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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Methods of Injecting PoE via Phantom Power
Schematic 4 in Fig. 1.11 is the basic way of inserting PoE by phantom power. The key difference, or rather addition, is a pass-FET in series with the battery on the left side and another pass-FET in series with the load on the other. The purpose of these pass-FETs and related circuitry is basically to impart some intelligence to the power-delivery scheme, and that is discussed in great detail in the following chapters.
In Fig. 1.12, in the upper portion, we have drawn this popular way of injecting (and extracting) PoE (DC power) on an Ethernet cable via the center-taps of the data transformers. We have learned that this is the way telegraph and telephone circuits have been historically combined for ...

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