Replacement DC psu but wrong polarity

Discussion in 'General Electronics Chat' started by goodbyegti, Apr 28, 2008.

  1. goodbyegti

    Thread Starter Well-Known Member

    Apr 28, 2004
    59
    1
    I've got a bit of equipment running from a 12V DC unregulated PSU. It connects via a standard jack type plug. The inner core is negative with respect to the outer. It's rated at 500mA.

    Since it's stopped working i need to replace it and i have a 12V DC switch mode power supply (max current 1.5A) but the polarity is the other way round, i.e the inner core is positive with respect to the outer on the plug.

    From my possibly naive point of view I was planning on just switching the wires round, or soldering a new plug on but i have been advised by an electronics tech that this wouldn't work and would probably end up damaging something like the PSU.

    I cannot understand why this would happen and would be very grateful if someone could point me in the right direction.
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Check if the outer core of the switched mode power supply is connected to ground. If yes its ok to swap the wires.

    What is the equipment you want to supply?
     
  3. goodbyegti

    Thread Starter Well-Known Member

    Apr 28, 2004
    59
    1
    Which ground are we talking about here?

    It's a pH meter, nothing especially complicated, basically a multimeter.
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Its ok to use it for your multimeter. Just connect the positive wire of the switched mode power supply to the positive supply pin of the voltmeter and the zero (negative) wire of the supply to the negative supply pin of the multimeter.

    Your old power supply has the polarities reversed because usually this small unregulated power supplies have a switch to change the polarity on their terminals. check it if you want to see if this is the reason.
     
  5. goodbyegti

    Thread Starter Well-Known Member

    Apr 28, 2004
    59
    1
    Thanks for your help, the old PSU has no switch so it must have been hardwired like it from the start. You've basically confirmed what i thought i could do. I would be interested to know if anyone else thinks otherwise!
     
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