Temporarily powering 5.5v board from 2.5v ethernet cable

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kzgrey
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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?
 
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kzgrey said:
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?
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?

Q2) Would charging the capacitor block traffic over the Ethernet cable?
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.
 
Have you looked at the PoE standard?

http://en.wikipedia.org/wiki/Power_over_Ethernet

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 separate unit.