Page 2 of 2

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Tue Sep 06, 2011 6:04 pm
by __Miguel_
Hmm, maybe I'm missing something here, but I have read about PoE and DIY versions of it a while back.

Long story short, and I think this was what the OP was getting at, the easiest, and possibly cheapest, way to achieve PoE on home environments is to just NOT crimping the unused pairs on a 10/100 run, wiring, at one of the ends of the run, one pair to the + line on the power converter, the other pair to the - line, and doing the same thing at the other end of the run, this time using the device lead.

Two quick tutorials (for both single cable or two small patch approaches) are here and here. Also, there is at least one Instructable on the subject.

Yes, blue smoke is still possible, but when isn't it, when you're DIYing? :P

CHeers.

Miguel

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Tue Sep 06, 2011 7:52 pm
by Svartalf
Quote from __Miguel_ on September 6, 2011, 19:04
Yes, blue smoke is still possible, but when isn't it, when you're DIYing? :P


Yeah, it is...but over the years, I try to take care, unless it's not a risk at all, to avoid a bluesmoke event.

I save those for doing stupid things like reversed electrolytics, quartershrinkers, and the like. Makes the mad science puttering more fun when you get more bang/bluesmoke for your buck... ;)

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Wed Sep 07, 2011 9:43 pm
by JRBeaman
Excuse me for butting in here, but it would seem a simple 4-diode bridge rectifier on/near the RasPi would solve the polarity protection issue. 1-Amp die are available at tiny prices. Feed that into the v-reg chip should give you what you need for a very low price.

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Sat Sep 17, 2011 11:25 am
by scologic
There are a number of POE adapter modules on the market and voltage divider is essential(is it 15000 volt protection or something like that) and your nuts working on an system without it already being approved - remember safety first, and youve got EMC compliance to consider also. But for the sake of a tenner - honestly...(and thats from a tight fisted scotsman)

POE sends 48v from the POE host and then you use a 5v or 12v splitter. All of which is nuts to try and recreate.. i think my single unit price for a high end module was £14 with divider, but it also had a secondary power socket option, and auto detected if POE was available or the power connector was supplying the juice.

if you need the detail pm me and I can dig it all out again

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Tue Oct 18, 2011 8:08 pm
by jewesque
There was an article long ago in 2600 if I'm not mistaken where a guy had the same exact issue. He wanted to power a wireless access point over the ethernet cable because he was putting the AP into a weathertight box and then hiking that sucker up on a 20' mast over his house so he could get long range outdoor connectivity on his property.

Well, as you just figured out, this is the issue with this, he didn't need more holes in the project box and also couldn't run a 30' power cord to the box as it was. So he pulled the power wires out of the ends of the ethernet sleeve, and then on the inside end connected them to the device's spliced power supply, and on the other end, to the power supply connector. Voila, power over ethernet for the redneck! ;)

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Tue Oct 18, 2011 8:28 pm
by nullstring
(dup)

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Tue Oct 18, 2011 8:30 pm
by nullstring
Quote from abishur on August 26, 2011, 15:18
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.

For the record, you can't just use "a couple of resistors" to lower the voltage down. It doesn't work this way.

The correct way to do this would be to use a transformer. A switching regulator would also work.

You could use a voltage divider circuit to lower the voltage, but this is really not the best way to do this. http://en.wikipedia.org/wiki/V.....ge_divider
You'll waist a ton of energy going back to ground... You'll end up using double or triple the energy powering the device.

^Sorry, thats wrong because the device idling would be using alot less than full load. Anyway, you will waste alot of than you'd want to.

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Wed Oct 19, 2011 5:09 am
by abishur
actually, you could. It's not recommended by 4 out of 5 dentists, but if I was working with a 24V source, it would be fine.

Regardless, since this is a DIY solution, it would be just as easy to use a power supply already in the 6-20V DC range.

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Wed Oct 19, 2011 4:06 pm
by Neil
PoE supplies -48V (yes, negative, that's telephone people for you *sigh*) on either the data pairs OR the unused pairs. The standard defines both, and some PoE hubs do it one way, and others do it the other way. Best to support both modes otherwise half the users will complain it doesn't work for them (yes, been there, done that, got the T-shirt, etc)

The analogue protocol of PoE sniffs the device at the other end of the UTP cable with a small voltage first, measuring the signature resistance at the other end (in the device) which should be 25k. If the resistance is not within a certain range (too high or too low) no power will be delivered. The device can then optionally request a maximum current by setting its identification resistor (different to the identification stage) to a specific value.

