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So I'm wondering if anyone has any information about the Dell power supplies which have 3 contacts on the barrel.

enter image description here

I know the 3 are power, ground, and some sort of communication link. Does anyone know what the communication line is for and/or what kind of protocol it uses?

Thanks

EDIT The reason I ask is I'm considering trying to build my own semi-advanced charger (not for a laptop but something else... it's a long story) and while I'm sure I could figure out a charging circuit given the internet as a resource, I was wondering what they use the communication line for and if it's necessary for my purposes. It seems the answer is no since I will be the only one using my device and thus don't need to worry about people plugging in not kosher chargers...

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  • \$\begingroup\$ It might be patented, or it might just be undisclosed. I bought a knock-off adapter like this for my Dell and it worked fine. \$\endgroup\$
    – endolith
    Commented Jun 27, 2011 at 15:58
  • \$\begingroup\$ Your wording of your original question seemed very off-topic. Now that you have added your edit I think it is a good question to be asking. In the future try to ask questions from a design perspective to start with to avoid down votes :-) \$\endgroup\$
    – Kellenjb
    Commented Jun 27, 2011 at 17:38
  • \$\begingroup\$ @Kellenjb will do \$\endgroup\$
    – NickHalden
    Commented Jun 27, 2011 at 17:40

4 Answers 4

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The protocol is 1-wire; it's used to read a DS2501 write-once memory chip: http://www.laptop-junction.com/toast/content/inside-dell-ac-power-adapter-mystery-revealed

I have no idea what data is in the chip, but you can probably read the data out of a working power supply and see if you can decipher it.

The data might contain information about the max current the supply can output.

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  • \$\begingroup\$ some kind of vendor lock-in would be my guest that they hide behind being smart and negotiating chargin power level \$\endgroup\$
    – kenny
    Commented Jun 27, 2011 at 18:28
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I don't know the protocol, and it seems it's being kept very hush-hush by dell, but there is this very interesting thread on tom's hardware.

It seems the third pin is purely to tell the computer that the charger is a) genuine dell, and b) powerful enough to run the system.

Without that data the computer will not charge the battery and will not run at full speed.

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Dell power adapters generally come in two power ratings. Lower wattage for the laptop only and higher wattage to power the laptop and the docking station. If a lower wattage is plugged into the docking station, it knows to limit the power draw.

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  • \$\begingroup\$ Some ACER laptops implement this feature with the size of the power plug. The higher power charger's plug fits both laptop and dock while the lower current version has an outside diameter just a bit too big to fit into the dock but fits the laptop just fine. \$\endgroup\$
    – KalleMP
    Commented Aug 13, 2018 at 6:11
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I've seen Dell power supplies using the DS2501 chip from Dallas semiconductor. It's used to communicate with the laptop. It is a "Unique Ware" "one wire" chip. It tells the laptop that the correct power supply is connected.

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