If got this cable as a power plug for my DELL XPS 15. I want to hook it up to a different power source.
But what are the 3! wires for on a bipolar plug?
There is two reasons. Remote voltage sense as tony suggests.
And because some of these chargers are actually both barrel and center pin, making them have 3 conductors. The third conductor is typically a 1 Wire type data connection, and is used to signal what type of charger is being used.
Dell and Lenovo use these extensively.
Based on a few searches for replacement XPS 15 jacks, you have the data pin. Based on your 4 conductor wire, it uses both reasons.
They do this to regulate voltage at plug using lossy flexible cable wire. It is a common practise.
It is also useful to detect voltage drop by current in wire for OCP and preventing fire in plug shorts and broken strands creating a hot spot worst case failure.
I am not certain we know yet , which plug you have, but this is what I found to be true. 19.5V@ 6.7A which on AWG18 wire @22mOhm/m*6.7A drops ~300mV/m per pair.
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