O'Reilly logo

Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

PD Preloading
What is the effective PD load-resistance corresponding to 10 mA at, say 50 V? It is V/I = 50/0.01 = 5 kΩ. The wattage is V × I = 50 V × 0.01 A = 0.5 W. If the output of the PD was 12 V, then assuming 100 percent efficiency for the DC-DC converter, we need to preload the PD for 0.5 W minimum load. That leads to a preload resistor of V2/W = 12 V2/0.5 W = 288 Ω. So we can provide the MPS by connecting a 5-k resistor directly across the port, but that would obviously interfere with detection and classification. So the correct option is to include a resistor of less than 288 Ω across the 12-V output rails.
If the efficiency of the DC-DC stage is, say, 80 percent, then the output power corresponding to 0.5 W at the input is 0.5 W × ...

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required