Electronic – In PoE: is alternative A compatible to all PDs

Don't do it! Here's some reasons why you shouldn't:

• The isolation transformer is not likely to be able to handle the current that PoE will push through it.
• You can't support the full PoE discovery protocol without modifying the firmware of the Ethernet switch. I am referring to LLDP.
• Not supporting the PoE discovery protocol could allow you to power a device that isn't able to handle it-- causing it to damage the device.
• Careful current regulation is required to prevent creating a fire hazzard.

I have to admit that I am making a couple of assumptions about your ability to design this thing properly-- assumptions that might be wrong. I am assuming that you don't know how to design it properly because you don't apparently know how the Ethernet signaling, isolation transformers, and PoE discovery works.

If you knew how PoE and Ethernet actually works then this project might be barely possible-- but hardly cost effective since there are other PoE switches and injectors that are not super expensive. Even as a learning experience this has limited value, since there are better ways to learn PoE.

I think this document helps Introduction Basic principles of PoE Power over Ethernet (Page 4)

Spare-pair power feed

Spare pair power feeds utilize wire pairs that are not otherwise used, the voltage being placed directly on the spare pairs 4/5 (+) and 7/8 (-). This method can be used only on networks with eight-wire cabling. This does not apply to Gigabit Ethernet, because here all eight wires are used for signal transmission and there are therefore no spares.

Phantom power feed

A phantom power feed has the voltage coupled to wire pairs 1/2 ( ) and 3/6 (+). This method can be used for networks with four-wire or eight-wire cabling.

So you can check with a voltmeter, if there is a voltage in pins 4/5 - 7/8 then the method used is Spare-pair power feed.