Power over Ethernet power ratings

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funkyguy4000

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Aug 14, 2011
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Hello,

I am attempting to power a board that requires 2 amps at 5V to run. I would love to do this over PoE although I've never done PoE and quite frankly I don't know if it can handle that.

A quick wikipedia search shows that PoE can handle about 31.4W of power so I would assume 5*2=10W would be just fine although seeing as how I've never worked with it before, I just want to make sure if this is alright.
 
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If you are building something with PoE to sell, please please make it IEEE 802.3af/802.3at compliant. It's easy - there are devices that will handle the signaling for just a few dollars.
(http://www.linear.com/product/LTC4265).

Over the years, roll your own PoE has been a real pain in the a&& to me countless times.
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bill001g

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Aug 9, 2012
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Be very careful with the term PoE. There are very clear standards for are 802.3af and 802.3at versions of PoE and these are the common ones used in networking. Anything else that calls itself PoE is not anything standard.

Those standards can deliver 13w and 50w at up to 100m. Problem is they do it by using 48v which only specialized equipment can use.

Lower voltage greatly decreases the cable length and you can only put so much amperage though before you start to damage the wires.

You are going to have to very carefully read the manufactures information what ever unit you buy they are all a little different. If you are going a short distance it should cause no issues but longer distance you may need to increase the voltage to make up for the loss. Since you can not just change the voltage on a power block most times this means buying a different one.
 
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DeauteratedDog

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If you are building something with PoE to sell, please please make it IEEE 802.3af/802.3at compliant. It's easy - there are devices that will handle the signaling for just a few dollars.
(http://www.linear.com/product/LTC4265).

Over the years, roll your own PoE has been a real pain in the a&& to me countless times.
 
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funkyguy4000

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Aug 14, 2011
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I don't think the power supply will be a problem for me. I am a power supply designer. it is more so the IT side of PoE. I don't really get it. Its my understanding that you just use 2 twisted pairs for the communication, and then the other two for power. I don't really see where the 802.3 comes in to play for PoE. I can't seem to find an IC that handles straight up PoE from a lower voltage (12V, 5V) source

Is this something that there is a single chip that will handle the communication and the power at the same time?
 
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bill001g

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Aug 9, 2012
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The way the fake PoE stuff works and the way I have made home built ones years ago is you insert devices in between that divert a pair away so you only use 3 pair...really 2 connect to the data equipment. What you do is take a rj45 coupler apart and take one pair and cut it and connect it to a power jack on both ends. What this does is allow 1 pair to carry the power between the 2 couplers but since this pair does not connect to either end equipment you can't fry it. You can use standard rj45 cable with this. If you know how to build power supplies then it is pretty much doing the calculations for loss due to the wire resistance and compensate for it by increasing the voltage.
http://www.amazon.com/HCP05-Passive-Injector-Splitter-Connector/dp/B00DZLSRJC/ref=pd_cp_pc_1

Now real PoE is special and what I may post here may be not 100% true I am too lazy to find and confirm. First the PoE detects a 25k resistor on the PoE pair(s). If it see this is delivers some power..i forget how much. But the the devices use LLDP protocol to actually request how much power it really needs. But there is some older method to do this that does not use LLDP messages I assume it is detecting resistance again.
Real PoE stuff can tolerate running the 48 volts over at the same time running signalling over it.

 
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DeauteratedDog

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I worked on a project a couple of years ago that used lots of PoE devices, some 802.3af, some 803.2at, and about every possible voltage, polarity, and pair choice for roll-your-own PoE. Given a couple of hundred sites - some pretty complex - somebody was always using the wrong PoE adapter. Sometimes it worked, sometimes it didn't, and one combination destroyed a $10k radio.

I pretty much spent the whole summer of 2012 cussing anybody that ever used non-standard PoE.
 
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