Wojciech Zabolotny wrote:
> 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.

One 120W replacement PSU was recognised as a 75W PSU as the supplied 
connector only had two pins connected. I replaced connector with the 
original and the i8200 didn't complain of a low power. I eventually used 
that PSU as 24V PSU and now use a different brand "2-power" 90W that is 
identical to that supplied with a new Acer, but with the correctly wired 
3 pin plug for Inspiron 8200.

Can you "open" the existing cable and see if it has actually any more 
than 2 wires?

So I suspect it's either (2) above or same function as the Dell, a 
simple indication of lower or higher rated units that have same connector.

I don't believe (1) or (3). That adds too much cost :-)

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