I generally understand how transformers work, and why they are important in POE, in particular for isolation.

I don't understand it how you can put a DC voltage on the centertap of one transformer, get that voltage at the centertap of another transformer on the other side of a cable, and keep it all isolated. What wires is the DC running on and if it doesn’t go through the transformer, how can it be isolated?

I guess I don't understand how a centertap works. I've read and watched a number of videos but still confused. I had thought the centertap was always half the voltage of the full coil, but in POE it's can be differential data on the coil and V+ or V- on the centertap.

 

Your question is confusing.
Transformers only carry AC.
DC cannot go through a transformer.
If there is DC on the transformer, it may be there to power an inverter circuit that drives the transformer with AC.


So show a diagram of what you are referring to because that's not at all clear.

 

Here is a schematic I found showing a DC voltage inserted onto the centertaps and being picked up on the other side of the cable (red and blue):

 

Okay, that's just feeding some DC power, that's riding on the AC signal, from the output of one transformer to the input of the other, to power some device at the right end.
The transformer winding looks like a short to the DC.
And obviously there is no DC isolation.
It's just to save a couple wires.

 

Used to come in handy when doing a remote via telephone wires. The cetertap can be used for communications with the studio and the earth ground or the center tap of another pair can be used for the return.

The waveform on the transformer secondary cancel at the center tap since the two outputs other than the center tap are 180° out of phase with one-another.

 

Okay, that's just feeding some DC power, that's riding on the AC signal, from the output of one transformer to the input of the other, to power some device at the right end.
The transformer winding looks like a short to the DC.
And obviously there is no DC isolation.
It's just to save a couple wires.

So are you saying both red wires are at the same potential as set by the centertap (call it 48V) and both blue wires are at the GND potential and when you pull them off on the other side they are no isolated?