Lab power supply - Voltage on negative terminal

Discussion in 'General Electronics Chat' started by Ndxx, Feb 12, 2016.

  1. Ndxx

    Thread Starter New Member

    Feb 12, 2016
    3
    0
    Hello,

    I noticed today that my Lab Power supply gives output on negative terminal ?

    I put multi meter lead to Black hole ( - ) and touched screw on my PC case and I got reading of 12V is that normal ?? ( Power supply was set for 12V )

    Thank you!

    POmiar.jpg
     
  2. skeptic

    Active Member

    Mar 7, 2010
    51
    9
    Provided that your PC case is grounded and that your power supply is not floating i.e. both positive and negative voltages are with respect to ground then yes, that would be normal.
     
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  3. Ndxx

    Thread Starter New Member

    Feb 12, 2016
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    I't bothers my mind that negative from power supply to ground will give same reading as positive to ground ... :)

    What does floating means ?

    Thank you
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    Something everyone in electronics runs into at some point is that "Ground" is not an absolute term. In your case, the power supply has an output that is the more negative of the two, probably a black terminal. It also has other parts of its circuits connected to "earth ground" through the third wire in the AC power line cord. If the power supply does not connect these two circuit points internally, then the output is said to be "floating" with respect to earth ground. Floating means that they are galvanically isolated from each other, in this case by the insulation in the power transformer. Lab supplies frequently have floating outputs so that multiple supplies can be connected together in different ways.

    What is the make and model o your power supply, and can you post a photo of its front panel?

    ak
     
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  5. Ndxx

    Thread Starter New Member

    Feb 12, 2016
    3
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    1510d-b.JPG
    GWR-1510D ( Linear)
    Input voltage AC 110V ±10% 50 /60Hz
    Output voltage 0 - 15 VDC
    Current output 0 - 10 A
    Working conditions Temperature: -10°C to 40°C
    Relative humidity < 90%

    Here it is ... so its normal when I ground Negative terminal to get +12V reading
    And obviously normal when I ground Red terminal positive I get +12V as well ...
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
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    Yes, and the fact than you can ground either output terminal is a sure sign that they both are floating with respect to earth ground and the power supply case. The earth ground terminal is the yellow one in the middle. Green is a more common color for it.

    ak
     
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