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All commercial POE products I have seen feature DC current to power endpoint devices. What are the reason(s) why AC is not used for POE products?

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    \$\begingroup\$ Most of the electronics is usually working from DC. Network products are usually low power electronics based. \$\endgroup\$
    – Eugene Sh.
    Jan 31, 2018 at 21:28

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AC is only 70.7% as efficient as DC at transferring power at a given peak Voltage. That is, to provide the same power as 48V DC, you would need 48/.707=68 V peak-to-peak.

Sure, you could increase the voltage, but the PoE voltage is specified at 48V to avoid electrical shock, as dry human skin begins to break down and conduct at about 48V. This is why the plain old telephone system (POTS) over Copper wire was 48V as well.

Another reason is that you don't want AC to couple into your receiver and negatively impact the ability to receive data. Usually DC blocking caps are used to block the DC before the transformer.

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  • \$\begingroup\$ This right here is the correct answer. Even power line owners know that 6 wires can be switched to DC and carry 41% more, the problem is most circuits are 3 wires, and a DC scheme can only use 2. \$\endgroup\$ Feb 1, 2018 at 1:28
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The reason is some of the exiting PoE schemes use the communication wires as power wires, too.

See e.g. https://en.wikipedia.org/wiki/Power_over_Ethernet#Pinouts

Transformers are used to avoid galvanic connection between ethernet devices, and this also comes in handy for PoE, as DC power cannot pass these transformers. AC would pass and required additional circuitry to separate power from signals.

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  • \$\begingroup\$ The way the ethernet isolation trsnsformers are configured, AC will pass to the circuit and be rejected from comms exactly the same as DC is. \$\endgroup\$
    – Neil_UK
    Jan 31, 2018 at 22:07
  • \$\begingroup\$ Yes, DC will not pass the transformer from primary to secondary. However, for PoE over a single pair without DC blocking caps, the transformer will present a dead short and burn out. This isn't an issue for multi-pair Ethernet as two pairs are used for power, one pair for D+ and the second for D-. \$\endgroup\$
    – TimB
    Jan 31, 2018 at 23:16

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