DC power jack shows voltage in 3rd pin when plugged in?

Discussion in 'General Electronics Chat' started by geratheg, Aug 22, 2014.

  1. geratheg

    Thread Starter Member

    Jul 11, 2014
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    I have a few DC power jacks of various sizes and shapes with 3 pins, 2 of them are connected when not plugged in, 1 is isolated for positive. When a plug is plugged into the jack usually the 2 that were connected disconnect, one goes to negative, the other - which I will call the third pin - isn't connected to anything. This is true for all power jacks and none of them show voltage in the third pin except for one.

    That's strange, where is this voltage coming from to the third pin if the pin is disconnected? Is this something to worry about and should I get a new adapter?

    This is a picture of the adapter that shows voltage, I only have one piece: 5.5mm x 2.5mm DC Power Jack
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You would need to show all the connections? if only 2 wire DC is used then what is connected to the other jack connection, if any?
    Max.
     
  3. geratheg

    Thread Starter Member

    Jul 11, 2014
    107
    3
    Nothing. I just tested it bare with no circuit. Multimeter straight onto the jacks terminals.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If it is not connected to anything why worry?
    It is probably not opening the third pin for some reason.
    Max.
     
  5. geratheg

    Thread Starter Member

    Jul 11, 2014
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    Oh I don't know, maybe I should just use it without worry. Third pin isn't going to be connected to any circuit or casing. I really don't know what the third pin is there for? What are some applications of it?
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It is sometimes used on a power jack socket to disconnect the internal power when the external power jack is plugged in.
    Max.
     
  7. to3metalcan

    Member

    Jul 20, 2014
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    It's for battery power. You connect the positive battery terminal there. The jack automatically disconnects it when you plug in. If you see voltage there, it's broken, but if you don't have a battery or other power source, it doesn't matter.
     
  8. geratheg

    Thread Starter Member

    Jul 11, 2014
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    Thank you for the responses. I don't have any use for it, and no longer need to worry.
    As for battery power, it would make more sense to connect the negative terminal of the battery there? Since the third pin is connected to the negative terminal when the power plug is disconnected.
     
  9. to3metalcan

    Member

    Jul 20, 2014
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    Depends which way around you're wiring it! Most devices use a negative tip, if they use batteries.
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    The jack your talking about is more likely to be an audio jack instead of a power jack.
    The pin disconnecting shuts off the devices internal speaker when an earphone is plugged in.
     
  11. to3metalcan

    Member

    Jul 20, 2014
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    Hmmm, I doubt it. The form factor might be slightly different, but I'd guess the OP is talking about something like this:

    [​IMG]
     
  12. MaxHeadRoom

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    Jul 18, 2013
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  13. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Didn't see the link to a picture, sorry.
     
  14. MaxHeadRoom

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    Jul 18, 2013
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    The pic displays in the body of the post?
    Max.
     
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