> > 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
> >
> It might simply be open or short.
> 
> The Dell Inspirion 7500,  8200 and others use a 3 pin connector. On
the
> 75W PSU for 7500 the extra pin is open, on the 90W 8200 PSU it's short
> to +20V.

Most of the Dell power supplies have a dallas 1-wire device connected to
the 3rd pin. This gives the laptop information about the capability of
the attached power supply. See
http://www.howtofixcomputers.com/forums/dell/dell-pa-10-ac-adapter-cente
r-pin-108608.html for details, including the ROM contents of one version
of the chip.
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