2 ms·

PoE power supplies need to be isolated (except in rare exceptions) and handle much higher voltages than common USB-C or wall wart power supplies. They have to

PoE power supplies need to be isolated (except in rare exceptions) and handle much higher voltages than common USB-C or wall wart power supplies. They have to use a transformer and a more complex control strategy, not a simple buck regulator with an inductor. PoE inputs need to tolerate voltages several times higher than the highest USB-C voltages, so more expensive parts are used everywhere.

cruffle_duffle 7mo ago
It sounds like the PoE spec was designed before the arrival of “IoT” type things like the esp32, raspberry pi’s, etc. How much of the complexity is a “fundamental electrical engineering problem” and how much of it is just a spec written to solve a different set of problems?
- cpgxiii 7mo ago
  Almost all of the complexity of PoE is fundamental. To get enough power over 100m of ethernet cable (10x longer than USB) you have to run at much higher voltages like 48V. The same has eventually come to USB: for USB-C Pd to reach 240W, it also has to use 48V. There have long been lower-voltage "passive PoE" systems which expect a lower always-on voltage on some of the ethernet pairs (usually 12V, 24V, or rarely 48V), which can be very easy to implement so long as your users can handle the setup and incompatibility with other ethernet devices (in the most extreme case of passive PoE on 100Mb/s ethernet, you simply connect the positive pair to the input of the device and the negative pair to the ground, no additional hardware needed).
namibj 7mo ago
Any Ethernet (well, any RJ45 you expect in a home/office) has to have at least 1500V isolation from the RJ45 wire to anything metal that can be touched or is a connector on the device. A PoE-only device with no electrical connectors besides the RJ45 can just use a very cheap RJ45 port with integrated magnetics and PoE allowance (tiny bit bigger wires and a center pin exposed, less than 50ct more than the cheapest RJ45 with integrated magnetics) and a cheap buck from 40~80V to e.g. 5V. Oh, and a cheap bridge rectifier and some signaling resistors to take care of input polarity and signal to the source that we in fact want the approximately 50V that could hurt a device not made for it.
- shadowpho 7mo ago
  > cheap buck from 40~80V to e.g. 5V. That’s not a cheap buck lol. Order of magnitude more expensive then 12v not even mentioning capacitors that can withstand 80v is $$$ and your derating goes to shit

cruffle_duffle 7mo ago

It sounds like the PoE spec was designed before the arrival of “IoT” type things like the esp32, raspberry pi’s, etc. How much of the complexity is a “fundamental electrical engineering problem” and how much of it is just a spec written to solve a different set of problems?

cpgxiii 7mo ago
Almost all of the complexity of PoE is fundamental. To get enough power over 100m of ethernet cable (10x longer than USB) you have to run at much higher voltages like 48V. The same has eventually come to USB: for USB-C Pd to reach 240W, it also has to use 48V. There have long been lower-voltage "passive PoE" systems which expect a lower always-on voltage on some of the ethernet pairs (usually 12V, 24V, or rarely 48V), which can be very easy to implement so long as your users can handle the setup and incompatibility with other ethernet devices (in the most extreme case of passive PoE on 100Mb/s ethernet, you simply connect the positive pair to the input of the device and the negative pair to the ground, no additional hardware needed).

namibj 7mo ago

Any Ethernet (well, any RJ45 you expect in a home/office) has to have at least 1500V isolation from the RJ45 wire to anything metal that can be touched or is a connector on the device. A PoE-only device with no electrical connectors besides the RJ45 can just use a very cheap RJ45 port with integrated magnetics and PoE allowance (tiny bit bigger wires and a center pin exposed, less than 50ct more than the cheapest RJ45 with integrated magnetics) and a cheap buck from 40~80V to e.g. 5V. Oh, and a cheap bridge rectifier and some signaling resistors to take care of input polarity and signal to the source that we in fact want the approximately 50V that could hurt a device not made for it.

shadowpho 7mo ago
> cheap buck from 40~80V to e.g. 5V. That’s not a cheap buck lol. Order of magnitude more expensive then 12v not even mentioning capacitors that can withstand 80v is $$$ and your derating goes to shit