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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?

enter image description here

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    \$\begingroup\$ Please check to see if it's a tri-polar plug. The Dell jacks I'm familiar with have separate contacts outside and inside the barrel. See Passerby's answer. \$\endgroup\$ – Transistor Sep 6 '17 at 22:47
  • \$\begingroup\$ is this the 4.5x3.0mm plug with center pin 3rd contacts or only inner/outer barrel? \$\endgroup\$ – Sunnyskyguy EE75 Sep 6 '17 at 23:20
  • \$\begingroup\$ Will check that with a meter. Didn't though that plug might actually be tripolar ... \$\endgroup\$ – haemse Sep 6 '17 at 23:34
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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.

enter image description here

Dell and Lenovo use these extensively.

enter image description here

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.

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  • \$\begingroup\$ So how do I hook this up to a battery? Just without the control pin? \$\endgroup\$ – haemse Sep 6 '17 at 23:31
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    \$\begingroup\$ Depending on how they programmed it, it may or not work without the control pin. See hackaday.com/2014/03/03/… \$\endgroup\$ – Passerby Sep 6 '17 at 23:32
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    \$\begingroup\$ 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! \$\endgroup\$ – Kevin White Sep 7 '17 at 0:28
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    \$\begingroup\$ @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. \$\endgroup\$ – hobbs Sep 7 '17 at 4:01
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    \$\begingroup\$ @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). \$\endgroup\$ – Jules Sep 7 '17 at 4:09
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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.

enter image description here

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  • \$\begingroup\$ 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 ? \$\endgroup\$ – VanGo Sep 6 '17 at 22:13
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    \$\begingroup\$ 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" \$\endgroup\$ – Sunnyskyguy EE75 Sep 6 '17 at 22:16
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    \$\begingroup\$ 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. \$\endgroup\$ – Transistor Sep 6 '17 at 22:39
  • \$\begingroup\$ This is a 130W charger intended for many versions. Inspiron 7347, 7348, 7459, Precision M2800, M3800, XPS 9530, 9550 \$\endgroup\$ – Sunnyskyguy EE75 Sep 6 '17 at 23:22

protected by Community Feb 6 at 13:34

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