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Power over Ethernet

Discussion in 'Microcontrollers, Programming and IoT' started by Mikey Lee, Aug 30, 2015.

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  1. Mikey Lee

    Mikey Lee

    Aug 30, 2015
    Hi. ! I have read that a port of a PoE switch has a 44VDC – 57VDC output voltage. I have a microntroller that works on 5VDC. My question is, if I use one of the ports as the supply for the microcontroller, will it deal any damage to it or does it depend on any other factors such as current rating? Thank you
  2. avsh


    Aug 30, 2015
    Hi.! it may depends on power rating and current rating.
  3. HellasTechn


    Apr 14, 2013
    Most likely it will burn your microcontroller chip but i bet you can use an 7805 regulator to get 5V
  4. BobK


    Jan 5, 2010
    44V is too much for most regulators.

  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    the use of high voltages in POE is to enable the use of a high power devices. It is done for the same reason that power transmission lines use high voltage.

    For normal cat 5 cable, "high" voltage is around 55 volts.

    it is normal in this case to use a switch mode buck converter to reduce the voltage to an appropriate value. These are often provided by the danger manufacturer as the switch, but there's no particular reason you can't lash one up yourself.
  6. jayanthd


    Jul 4, 2015
    Hi Mikey

    Use a LM2576HVT-5.0 (TO220 package) Buck regulator to get 5V from 44V and then power the microcontroller from 5V. You need one 100 uF 70V, 1000 uF 50V, 100 uH 3A for the LM2576 circuit. The circuit is available in the LM2576HVT datasheet. If you want I can provide the circuit for the power supply. HVT version of the regulator handles 60V input but make sure you put a SK104 heatsink. The most important thing is Whether your PoE is able to provide the required current for your microcontroller circuit. First check the current PoE can provide and also the max current required by your microcontroller circuit. LM2576HVT-5.0 is a 3A regulator. It can handle 3A current.
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    If you're using a buck regulator to drop 50V to 5V with 85% efficiency, then for every amp at 5V, the 50V power supply needs only to provide about 120mA.
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