Powering a switch via PoE


So the story goes like this, my roommate needed to tap some IP phones for diagnostic purposes. I found a netgear HUB that would serve that purpose. This was not ideal as the IP phone needed power from the network, or a 48V wall-wart. He asked if I could get PoE to pass through the hub. Easy, I’ll just loop the unused pair and viola! Wrong.

What really happened is I had to do a lot of reading, and found a few key things. According to 802.3 af standards, the PSE or Power Supply Equipment, decides how power is going to be sent, while the PD or Powered Device has to accept BOTH modes.

This might be a good time to explain that 802.3af can send power one of two ways. Using phantom power through pins 1,2,3 and 6 (mode A) or through the unused pairs, pins 4,5,7 and 8 (mode B). In all my testing, Cisco favors mode A power. Probably something about gigglebit standards?

[Kajer] was doing some work with IP phones that use Power over Ethernet. While trying to get this to work with a network switch he decided to use PoE to power the switch itself. The best thing about this is he managed to shoehorn all of the necessary bits into the stock case. Those bits include a bridge rectifier, transistor, resistor, and a 5v power supply. Along the way he discovered he can now power the switch off of USB if he wishes.

source: hackaday

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One Response to “Powering a switch via PoE”

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