I use the HP/Compaq nx7300, which I need to power from the car power
supply. Unfortunately, this laptop uses a nonstandard power supply
jack.
It features the inner and outer ring, which provide the high current
power supply, and also the thin central pin, which is used for ???
I have three hypotheses:
1. It may be used to control the operation of the AC charger from the
laptop (forcing the "power down mode"?)
2. It may be used as a feedback connection to provide better voltage control
3. (the conspiracy theory ;-) ) It may be a kind of serial interface
used to authenticate the original HP power supply to prevent using
other, cheaper adapters.

Does anybody know what is the real function of this central pin?
The original power supply looks like this:
http://www.topfreebiz.com/product/469522/Original-New-Laptop-Adapter-For-HP-18.5v-3.5a-Center-Pin.htm
-- 
TIA & Regards,
WZab
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