Power over Ethernet (Difficulty: Moderate)

[abishur]

Project Scope: Use POE to power the r-pi

Project Materials:
R-pi
POE capable switch
RJ-45 Coupler
Coax power plug and wire
Soldering Iron
Solder

Project Description:
We know that at this point in time the R-pi is not capable of being powered off USB, but why shouldn't it be? The goal of this project is to take a POE capable switch/router (or make one yourself, I've seen a few on Google), send power down the ethernet connection and then spice that to the power port!

Project steps
1) Set up your POE switch/router
2) Plug one end of ethernet cable to POE source
3) Modify RJ-45 Coupler
3a) Open up the RJ-45 coupler
3b) Find wires that POE sends power down and cut them!
3c) Drill a hole in the side of the coupler and slip in your wire coming from the coax power plug
3d) Solder the positive and negative wires from the coax power plug to the positive and negative wires in the RJ-45 coupler. then cover with tape
3e) Take the wires loose wires from the r-pi side of the coupler (the wires that were once attached to the POE power but we cut) and tape them off (we don't want them touching and causing problems)
4) Attach ethernet cable from r-pi to the coupler

In theory this should allow you to power the r-pi from a POE device and still also be able to receive data over the cord as well!

[pieter]

Sounds cool but...
POE (Wikipedia) is specified to deliver 37V(42V) - 57V. The Rasperberry (Wiki) can take 6v-20v (or 5v - 16v).

I don't know much about electricity but that seems like a bit of a problem

[abishur]

huh, that weird I could have sworn I had powered some 12V devices before. Oh, well. It's not actually an issue we'll just have to hide some resistors in the ethernet coupler to lower the voltage down to 20V. Hopefully the mA it sends will be enough for the r-pi, over in this thread Gert says the alpha boards use 300 mA at full tilt so 350 mA should be enough for it to work properly.

[Svartalf]

You probably powered them using the wiring spec or the 12v device had regulation to handle POE's actual nominal voltage (48v, to match all the telecom setups, as much to support VoIP phones as anything else... )

Simplistically, you can do POE by doing Mode B, basically stealing the dead, non signalling pairs, and using them to carry current for you. It's not real POE, but if you want that, it gets more complex, requires an injector and splitter, or custom PHYs on the remote end in addition to a switch or injector.

[abishur]

The not real PoE is exactly what this project is aiming for It uses a PoE source, then using the 4,5,7,& 8 pins and attaching them to a coax power plug. It's not true PoE, but it's a good and cost effective solution that still allows the ethernet cable to work as an ethernet cable.

I want a $25 computer, spending a bunch of cash for an injector/splitter seems silly to me

[Svartalf]

Quote from abishur on August 25, 2011, 19:59
The not real PoE is exactly what this project is aiming for It uses a PoE source, then using the 4,5,7,& 8 pins and attaching them to a coax power plug. It's not true PoE, but it's a good and cost effective solution that still allows the ethernet cable to work as an ethernet cable.

Then we're on the same page here...


I want a $25 computer, spending a bunch of cash for an injector/splitter seems silly to me

Depends on your goals with that $25 computer, really- that and whether you can reliably prevent some joker not getting their wiring on something wrong and mis-hooking up the power to one of your signal lines on the board.

[Michael]

Or some joker plugging your not-IEEE-PoE Raspberry Pi into a (IEEE) PoE switch and releasing the magic blue smoke from the Raspberry Pi. Given that IEEE complant modules are around $3-4 in bulk or £11 individually and only requires a few wires and three other components to hook up, I don't understand why anyone would choose the non-compliant route.
[img=http://img.directindustry.com/images_di ... 494280.jpg]module[/img]
[img=]link to wiring diagram[/img]

[abishur]

Quote from Svartalf on August 25, 2011, 23:00
Depends on your goals with that $25 computer, really- that and whether you can reliably prevent some joker not getting their wiring on something wrong and mis-hooking up the power to one of your signal lines on the board.

