ethernetpoe
Hey guys what does the signal look like on the right of the transformer (T1) on the data lines {TXD_P, RXD_P}? Just trying to understand what it might look like…Is it a square wave? sine wave? Vmax? Vmin? Assuming a typical PoE signal from the PSE (48V).
I used to run USB and Ethernet in the same cable, to connect a router to a printer. I made two custom adapters to inject the 4 USB signals to the unused 2 pairs in 100Mb Ethernet. It worked very well, until an electric storm fried the USB port of the router; I don't know if the setup contributed to the failure. If you try it, make sure the USB data lines use a twisted pair, and 5V+GND use the other. In my case the distance was about 4 meters.
Edit about the failure: There was also a telephone line running next to the USB+Ethernet cable in the last 2 meters. Since the electrical storm also fried the ADSL modem, I think a voltage spike in the telephone line coupled into the USB wires and fried the port on the router.
T3 is used to check the maximum current flowing into the mosfet drivers - it's a current limiting device. If you look in the normal data sheet it replaces the component below: -
Can I combine both designs, use a current sensing resistor on the primary side and a perfect diode on the secondary, and leave out the second transformer completely?
There's no connection between current monitoring on the primary (via T2, a current transformer) and secondary side rectification.
Is this really proportional to the current on the primary side of the main transformer?
It's a 1:100 CT so it's going to work fine even though the burden (R12 @ 11 ohms) is after the rectifer D9.
Best Answer
Ethernet 100BASE-T family of standards uses a general signalling scheme called PAM (Pulse Amplitude Modulation), sent at about 2V peak-to-peak differential (1V single-ended).
Different 1000BASE-T versions use different variants of PAM, but work at similar voltage levels. More here: https://www.edn.com/electronics-blogs/absolute-eda/4441982/PAM---Ethernet--A-perfect-match
100BASE-TX, the most common 100BASE in use today (and what's likely being used in your setup), uses a PAM-type variant called MLT-3. Below is a typical MLT-3 waveform.
Power over Ethernet (PoE)
Power over Ethernet (PoE) injects a DC bias of about 48V onto the signal pairs to carry power to the far-end device. There are two modes for this: Mode A, which uses the center tap of the transformer, or Mode B, which uses the unused signal pair.
Mode ‘A’:
Mode ‘B’:
From here: https://kintronics.com/how-power-over-ethernet-works/
Ethernet When PoE Is Present
The Ethernet signal rides on top of the PoE DC bias (Mode A). Since the DC bias is common mode to the pair, the transformers at each end block it while still passing the Ethernet signal. The recovered signal is what you will see on the right (PHY) side of the transformer. So there is no difference on the PHY side.
To view the signal on the left (cable) side with PoE injected, you will need to use AC mode on the scope to reject the bias. Likely also you will need two probes, set up as differential to reject the variation on the PoE voltage. The signal you will see should be the same as the PHY side.