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 *** 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 data*** refering me to the DS2502 which is a 1kbit
one-wire EPROM version. The "2501" was a DS2501 of 512 bit data space.
The data*** 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.)
New update....since my last post on this subject I've written a
firmware for a microcontroller to read the memory chip in these power
supplies...this is the eeprom dump from it
at memory address 2130, D0, is a CRC (8-bit, x^8+x^5+x^4+1 polynomial)
of address locations 2110 - 212F that was calculated by the DS2501 as
the data was read in by the bus master.
I was unable to get any useful data after 213C but then it read a byte
at location 2155. The cause of this is unknown and there was no valid
CRC calculated...this is still under investigation.
As far as the memory dump....Dell (manu.), 00 I'm guessing is an
identifier, AC describing what type of adapter, 0 another identifier,
195046 being 19.5 volts and 4.6 amps. From CN to 62 this is the
product identification number found on the barcode and A00 is the
revision code 00 being the important part b/c I obtained a second 90
watt power supply and it too and an 'A' appended before the numerical
portion of the revision number.
Hope anyone reading this finds the information useful.
(Same as before insert disclaimer here....)
After doing some detective work and general head scratching I rewrote
the firmware to read the whole memory space instead of a page at a
time, this let me gain access to all the information (consecutive
addresses throughout) allowing me to find that the whole barcode
number is stored on the DS2501 followed by the revision number. The
following is the memory dump from the 12F683's internal EEPROM....
002100: 11 36 14 6D 02 00 00 B8 .6.m.... = ROM
002108: F3 FF FF FF FF FF FF 00 ........ = STATUS
002110: C3 FF FF FF FF FF FF FF ........ = STATUS CRC (=C3)
002118: 44 45 4C 4C 30 30 41 43 DELL00AC = Start of page 0
002120: 30 39 30 31 39 35 30 34 09019504
002128: 36 43 4E 30 43 38 30 32 6CN0C802
002130: 33 34 38 36 36 31 36 31 34866161 = End of page 0
002138: 52 32 33 48 38 41 30 33 R23H8A03 = Start of page 1
002140: 4D 7C FF FF FF FF FF FF M|......
002148: FF FF FF FF FF FF FF FF ........
002150: FF FF FF FF FF FF FF FF ........ = End of page 1
002158: FF FF FF FF FF FF FF FF ........
So here we have the first line being the ROM. 11 is the family code,
the serial number is: 0000026D1436 and B8 is the hardcoded CRC reality
check byte
The next line is the EPROM status register. The first byte is the code
protection bits (bits 0-3) and memory usage map (bits 4-7, reserved
for TMEX) the next 4 are page redirection bytes, bytes 6 and 7 are
maintained clear/unprogrammed and byte 8 is programmed to 00 at the
factory. C3 is the CRC computed by the DS2501, this was checked by
using the program I wrote in windows for computing the CRC
(x^8+x^5+x^4+1), and was determined to be valid. The next five lines
are the actual data space information. Dell is the manufacturer, 0 is
a field identifier, AC is the type of adapter, 090 is the wattage
rating (my thought is that it allows for 99+ watt power adapters),
195046 is the nominal voltage rating (19.5 Volts) followed by the
nominal current rating adjusted to one decimal place (4.6 Amps). From
C to H8 is the barcode number of the power adapter and A03 is the
revision of the power adapter. 4D7C is an end of information
identifier as I have found this on another adapter I own. The one
thing I was unsuccessful in acquiring was the computed CRC at the end
of the data space that the memory chip is supposed to send as a 65th
byte (which is sent at the end of the data space, this was assumed to
be the same communication protocol as is used for the DS2502).
Again I hope this helps answer any inquiries anyone may have as to the
actual data stored in the memory chip.
I'll be happy to answer any technical questions anyone may have about
this. Please only technical questions, I am not offering design
services and any inquiries of that type will be immediately ignored
and any additional blocked permanetly if I so deem them to be a
nuisance.