Fair enough I am assuming that with all these projects that there is a level of "you're doing this at your own peril" That also why I try to add a warning to the difficulty in the title... but I'm sure at least one person (me?) will mess up

[Svartalf]

Quote from Michael on August 25, 2011, 23:17
Or some joker plugging your not-IEEE-PoE Raspberry Pi into a (IEEE) PoE switch and releasing the magic blue smoke from the Raspberry Pi.


<chuckle...> I think I was implying that when I said the other.


Given that IEEE complant modules are around $3-4 in bulk or £11 individually and only requires a few wires and three other components to hook up, I don't understand why anyone would choose the non-compliant route.


Kits and pre-made circuit boards run about 2/3rds of the cost of the cheapest compliant injectors/splitter pairs- so, unless you're fairly skilled, you're not saving much- whereas you can go the lazy route and spend $2 on the whole lot.

I agree with your sentiment, really- but the problem lies in getting the lashup down to the $5-10 total cost range to keep people from contemplating a bluesmoke failure looking for a place to happen.

[abishur]

Quote from Michael on August 25, 2011, 23:17
Given that IEEE complant modules are around $3-4 in bulk or £11 individually and only requires a few wires and three other components to hook up, I don't understand why anyone would choose the non-compliant route.


Mainly because as long you follow the instructions I've outlined, then you'll be fine, the whole point is to take the power lines and make sure they're not even going to the r-pi anymore. Besides, most of us aren't going to need to order these in bulk, but being able to spend a couple of dollars to build this for the geek value or simplicity of powering devices over ethernet makes it worth it. You just have to use a modicum of intelligence, but that goes for any of these projects we've been suggesting.

[Michael]

I'm really hoping that for the Model-B, Gert will be able to accomodate the four through-holes for something like the above module, plus pads for the cap and two bridge rectifiers. Then they could offer a $5 PoE kit containing the four extra components needed. A $5 PoE kit compares well to a $5 PSU brick if you need the feature and have the appropriate soldering skills (or know someone who does).

[Lob0426]

I don't know though, the magic blue smoke sounds cool. And you can repeat it as long as you can afford $35 for each puff. LIKE high price electronic CRACK.

you could run something lower that is within the 6v to 20v that is already there (hopefully they don't change it to 5 volt). Modify a 10/100 router to carry it. and use it as a home based PoE. Still has to be done right. will you carry two positive and two negative (ground) lines on the open lines (4,5,7,8)? if you did you could "Y" into the lines as they enter the RJ45 connector and run then to the Barrel connector for power. They would never enter the board except through the regulator. I know this sounds kind of redneck, but a lot safer for the equipment. Much more likely to prevent the Magic Blue Smoke. You would have to assemble at least one end of the cable. No wires to the uneeded pins.

[abishur]

That's exactly what the original post is describing except I was planning on using a couple of resistors to lower the voltage down to 6-20V rather than using a 6-20V source.

[Svartalf]

Quote from Lob0426 on August 26, 2011, 05:09
I don't know though, the magic blue smoke sounds cool. And you can repeat it as long as you can afford $35 for each puff. LIKE high price electronic CRACK.


Heh... You can get that fix at slightly cheaper rates with feeding 2-3 times the voltage reversed into an el-cheapo electrolytic. Bonus is that it goes **BANG** at the same time as the bluesmoke event.

[Michael]

You must have been a nightmare at school

[Svartalf]

Quote from Michael on August 26, 2011, 18:29
You must have been a nightmare at school


<*chuckle*> Heh, I could have been accused of being one of those from time to time...

[Svartalf]

Quote from Michael on August 26, 2011, 00:43
I'm really hoping that for the Model-B, Gert will be able to accomodate the four through-holes for something like the above module, plus pads for the cap and two bridge rectifiers. Then they could offer a $5 PoE kit containing the four extra components needed. A $5 PoE kit compares well to a $5 PSU brick if you need the feature and have the appropriate soldering skills (or know someone who does).


