# How do you measure power consumed by a PoE-powered device?

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:

1. 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

1. 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.
• Please show how the ethernet wires combine to produce a single DC voltage supply. Jan 16, 2023 at 19:15
• Should you decide to go back to the off the shelf route, PoE meters are quite easy to find, see for instance amzn.eu/d/ar9AmVX Jan 16, 2023 at 20:58
• Reading this article on Power over Ethernet would be a good start. I would suggest you stick to the defined standards and ignore all the proprietary methods. Jan 16, 2023 at 21:06
• I think you need more research. You really don't need a 2 Gsps ADC just because you want to measure DC current. And many PoE switches can show how much each device consumes, so they have this built-in already. And for a makeshift device, you's just need two gigabit PoE magjacks and maybe 2 multimeters, and you can expand up from there to fit your requirements of all obscure corner cases. Jan 16, 2023 at 22:48
• I don't think that there is such a thing as 1000 MB PoE connection ... 1000 Mb refers to ethernet data rate ... it is not related to the PoE Jan 17, 2023 at 2:25