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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Practical Limits of AC-DC Power Supplies
The underlying reason for desiring classification in the first place is that AC power is neither free nor unlimited. The incoming “48V” rail from the AC-DC power supply has practical limits as to its maximum-output power. For example, if a standard-household AC outlet is used as the input to the AC-DC power supply, we must keep in mind that the outlet has a maximum RMS rating of 15-A. Engineers usually derate that by applying a multiplicative factor of 80 percent to get the safe, continuous rating of the AC outlet as 12 A. Then they multiply that (safe) number with the incoming AC (RMS) voltage (say 110 V), and further multiply that by the “power factor” of the power supply (say 0.9), and also the efficiency ...

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