February 2013
Intermediate to advanced
480 pages
13h 35m
English
Content preview from Power Over Ethernet Interoperability GuideBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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Detection Signature Resistor Disengagement Concerns
At this point it is a good idea to recall/repeat the discussion in the previous chapter on a very basic issue: the presence of the 25-k detection resistor. In almost all cases discussed so far, and also in related literature, this resistor is shown as a stand-alone component just past the bridge on the PD-chip-side. It is “always present.” So when the voltage ramps up into the classification range (~18 V), it continues to draw current, of about 18/25 k = 0.7 mA. Not a big deal, and though it adds some current to the constant class-current-sink, its effect is less than 1 mA. When the port is fully powered up, it continues to draw about 50 V/25 k = 2 mA. In terms of wattage this is 50 V × 2 mA ...
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