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If got this cable as a power plug for my DELL XPS 15. I want to hook it up to a different power source.

But what are the 3! wires for on a bipolar plug?

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haemse
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2 Answers2

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There is two reasons. Remote voltage sense as tony suggests.

And because some of these chargers are actually both barrel and center pin, making them have 3 conductors. The third conductor is typically a 1 Wire type data connection, and is used to signal what type of charger is being used.

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Dell and Lenovo use these extensively.

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Based on a few searches for replacement XPS 15 jacks, you have the data pin. Based on your 4 conductor wire, it uses both reasons.

Passerby
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  • So how do I hook this up to a battery? Just without the control pin? – haemse Sep 06 '17 at 23:31
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    Depending on how they programmed it, it may or not work without the control pin. See https://hackaday.com/2014/03/03/hacking-dell-laptop-charger-identification/ – Passerby Sep 06 '17 at 23:32
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    I learnt this the hard way a number of years ago while trying to measure the voltage on the centre pin - my probe slipped and I shorted the pin to the inner part of the barrel - the one wire memories in the PSU do not have any protection against that (at least then) and was destroyed. After that the PSU was not recognized and the computer would only run at slow speed and not charge the battery - so be careful! – Kevin White Sep 07 '17 at 00:28
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    @haemse likely you can't just do that. Possibly the laptop will *run* off of such a supply but refuse to *charge* from it; possibly it will refuse to draw any power at all. AFAIK no Dell laptop from the past 10 years or so will be fully functional with a "dumb" power supply. – hobbs Sep 07 '17 at 04:01
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    @hobbs Is definitely correct. Dell laptops attempt to detect valid chargers and *will refuse to charge* if they aren't connected to something they recognize as an original charger, to the point where if a fully working (electrically speaking) charger has a damaged data connection, it will simply not work to charge the battery in the laptop (although you may be able to use it to power an external charge circuit if you take your battery out of the laptop). – Jules Sep 07 '17 at 04:09
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They do this to regulate voltage at plug using lossy flexible cable wire. It is a common practise.

It is also useful to detect voltage drop by current in wire for OCP and preventing fire in plug shorts and broken strands creating a hot spot worst case failure.

I am not certain we know yet , which plug you have, but this is what I found to be true. 19.5V@ 6.7A which on AWG18 wire @22mOhm/m*6.7A drops ~300mV/m per pair.

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Tony Stewart EE75
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  • I always thought it was a connection to earth - could you explain a bit more about how they use the 3rd wire to regulate the voltage ? – VanGo Sep 06 '17 at 22:13
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    It is used for voltage feedback in the regulator. I have used this principle to adjust the output voltage with external components. Usually feeds back to a 2.5V voltage divider on V sense. aka "Remote sensing" – Tony Stewart EE75 Sep 06 '17 at 22:16
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    Not so. The Dell connectors have an outer barrel and an inner barrel and centre-pin. I can't remember the pinout but the extra wire is used for comms between the charger and laptop. In this way a large laptop might run from a 60 W PSU but it won't charge the battery unless a > 90 W PSU is detected. See @Passerby's answer. – Transistor Sep 06 '17 at 22:39
  • This is a 130W charger intended for many versions. Inspiron 7347, 7348, 7459, Precision M2800, M3800, XPS 9530, 9550 – Tony Stewart EE75 Sep 06 '17 at 23:22
  • @Transistors answer is correct, the 3rd pin is connected to an e. g. DS2501 Add-only memory with a 1 wire protocol - thanks to the schematics found [here](https://320volt.com/en/dell-pa-12-laptop-smps-19v-3-34a-adaptor-semasi/) and details of its [1 wire protocol](https://hackaday.com/2014/03/03/hacking-dell-laptop-charger-identification/) / oh last link already mentionned by Passerby. – BitLauncher Dec 22 '22 at 21:32