Ground noise from the wall

Thread Starter

tpny

Joined May 6, 2012
220
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!
 

Brownout

Joined Jan 10, 2012
2,390
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.
 

Thread Starter

tpny

Joined May 6, 2012
220
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.
 

Kermit2

Joined Feb 5, 2010
4,162
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.
 

Thread Starter

tpny

Joined May 6, 2012
220
Found the problem. The laptop adaptor on the same power strip is causing the noise. Removed it and line is clean.
 
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