Patent ID: 8014412

Claim:
A system for supplying power to a powered device over a communication link, comprising: a power supply device having a controller and an output port coupled to the communication link, a bipolar junction transistor (BJT) controlled by the controller to provide power to the output port, the BJT being controlled to increase impedance of the BJT so as to enable the controller to detect an AC impedance at the output port, wherein the power supply device further comprises an AC disconnect-detection pin for detecting the AC impedance at the output port, the BJT is configured to turn off to support detection of the AC impedance at the output port, and sensing capacitance is coupled to the AC disconnect-detection pin for supporting detection of the AC impedance at the output port, the sensing capacitor further being coupled to a first electrode of the BJT to turn the BJT off in response to a probing signal supplied to the sensing capacitance.