Ground noise from the wall

Discussion in 'General Electronics Chat' started by tpny, May 14, 2013.

  1. tpny

    Thread Starter Member

    May 6, 2012
    216
    0
    Hi, my circuit board is powered by a switching power supply which outputs 12VDC. I put the scope probe at the COM output terminal on the power supply (setting scope to 1sec/div on the timescale) and it shows a very noisy ground - zigzagging nearly +/-1V from 0V. As a result my whole board is noisy. When I move to a different wall outlet (closer to the subpanel) it's so much cleaner. My circuit cannot depend on how noisy the wall outlet is. How do we "filter" ground noise on the circuit side? Thank you!
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    If your probe was on your circuit's common, where did you attach the probe's ground?
     
  3. tpny

    Thread Starter Member

    May 6, 2012
    216
    0
    same - probe's ground = circuit's ground = power supply's output COM.
     
  4. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    So your probe and ground were at the same point (electrically)?
     
  5. tpny

    Thread Starter Member

    May 6, 2012
    216
    0
    yes, I want to see what the ground looks like on the screen.
     
  6. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Well then the only thing you measured was the magnetic field that penetrated the loop made by your ground wire. You haven't measured the actual common of your power supply because both your probe tip and ground wire were at the same potential. Considering you're getting so much noise despite not measuring, I have suspicions about your measurement setup.
     
  7. tpny

    Thread Starter Member

    May 6, 2012
    216
    0
    where do you propose I put the scope's ground? I move the whole setup to a differnt wall outlet and noise is down by half, < +/-500mV around 0V. My wall plug is 2-prong, no earth prong.
     
  8. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,789
    945
    If your house is old enough to have copper plumbing and not contain PVC, try grounding to the cold water pipes. Pay attention to the DC offset you see.

    A good fluke voltmeter on the lowest DC voltage scale might tell you something about the integrity of your grounds in the wall outlets too. The screw terminals tend to work loose over the years from just the temp. cycles of the 4 seasons. re-tightening them is good idea.
     
  9. tpny

    Thread Starter Member

    May 6, 2012
    216
    0
    Found the problem. The laptop adaptor on the same power strip is causing the noise. Removed it and line is clean.
     
Loading...