert, but would this cable set really work for a PoE solution for $5 ? (plus a power supply, of course)
http://www.seeedstudio.com/depot/passiv ... ?cPath=207
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by thelightguy » Thu Jul 19, 2012 4:41 am
You also have to factor in a 48VDC power supply for the injector and something to step that down to 5VDC for the Pi.
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by Burngate » Fri Jul 20, 2012 9:31 am
Yes it'll work, but ...
1) the Pi end will need some work to get the power out of the coax plug into the micro USB socket on the Pi
2) depending on cable length, you might find 5v injected in one end comes out rather less at the other. That's why proper poe uses 48v - higher volts means less current means less loss due to resistance. But then you'll need a dc-dc converter of some sort.

If the Pi is at the top of a tree and the power source at the other end of the garden, it'd probably be worth it.
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by piglet » Fri Jul 20, 2012 9:58 am
Burngate wrote:you'll need a dc-dc converter of some sort.

They had one on that same page: http://www.seeedstudio.com/depot/cptc5- ... -1066.html
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by nil0lab » Sat Aug 04, 2012 3:28 pm
25 watt will power quite a few raspberry pi.
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by derekk19 » Sun Sep 16, 2012 2:13 am
I would probably start with a regular ethernet cable then I'd cannibalise a micro-usb cable and graft it into the ethernet cable - I saw a sparkfun page explaining how to do this. Then I'd go with a midspan injector (I already have one for powering an arduino). At the moment my injector is outputting 9v so I'd look at running that straight through to the Raspi. I would first check that the Raspi can cope with 9v input. If it couldn't I'd use a voltage reducer (again sparkfun have such a part and I've used them in the past for arduinos). In my case the Raspi is only a foot away from the midspan injector so voltage drop is not an issue - all I'm saving is one power adaptor. I realise I've now gone over the $5 cost limit and am probably looking at $20, though I do have some of the parts already. I'm thinking of going this way in the next few weeks so I'll keep you posted how I get on.
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by derekk19 » Sun Sep 16, 2012 3:50 am
My solution (though it would cost more than $5) is:

Freetronics 4 channel power over Ethernet mid span injector $39.99 NZ. I already have one of these driving an Arduino over Ethernet. It's outputting 9v on the Ethernet cable.

Homemade Ethernet to 2.1mm barrel plug? There are various How-tos on the Net for this. Basically you break out the power wires from the cable and route them to a barrel connector, or you use the cable shown the OP.

I'd use Sparkfun's breadboard power supply to take my 9v and step it down to 5v. It takes the barrel plug as its input. Fortunately I have one of these too, so can save myself a further $16.87 NZ

I'd cannibalize the micro USB plug from an old cable, since buying and wiring one from scratch is not trivial - the connectors are very small. Then I'd connect this cable to the 'out' on the power supply.

My Raspi is a foot away from my mid span injector so voltage drop is not an issue. I could go up to 12v, which my breadboard power supply can deal with, then I think I'd be good for a bit more distance, maybe 10m

I'm thinking of putting this into practice in the next few weeks. Really it's not going to save me much, just one wall wart power adaptor.
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by jbeale » Mon Sep 17, 2012 5:08 am
If you believe the specs, $5 will get you this 48V -> 5V converter:

http://www.ebay.com/itm/DC-DC-HRD-Conve ... 0777752555
Input voltage: 8-50V
Output Voltage: 5V
Output current: Rated current 3A
Conversion efficiency: Up to 80%
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