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Power over ethernet

Discussion in 'Electronic Design' started by IanM, Apr 9, 2009.

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  1. IanM

    IanM Guest

    Anybody got an experience of POE?

    I want approx. 9V 400mA out of it. So far I have looked at solutions from
    LTC and Nat Semi and they are going to take up a lot of board space.


  2. Quoted:

    The 802.3af PoE specification details all of the requirements for
    designing PoE equipment. Two types of devices are specified in the
    standard: Power-Sourcing Equipment (PSE) and a Powered Device (PD). The
    PSE provides 48-V DC power, with a current limit of 350 mA, to the PD—be
    it a VoIP phone or wireless access point. The PSE is limited to a
    continuous maximum power output of 15.4 W.

    End Quote

    If it can provide 48 V at 350MA, you should surely be able to get your
    done. All of the routers I have seen were normally sized, so unless you
    are making a device that is not a 1U rack form factor, I cannot see how
    you would be so concerned.
  3. IanM

    IanM Guest

    It's an access control terminal, pcb is about 3 in by 3 in.

    The design in the link below is similar to the ones I have been looking at
    on the LTC and Nat Semi websites
  4. That alone looks like it would take up more than 3 x 3.

    An "access control terminal" sounds more like a device, than a source.

    A PSE is a source equipment device. A PD is a powered device in PoE

    Perhaps you could simply use a PoE router so the power is generated
    before your device, and then passes said power downstream.

  5. There IS a standard. 802.3af
  6. IanM

    IanM Guest

    Yes it is a PD. The goal is to take whatever it is a PoE router passes down
    the liine and convert it to 9V @ 400mA.
  7. Put the power conversion inline. There are likely even off the shelf
    solutions for DC-to-DC power sources that are inline.
  8. IanM

    IanM Guest

    Yes, I like that idea. Thanks.
  9. Only applies to 10/100BASE T.


  10. There is one for GbE then as well, because we have GbE Poe Cisco routers
    in our buildings. They are fully addressable ports, which only work when
    activates via administrative hooks.
  11. I've designed a PSE (Power Sourcing Equipment) Gb Ethernet interface.
    From memory, the powered device must correctly identify itself as a POE load
    (by means of a correct start-up load sequence), if the PSE chipset does not
    see the correct load then power is switched off.
    So you can't just connect a DC-DC converter to the end of the line, you need
    a specifc PoE load chip to take care of the smart power-up mode.

    There are simpler non-stand solutions that simply couple 48V down the line,
    but they are not part of the PoE standard.

    The chip solutions generally don't need a large PCB footprint, and a 48V to
    9V DC-DC converter can be made really small as well.

  12. IanM

    IanM Guest

    Thanks Dave,

    thats the lowest parts count solution I have seen so far. The application
    examples in the datasheet with the wall wart adapter as alternative power
    source are exactly what I want to do.

  13. Jasen Betts

    Jasen Betts Guest

    IIRC the standard is 42V(ish) at upto 400mA between two pairs.

    If you don't care about standards and are prepared to just use unused
    pairs in a 100Mb/s CAT5 run the you can do anything you want.

    where they are quoted the connectors seem to be specced for conducting
    upto 2A current.

    if you want to go the standards way you need centre-tapped ethernet
    transformers that can handle the common-mode current. and apropriate
    regulation and signalling circuits to make it all work.

  14. Ever heard of google? It would save your foot from teeth marks.

    48V @ 350mA.
  15. Nico Coesel

    Nico Coesel Guest

    You better be concerned. The power isn't applied unless you enable it
    using some kind of protocol. You'll need switching controller with POE
    negotiation. Freescale has some pretty simple, reasonable priced
    controllers for this purpose.

  16. Maybe you should try reading the entire thread next time, dipshit.
  17. Nico Coesel

    Nico Coesel Guest

    I'm sorry that I didn't read all the episodes of your answer. Maybe
    you should try to squeeze it all in one -not so boring- episode :)

    At least I point to a working solution for the OP's problem.

  18. Our routers were GbE. Likely the same spec there as well.
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