The Natural Philosopher wrote:

Andy Burns wrote:

The Natural Philosopher wrote:

You can't supply DC via a transformer

You don't need to;Â* PoE uses a phantom* power arrangement so the DC is
applied across different twisted pairs, not within each twisted pair.

there are no spare pairs in gigabit ethernet

We're not talking about the spare pairs, that was the "dumb" scheme I
mentioned, "proper" PoE doesn't need separate pairs for data and power.

Equipment that recovers the power will pass the DC through a diode
bridge, not because there is any AC which needs rectifying, but just
to ensure correct polarity in case it has passed through a odd number
of crossover cables.

[*] ignoring the "dumb" PoE method that simply applies DC to the
unused pairs of 10/100 ethernet, but that doesn't use transformers
either.

Like I said., POE doesnt use transformers

Not sure we're saying the same thing.

I think you missed the point. Ethernet is inherently isolated because it
uses transformers to couple input and output pairs.

POE inevitably breaks that isolation

depends whether the PSU itself is isolated?