Voltage between grounds

Discussion in 'Homework Help' started by Goddamnox, Oct 22, 2011.

  1. Goddamnox

    Thread Starter New Member

    Oct 22, 2011
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    Hi, I happened to notice today that when I put a meter between the BNC shield on my oscilloscope and the one on my frequency meter(running off wall wart) I get around 10v AC at 50Hz showing up. This does not happen anywhere else in my setup.
    Could anyone enlighten me as to what is happening and if I should be worried!?

    http://forum.allaboutcircuits.com/images/smilies/confused.gif
    Thanks
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    It may be capacitive leakage through the wall wart, via deliberate filter capacitors or otherwise. This assumes that it is not supposed to have its own mains ground though.

    If it is intended to be grounded, something would seem to have dropped off and needs to be fixed. If not it's probably not surprising: modern meters have typically 10Mohm input impedance, so you would not need much coupling to mains live to get 10V.

    What would not be OK would be if this voltage is capable of driving any serious current: that might indicate a wiring mistake like Earth/neutral transposed. You might not want to join the shields together again until you are sure that this is not so.
     
  3. Goddamnox

    Thread Starter New Member

    Oct 22, 2011
    2
    0
    The ac/dc adaptor does have a metal earth prong not a plastic one, this does not seems to have continuity through to the dc plug end though.
    I have checked this adaptor is the correct polarity and voltage and it has very low ripple. It is not overheating and seems to be working fine.

    I have tried running the frequency meter from a bench supply and I still get about 3.9v ac 50hz across the screens?

    Could you advise on how best to check if this voltage can drive serious current, as you say?

    Thanks
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    Measure the AC current with the multimeter.
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
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    And kiss goodbye to the meter, or at least its fuse if there is a large amount of current available!

    A safer way would be to get a resistor rated for the full local mains voltage (for safety), and use a VOLT meter in parallel with that. A 220kohm 1/2W might be a starting point.

    In practice this is may be a tiny leak and the current range might be fine, but I prefer to err on the side of caution, especially when giving advice.
     
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