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I am working with a 48 VDC switching power supply that provides power to a PCBA that derives onboard DC voltages via a 48 VDC power input module followed by a 12 VDC switching regulator that is followed by a half dozen linear regulators.

I was using an oscilloscope to look for undesired signals on the outputs of the linear regulators. When I probed one of these onboard linear regulators from its output pin to its ground pin, I saw a 10 Vpp 60 Hz non-sinusoidal voltage. I checked the 48 VDC switcher and found that the 10 Vpp voltage is common mode on its outputs, it shows up between both the positive and negative outputs and ground, but not between the outputs.

A web search showed that common-mode 60 Hz is often present on switching supplies. I suspect that something is wrong with the PCBA, but I could be probing incorrectly. What I would like to understand is when can I use a power supply that has a large common-mode 60 Hz voltage to power something, and how that is handled on PCBA designs?

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    \$\begingroup\$ You might see that common mode signal on a scope but is it actually causing you a problem? I would doubt it. \$\endgroup\$
    – Finbarr
    Commented Aug 23, 2023 at 17:01
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    \$\begingroup\$ I suspect it's actually 60V, just that when loaded by the scope probe's 11M resistance, you only see the 10V. What voltage would it be into 1M, how could you test and verify it? Or if the output were grounded, what current would it be? \$\endgroup\$ Commented Aug 23, 2023 at 19:05
  • \$\begingroup\$ @TimWilliams I was probing this with the on-board linear supply loaded. I should have added above that I was seeing the 60Hz on a lot of the ground points around the PCBA, and it varied from one side of the PCBA to the other. \$\endgroup\$
    – DavidG
    Commented Aug 23, 2023 at 19:16
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    \$\begingroup\$ Are these "ground points" actually grounded? To where? I think you need to show a diagram of how these things are all connected and how you're measuring this common mode signal. \$\endgroup\$
    – Finbarr
    Commented Aug 23, 2023 at 20:11

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After thinking about it I realized what is going on. My board has positive and negative rails that are moving together at the common-mode 60Hz coming from the switching power supply. When I probe between the rails the scope probe is grounding the negative rail, and I am seeing the commom-mode 60Hz that is between the positive rail and earth ground. Now I need to determine an effective way to look at noise between the rails that won't cause damage, but it looks like a lot is written about that.

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