I just tested a hardware setup yesterday which consists of a 75m cat5e and a tp-link cpe210 (v1) and the original poe injector (24V, 1A). The manufacture "only" supports up to 60m.
Any how the thing works good with openwrt. Just before I flashed it I had the feeling the stock firmware did some reboots - but not sure.
Thing is, can I somehow read the voltage arriving at the poe port from the cpe210 with openwrt?
Surely I can measure with a multi meter but only without the load so the voltage drop measured is not very useful. The poe injector itself also features two leds, a green one if the load goeas up to 0.8A and a red one if the load is over 0.8A. So far only the green led was on.
I plan to active the poe pass through to harvest the 24VDC (or whatever is arriving), tackle it down to 12V and use it for a motorized valve. Just upfront I wanna see if their is still enough "reserve".
The thing is that you don’t have any measurable voltage in the cable to begin with. PoE is distributed in pole pairs as differential voltage per pole so you need a PoE LAN transformer to get any meaningful voltage at all after the cable.
So you need a PoE splitter to drive anything at all that isn’t built for PoE operation.
That's not true. The passive poe injector put's 24DCV (+4,5pins; -7,8pins) and with the 75m meter cat5e cable their is no problems measuring the voltage (it's "still" 24V without load).
This might be correct for a active poe installation but this is not my case.
Again false, nothing like that is necessary for my setup
Nothing to electrocute here. The isolated 24VDC PSU is already extra low voltage and even get's down to 12VDC (Yes, that's the voltage in your car you don't need to fear!). The motor turning the water valve is again isolated so let's call it double safe
I have used these. It's not shown on the diagram but there is a simple internal modification to have both ports powered. So modified it can be used to tap DC out of the line while still powering an AP at the end.
Make sure you're not using CCA (copper clad aluminum) cat5e Ethernet cable. This cheap cable has much higher DC resistance than solid copper, among other problems. Cat6 best for PoE.
Since the CPE210 is not gigabit, the 4,5 (blue) and 7,8 (brown) pairs of wires are used only for power and you could just splice into them. That of course will not work if you upgrade to a gigabit link.
23V ~ w/o extra load (only the cpe210 with wifi disabled maybe ~2W)
19V ~ 4W (pwm led)
16V ~ 8W (pwm led)
going higher the voltage drop will force the cpe210 to restart and the poe passthrough will be disabled upon the cpe210 is booted up again.
I actually suspect I ended up with a CCA cable because the PoE injector (24V,1A) never get stressed at all but instead the voltage drops heavily on the other end with even moderate load. I measured a 20 ohm resistance on each of the (8) wires packed inside the 75m cable.
In the end I might need to install some (super) caps like suggested by @mbo2o. But let's see. I was already able to strip around 3m of cable, maybe I can get rid of a little bit more and than even with the heavy voltage drop it could be sufficient for my use case