Dell PA-10 ac adapter center pin

I've been looking around a lot to see if anyone had any information
about this but came up with nill...So I investigated it myself for
anyone out there that may be interested in what that center pin really
does. For those of you that think I don't know my butt from a hole in
the ground and feel like leaving any messages against what I did I'm
not asking for pointless opinions here so take them elsewhere. There
was a need for it to be reverse engineered and I did it. Enough
ranting lets get on to the beef of the post.

I too was wondering what this third center pin is about. No voltage
present to it, no resistance, no capacitance; got me wondering what
kinda surprise did Dell put in the power supply that they don't want
us to know about. So curiosity got the best of me and I very carefully
pried the glued case apart.

Standard switching power supply, but then near where the wire enters
the case and is soldered to the circuit board resides a small TO-92
device. First thought was maybe a transistor that sinks a certain
current upon attachment but after further inquiry, only two leads were
connected. This led me to reverse engineering this small circuit. It
turns out theres a 131 ohm resistor in series with the center pin wire
of the DC connecter and one pin of the 'mystery device' and then there
is a reverse biased diode going to ground. The other pin that is
connected of said device is also connected to ground (by reverse
biased I mean that the diode will only conduct if a positive voltage
were applied to the ground connection of the power supply and a
negative (ground) were connected to the center pin of the DC power
connector. This lead me to deducing, because of the series resistor,
that this diode was a zener of currently unknown breakdown voltage.

The next step was to determine the true identity of the 'mystery
device.' The part number read "Dallas 2501 (then a date code)." Dallas
being dallas semiconductor (aka Maxim IC). A search yielded only a
very incomplete datasheet refering me to the DS2502 which is a 1kbit
one-wire EPROM version. The "2501" was a DS2501 of 512 bit data space.
The datasheet gives specifications to a max programming voltage after
EPROM write instruction of 12V. This means to protect the device from
overvoltage this zener diode connected to the pins must be a 12V zener
and the sereis resistor being a current limiter protecting the diode
in the event that the inner barrel and center pin were to come into
contact.

The DS2502 and 2501 (1kbit and 512 bit respectively) use Dallas Semi.
1-Wire (R) communication protocol. It gets its power from the data
line and when the data line is low a diode protected capacitor supplys
power for its logic circuits, Parasite Power. This means that to
communicate with the DS2502/1 one only needs two lines, a data line
(logic high idle state) and ground. The power to the data/power line
is supplied by the master through a 5k ohm resistor for short cable
lengths.

Hope anyone reading this that wants to make his/her own power
converter finds this information usefull (insert disclaimer here; ie.
use this information at your own risk, I am not to be held responsible
if someone else's equipment gets fried b/c of poor design, I only
described how it works and make no claims to it being my own design
giving rights of design and operation to Dell and/or LiteOn (written
on power brick) and any other engineering firm/company/manufacturer
that was involved in the design of the motherboard, power brick and
any other associated equipment, etc.)