Question Is there a simple way to implement POE in an ethernet client device (10 or 100 only)?

peterh337

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Hi All,

I've been reading the wiki on POE and found this:

The common 100 Mbit/s passive applications use the pinout of 802.3af mode B - with DC plus on pins 4 and 5 and DC minus on 7 and 8 (see chart below) and data on 1-2 and 3-6. Gigabit passive injectors use a transformer on the data pins to allow power and data to share the cable and is typically compatible with 802.3af Mode A. In the common "passive" PoE system, the injector does not communicate with the powered device to negotiate its voltage or wattage requirements, but merely supplies power at all times.

It suggests that I could just connect a DC-DC converter to pins 4+5 and 7+8.

However, one cannot just short 4+5, and short 7+8, can one? I realise ethernet is protected from cable shorts but would that not upset that port on the powering device (typically a POE compatible switch)?

The next option is to use diodes. Two diodes from 4 and 5, that's your +ve, and two diodes from 7 and 8, that's your negative.

Or just use 4 and 7, and ignore 5 and 8?

At 50V or so, the current draw would be about 80mA. It seems that the passive scheme would easily cover this requirement.

And no software negotiation is needed.

I also understand that if somebody connects a crossover cable, that will reverse the supply :) So while auto-mdix will deal with that for the data, it will blow up the DC-DC converter unless a further diode bridge (4 diodes) is used.

Any tips and comments would be much appreciated.
 
You could use the common rj45 splitter devices that split pairs 1,2/3,6 and 4,5/7,8 into separate ports. You could of course use the brute force method and just cut those wires on both ends and connect them to the power and only connect the 4 data wires to the rj45 plugs.

If you look at the commercial passive injectors they disconnect the extra wires on each end from the equipment inside the boxes. The power never reaches the ethernet ports when you are using passive.

You have to be very careful with passive injectors. 12 volts likely will not hurt a ethernet port but 24 or 48 volts might.

I really wish they would not call all this crap POE since all the passive solutions are proprietary. 802.3af is a standard that just works for all brands of equipment that follow the rules of the standard.
 

peterh337

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I am developing the client device (doing the PCB design) and have no control over the powering device.

On the client, 4+5 are indeed shorted and go to GND via 75R. And 7+8 are shorted and got o GND via 75R. So they are already shorted, in what is apparently a standard 10/100 ethernet implementation.



So it looks like I could just take power from these pins. They go nowhere. I wonder what the 75R resistors do? They are probably terminators for any signals present in the 1000mbps+ modes. Clearly you don't want them if there is power there, because 50V across 150R is 16W :) If somebody connected a POE injector to my device, these resistors would instantly blow up.
 

peterh337

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Of course, I now realise that POE of any sort if impossible with the Hanrun HR911105A integrated RJ45 + magnetics module, because pins 4,5 and 7,8 are not accessible at all :)

All the stuff I found online that does POE uses external magnetics and a dumb RJ45 connector. And I don't have room for that on my PCB.

It is possible that a HR913382AE will do this but the Hanrun website is broken for datasheet downloads.
 
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peterh337

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Does anyone know if POE injection equipment applies power to 4,5 and 7,8 permanently, or whether it implements the "negotiation" using the resistor value?

I would be surprised if passive injectors implemented anything like that. That more or less needs a microcontroller, to briefly apply some voltage and see the current drawn.
 
Read the standard for 802.3af. Any other things called "POE" is proprietary and there are no rules everyone does their own thing. In many cases the equipment is not compatible between vendor.

From what you are posting it appears you are trying to create your own proprietary POE so how it works is up to you.

There are chipset that implement 802.3af I just was never that interest to build my own device.
 

peterh337

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I am trying to make a product which can be POE powered, and I want to avoid using the expensive POE chips.

It appears there is a "base" mode which presents power on 4,5 and 7,8. It seems that passive injectors permanently power these wires, while active injectors (switches, etc) check for the correct value resistor across them (22k, AFAICT).

I can take it from there.

The standard documents are huge and very hard to read. And expensive to buy :)
 

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