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i need to get out from PoE cable 1A of power in order to drive some LCD Displays and leds, i've digged around and i've found some interesting POE extractor that can fit my requirements (like the RT9400), basically, what i need is:

  • Extract power from POE Lan cable in order to use and drive my LCD Displays and led devices
  • At the same moment, reuse (with another female RJ45 female port - J2-OUT) the same POE and data to power up and use a olimex ESP32-POE-ISO device (Schematics Here)

enter image description here

Basically, J1-IN has a branch that will go to RT9400 PoE Extraction module, and J2-OUT will go to the ESP32-POE/ISO.

As you can see, i've only wired a part of the total H2019 transformers.

I'm not sure if it will work for two main causes:

  • Because the TD and RD transformers of the H2019 will not used
  • I don't know what can happen when the two devices will negotiate the POE power supply, the RT9400 is a class 0 signature device

This is the main block logical idea:

enter image description here

It can work? do you have some advices?

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  • \$\begingroup\$ In any case, i've added a schematic-like block image to explain better what i want to do \$\endgroup\$
    – VirtApp
    Apr 12, 2020 at 11:03
  • \$\begingroup\$ Which version or standard of PoE must be supported? Which power class the devices are? Is the power source passive? \$\endgroup\$
    – Justme
    Apr 16, 2020 at 19:58

2 Answers 2

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Both the RT9400 and ESP32-POE are Powered Devices (PD).

As standard Ethernet links are point-to-point, the PoE also assumes it is point-to-point link, meaning, one Power Sourcing Equipment (PSE) will connect to one Powered Device (PD).

It means that two Powered Devices cannot be paralleled. The detection of a PoE PD happens by setting a certain resistance on the power lines when the PSE tries to identify the resistance. When you parallel two PDs, the combined parallel resistance will be out of limits for detecting a valid device, and the power negotiation will not continue.

What might work is that with power extracted by RT9400 PoE extraction module, you feed a PoE PSE module that re-inserts power for the next device.

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As i understand the phy doesn't intervene in POE negotiations, it's just the POE device doing that. regarding the negotiation it should either work if you have a poe injector (assuming it can provide enough power) or do nothing ref, the poe extractor does the negotiation not the PHY, the datasheet of your device explicitly says it supports IEEE 802.3af so that shouldn't be a problem.

in terms of the transformer since the phy does no negotiation i would leave it there as a terminator for the differential lines (just be sure to put the proper resistors on the other side) and a common mode filter for the poe pairs maybe you can even do without it but just to be sure

also take a look at the Wikipedia article on POE, there you have more info, such as the wiring of poe, and how it makes sense

edit: the specific schematic will depend on the particular module you use, the logic it follows is the same, however in some modules it requires previous rectification as in page 4 of this datasaheet (and in the POE esp), in the case of the extractor you proposed you most likely would be able to use a rectification stage as a side effect having a small loss of efficiency but making the design more universal

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  • \$\begingroup\$ Thanks, can you give me a reference schematic starting from the first that i've posted? \$\endgroup\$
    – VirtApp
    Apr 15, 2020 at 10:24

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