Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Temporarily powering 5.5v board from 2.5v ethernet cable

  1. Apr 17, 2013 #1
    I'm not an EE. I'm a Software Engineer.

    I would like to build a small circuit that would leach power from an active 2.5v Ethernet line to charge a capacitor very slowly (over a timespan of 1-2 minutes). When the capacitor is fully charged, it would then discharge at 5.5v to power a circuit containing an Atmel chip and an I2C sensor.

    Q1) Is this possible?
    Q2) Would charging the capacitor block traffic over the Ethernet cable?
    Q3) Can anyone give me a rough idea what this circuit should look like and what the specs of the capacitor should be?

    Alternatively, can someone refer me to an EE who could design this for me?
  2. jcsd
  3. Apr 17, 2013 #2
  4. Apr 19, 2013 #3


    User Avatar

    Staff: Mentor

    Probably, providing your circuit doesn't draw much current. (To supply 10mA at 5.5V may equate to taking, say, 30mA from the Ethernet.)

    It's not clear for how long you wish to power your circuit, but I assume its for some minutes, at least?

    It would, but this could probably be prevented by incorporating a filter to block that data signal. An RF choke built around a ferrite core should do the job, allowing the DC through but isolating your circuit from the digital data on the line.

    Beyond this general discussion, I can't help much though.
  5. Apr 19, 2013 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Have you looked at the PoE standard?


    There are commercial plug and play adaptors designed to inject and extract the power from standard Ethernet cable. You can also build the circuit into your device if you don't want a physically seperate unit.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook