3 Contact Laptop Power Supply

The protocol is 1-wire; it's used to read a DS2501 write-once memory chip: http://www.laptop-junction.com/toast/content/inside-dell-ac-power-adapter-mystery-revealed

I have no idea what data is in the chip, but you can probably read the data out of a working power supply and see if you can decipher it.

The data might contain information about the max current the supply can output.

I don't know the protocol, and it seems it's being kept very hush-hush by dell, but there is this very interesting thread on tom's hardware.

It seems the third pin is purely to tell the computer that the charger is a) genuine dell, and b) powerful enough to run the system.

Without that data the computer will not charge the battery and will not run at full speed.

Dell power adapters generally come in two power ratings. Lower wattage for the laptop only and higher wattage to power the laptop and the docking station. If a lower wattage is plugged into the docking station, it knows to limit the power draw.