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4 Prong Power Adapter Plug

Discussion in 'General Electronics Discussion' started by PointyHairedBoss, May 16, 2011.

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  1. PointyHairedBoss


    May 16, 2011
    Hi, I am new to this forum & this is my first post.
    Our future retirement home is off grid. We have a PV system with invertor however the building is wired for both 120VAC & 12VDC. We prefer using 12VDC where possible to avoid the power losses & faint hum of the invertor.
    For example, we have an electric piano, TV & DVD player that all came with 120V-12VDC power adapters. I cut them off & added a fused, 12V car type plug to the low voltage power cord so I can plug them right into the 12V system (which actually provides between 12.0 & 14.0VDV). So far this has worked perfectly for everything I have tried.
    But now I need some help. I have an Ericsson W35 mobile broadband router which, according to their specifications, will accept input power anywhere between 10-28VDC so I thought it would be perfect for our situation. Ericsson includes a 120V-15VDC power adapter with the router but I assume any power supply providing 10 - 28VDC should be OK.
    My problem is the low voltage plug that is provided with the adapter - it has 4 conductors. There is a small diagram on the back of the transformer indicating that 1 of the pins is positive & the other 3 are negative. My ohm meter shows continuity between the 3 negative terminals. Why are there 4 conductors instead of 2? Can the 3 negative terminals be connected together? If so, I can make up a low voltage power cord with a 4 prong plug on the Ericsson end & standard 12V plug on the other.

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  2. davenn

    davenn Moderator

    Sep 5, 2009
    why? its just the way they decided to make it :)

    personally I would have probably gone the other way 3 x positives and 1 x negative
    or more commonly 2 x pos and 2 x neg.
    either of those 2 ways you have been better for reducing voltage drop across the lead length

    If you have measured continuity between the 3 negatives, then yes you have answered your own Q ... they are connected :)

  3. PointyHairedBoss


    May 16, 2011
    Thanks Dave.
    I can understand a manufacturer using a non standard plug in order to ensure that the transformer supplied with the unit is the only one that can be plugged into it. If there is no reason to do this from an electronics point of view, are they using a mechanical way to help protect their electronics or would there be any other reason?
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    There are many reasons they may use a "special" plug:

    1) to force people to purchase their expensive replacement rather than a cheap alternative.

    2) to make it difficult for people to use the wrong power supply which may damage the unit, or not permit it to operate correctly.

    3) As part of regulatory requirements, the power supply may need to be tested along with the rest of the device. With an alternate power supply the device may be technically non-compliant.
  5. Alchymist


    Apr 16, 2011
    4) Purchasing screwed up on another project, and there are 4000 4 pin plugs i stock. :D
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    It would never happen.

    "When you said you wanted 5, we assumed 5 boxes"
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