Hello,
I'm looking for a minimum circuit to request 48v power from a PoE switch (not based on a dedicated PoE IC).
I built the below circuit from
this article but it doesn't work. I both bread-boraded it and prototype boarded it and same result - the PoE switch seems to be sending an initial test voltage and then ... nothing.
Any tips / does anyone have an example of a minimum PoE circuit ?
Thanks
I do not have a complete circuit to share, but I do understand the problem.
PoE switches periodically present a test voltage, and expect to see a 24.9k signature resistor. So you're good on that front.
Then the switch will to see the PD present a low resistance to set the class (maximum power). I think the lowest possible class is approx 1800 ohms.
What that means is you need some sort of state machine to present the right resistance at the right time. The switch is prohibited from powering a purely passive load, so putting 1800 || 24.9k won't work.
I think the PoE PD chips are worth the cost. There are quite a few ways to mess up standards-compliant PoE, but the chips protect you from many of them.
Found the issue. Just needed a diode bridge on the input and it works ... even the polarity is not flipped by the bridge.
I guess the circuit is calculated to work only if having the voltage drop across the diode. If no diode, then the signature fails.
IMHO ... I would suggest adding appropriate fuses to both sides. A dedicated PoE Ic has some short protection that you would be missing. Better safe than sorry. Maybe set an appropriate max current on the switch as an alternative, but that is hardly a replacement for fusing long utp runs.