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.]