That'd be a win. I could see a LOT of people taking you up on the option there. Not enough to have it default, but enough that it's probably worth the board real estate to provide for it.

[Lob0426]

Kits like that would ease people into hardware mods. It would be nice to have a complete mod kit for school projects. That would be a good next step after the release of the education models are figured out.

A hardware experiments and microcontroller kit.
Would it be possible to emulate a microcontroller experiment without the hardware?

[Bloodred]

Quote from abishur on August 26, 2011, 15:18
I was planning on using a couple of resistors to lower the voltage down to 6-20V rather than using a 6-20V source.
What do you mean using a couple resistors, a voltage divider? Voltage drop on a resistor depends on current (so it depends on what load the RasPi is under), so how do you plan on doing this?

[abishur]

Well it helps to remember that at this point in time the r-pi will accept a range of 6-20V DC. So I set up my resistor(s) to hit the 20V (maybe play it a little safe and aim for 19 or 18 V) mark. If the load increases a little (or even a good amount) it wll still stay above the 6V mark. If they change this, however it would be easy enough to create a DIY PoE source router using anything in the 6-20V range and not have to worry about resistors

[Lob0426]

I don't know abishur, but if you make the magic blue smoke I want pictures. lol

Fixing up a 10/100 router shouldn't be to hard. There are two unused pairs. You would have to cut the ties to the board for those as on my router they are all soldered in. How do they carry the power on a Gigabit PoE? Over the power wires and then have a regulator to drop the voltage or does it all just run at 48v?

[Bloodred]

Quote from abishur on August 30, 2011, 00:10
Well it helps to remember that at this point in time the r-pi will accept a range of 6-20V DC. So I set up my resistor(s) to hit the 20V (maybe play it a little safe and aim for 19 or 18 V) mark. If the load increases a little (or even a good amount) it wll still stay above the 6V mark. If they change this, however it would be easy enough to create a DIY PoE source router using anything in the 6-20V range and not have to worry about resistors
I know that should theoretically work well enough since the RasPi has its own Vreg(s) but I'm not too sure it was meant to function with a wildly fluctuating power supply. It should be fine but I don't think I'd do something like that with my RasPi. I'd much rather just add another Vreg and maybe a filtering cap or two somewhere on the power lines.

[abishur]

Quote from Lob0426 on August 30, 2011, 00:33
I don't know abishur, but if you make the magic blue smoke I want pictures. lol

Fixing up a 10/100 router shouldn't be to hard. There are two unused pairs. You would have to cut the ties to the board for those as on my router they are all soldered in. How do they carry the power on a Gigabit PoE? Over the power wires and then have a regulator to drop the voltage or does it all just run at 48v?

Well, the good news is that the r-pi doesn't support gigabit so we've dodge that bullet for now But if you read wikipedia's page on PoE, they describe the Phantom power which allows the power lines to also carry data (I don't get the deep details of it myself, but apparently it works!)

[Svartalf]

Quote from Lob0426 on August 30, 2011, 00:33
I don't know abishur, but if you make the magic blue smoke I want pictures. lol

Fixing up a 10/100 router shouldn't be to hard. There are two unused pairs. You would have to cut the ties to the board for those as on my router they are all soldered in. How do they carry the power on a Gigabit PoE? Over the power wires and then have a regulator to drop the voltage or does it all just run at 48v?

Heh... Per the standard, it runs 48v. That's why we're talking about bluesmoke events if you don't wire it up according to the spec and play by the rules...

[jdarling]

My suggestion would be a standard POE interface and then feed the output into a MIC2941AWU low-dropout voltage regulator. With a bit of cheap trickery (read two standard 2n2222's) you could easily add battery backup functionality off a standard 9v batter. Kinda like your alarm clock

PS: I'm well aware that the 2941 has a peak voltage input of 26v, but that's what your standard interface does for you, brings it down to the 20v range. Also the 1.6a (peak) 1.25a standard current rating makes the regulator worth the few extra components.

- Jeremy