PoE power should be fed to the device through some sort of isolation, the same as for the data lines. This is typically done with an isolating DC-DC converter to drop the 48V down to a more useful voltage (5V, 3V3, etc). I've used Linear chips in past projects - I think it was the LTC4267 (but that was, ooh, 6 years ago now so I may be wrong).

If done carefully you can use the same chip to handle power inputs from USB and power jacks (5-24V) as well.

I guess if the device remains completely enclosed (e.g. netcam) then you don't need to isolate the supply, but for anything else you do need it for safety as mentioned above. Certainly for something like the Raspi. It will also be important to enclose the high-voltage converter in some sort of protective enclosure -- having a flyback converter running off 48V within finger-reach is a guarantee of a zappy time.

Neil

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Wed Oct 19, 2011 7:01 pm
by abishur
Yeah, that's why, as I keep mentioning :P , I had already figured the project should go more of a diy route for the POE source (i.e. something like this. Now in that link it's using the POE to power the hub, but I was thinking about doing something similar (which is to say, not 100% the same). My idea at this point is to get a single wall wart rated at 12V <1A. My reasoning is that 12V is a common voltage and will still be in the 6-20V DC range after any voltage loss caused by distance, and 1A would be capable of supplying power to 3 r-pis even under full load (Alpha board was reported to use ~180mA idle and ~300mA under high load) with a little wiggle room left over and anything above 1A would allow me to use more than 3 pis.

Anyway, so you take the wall wart, and attach however many wires you want to the + and - sides in Parallel the other side you can either attach by splicing the ethernet cable directly (which I think would look a little ugly) or by attaching it to a coupler of some sort (I was thinking about using those snap in couplers and putting them in a straight line enclosure, that's how I spell classy! ;) ). Then on the other end you reverse the process, if you do it right, then the power wires are physically disconnected before it reaches the router or the r-pi. I've seen some people actually wire the power directly onto the router, but unless I knew for a fact that the router could handle that, I'd want to use one of the methods above for safety. Also, I might even put a little fuse break between the PS and the connectors to protect from over voltage and to protect the PS in case one too many r-pis are placed on the line.

Of course, I realize that all this could be done much more easily by using an injector, but the point of this is DIY! Using a pre-made injector is just plain boring!

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Wed Oct 19, 2011 8:38 pm
by jewesque
So guys, just in case ya'll wanna love me now, here's a nice piece of hardware I just stumbled across for way less than you can even build them for. Obviously I haven't bought and tested it yet, but yeah. At $3.99 for an injector and splitter, you can't do better than that.

http://andahammer.com/poeis2/

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Wed Oct 19, 2011 8:54 pm
by abishur
Is the link right? It doesn't seem to be working for me.

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Wed Oct 19, 2011 8:56 pm
by jewesque
It's right, it seems that their site went down almost right after I started exploring it - they sell a lot more ARM related stuff on their home page - I guess now we just need to wait until the site comes back up.

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Wed Oct 19, 2011 8:58 pm
by jewesque
It seems that what we're going to be looking for here is going to be a "passive PoE injector" set which includes the splitter as well. There are tons of those for next to nothing available.

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Sat Oct 22, 2011 12:38 am
by abishur
So I looked at that link you provided, and it's exactly what I'm talking about doing on this thread ;) Of course, that's why he can offer it so cheaply, there's no sort of voltage control or fuse protection.

So now the power supply has been decided and it's a strict 5V. As such I'm a little weary of continuing this idea. It was one thing to be a voltage range, where it was less important if there was voltage loss due to distance, but it's another thing when it needs to be a consistent voltage, that would require a higher beginning voltage with 5V regulator... that would still be cheaper than buying a pre-made one, but a step farther than I'd want to go. :P

Re: Power over Ethernet (Difficulty: Moderate)

Posted: Sat Oct 22, 2011 3:22 am
by nullstring
nvm