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Author Topic: POE: Power over Ethernet confusion  (Read 3069 times)

techidave

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POE: Power over Ethernet confusion
« on: October 01, 2011, 07:56:12 PM »
I am confused about using POE devices.  Does the device like an access point, need to be specially made for POE?  I understand how a POE switch or POE injector pushes power over the ethernet cable.

What I don't understand is how the device on the other end takes the power from the ethernet cable to actually power the device.

I am sure someone here can clarify this for me.

Dave

4wd

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Re: POE: Power over Ethernet confusion
« Reply #1 on: October 02, 2011, 02:06:44 AM »
The device has to either be made to utilise PoE or you can use a breakout box to separate the power feed from the Ethernet cable before connection to the device.

Here's an example on ebay along with a connection diagram: PoE Passive Injector/Extractor

poe example2.gifPOE: Power over Ethernet confusion
« Last Edit: October 02, 2011, 02:10:17 AM by 4wd, Reason: Attached the image just in case it falls off of ebay. »

techidave

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Re: POE: Power over Ethernet confusion
« Reply #2 on: October 02, 2011, 05:32:12 AM »
Thanks, 4wd.  I was thinking I would need something on the device end but didn't know what.  When I googled it, I came up empty.  But didn't know what I would need is called a POE extractor.   :-[

40hz

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Re: POE: Power over Ethernet confusion
« Reply #3 on: October 02, 2011, 07:25:14 AM »
4wd beat me to it! ;D (4wd - the Man! :Thmbsup:)

There's a really good article on  DIY PoE projects done by the NYCwireless non-profit. It shows diagrams, photos, etc. And it provides everything you need to roll your own - or provide you with enough technical detail to be able to better buy one. For a one-off project, it's probably easier and not much more expensive to buy an injector/splitter combo unless you have an adequate quality 'wall wart' lying around. Link to article here.

Important Note: If you do build your own, pay close attention to voltage drop since sending DC current over any distance can create a fire hazard if you exceed safe maximums. Especially with something as thin as the 24AWG wire your standard CAT-5/6 cable uses. And if you don't know enough about electricity to know what voltage drop is, you probably shouldn't be building your own. ;)

PCRush has a good selection of PoE components on their website. They carry all the majors, so you can either shop there or use their site to get tech specs and comparative pricing numbers.

Luck! :Thmbsup:

-----------------------
Standard disclaimer: 40hz has no affiliation with PCRush.

4wd

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Re: POE: Power over Ethernet confusion
« Reply #4 on: October 02, 2011, 07:50:01 AM »
4wd beat me to it! ;D (4wd - the Man! :Thmbsup:)

The advantages of being in the light while ya'll in the dark ;)

Quote
Important Note: If you do build your own, pay close attention to voltage drop since sending DC current over any distance can create a fire hazard if you exceed safe maximums. Especially with something as thin as the 24AWG wire your standard CAT-5/6 cable uses. And if you don't know enough about electricity to know what voltage drop is, you probably shouldn't be building your own. ;)

Very good point  :up:

40hz

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Re: POE: Power over Ethernet confusion
« Reply #5 on: October 02, 2011, 08:33:03 AM »
Hmm...just thinking out loud for a minute...

You could also drag a length of inexpensive  interior-rated low voltage lighting (or doorbell) wire up to your remote point, splice on a plug, and use that for your DC supply. If you're pulling CAT5, it's not that much more effort (or that much bigger a hole) to bring some additional wire along for the ride. You could even run a second length of CAT5 along (you never know if you'll need a spare anyway), solder two sets of two wires together, and use those for your DC.

You'll still need to pay attention and calculate for voltage drop with whatever gauge wire you end up using. But it may be an easier thing to do than having to muck about  with a fancy PoE solution.

The only real advantage to PoE is that it uses a single wire bundle. So it's neater and easier for the installers. (Especially if the data cable is already in place.) And also a bit cheaper since PoE uses a technically "unassigned" set of wires you've already paid for.

That may be an important consideration when you're wiring up 20,000 square feet of office space in a city high-rise. But when you're only running a hundred or so feet of 'copper' in a standard residential wood frame building, the savings in wire really isn't all that much. So it's more a matter of convenience.

But again, if you're pulling a new wire, dragging something along with it at the same time doesn't add much work to the project.

Something to think about. :)


Shades

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Re: POE: Power over Ethernet confusion
« Reply #6 on: October 02, 2011, 09:06:58 AM »
Unfortunately there are no tricks to counter the DC voltage power drop...impedance only works with signals on a carrier wave (proved by your TV cable provider). 

AC power does not have so much trouble with this 'drop' effect (within commonly used voltage ranges).