For a weekend project I would like to make a Power Over Ethernet (PoE) power meter. There have been five situations where this could have come in handy during fault-finding in my projects. I finally decided to make one since I could not find one.
So for my parameters:
- Must measure the power consumption of the Powered Device
- Must handle all PoE standards and modes.
- Must adapt to T568A, T568B, and irregular wiring conventions.
I have two problems that I must address before I can even make a prototype:
- Ethernet wires come in differential pairs so there is no really common reference ground, add to that there are irregular wiring where they don't follow the standard wiring pattern it would be very difficult to design a circuit that constantly adapts which ones are the pairs. Instead what I have in mind is having a shunt resistor on each line — theoretically a pair of wires should have the same current reading (the other is a negative reading since PoE is DC) on them so I could combine them in software. After having four current values, the next problem would be how do I extract the voltage so I could calculate the power? I could not come up with a simple and reliable way of extracting the operating voltage of the PoE source.
simulate this circuit – Schematic created using CircuitLab
- Next problem is ADC selection. If I understand correctly, there are certain PoE modes that will both use the same wire for sending data and power at the same time. How would that look like? If no data is currently being sent there will be no power also? I don't think so. If it is the case I would need an ADC that has a sampling rate of at least 2 giga samples/sec for a 1000 MB PoE connection that would be at the same caliber as oscilloscope ADC, I might as well build an oscilloscope at this point.
1000 MB PoE connection
... 1000 Mb refers to ethernet data rate ... it is not related to the PoE \$\endgroup\$