In this document it's explained how to reduce the common mode noise.
In my case I have common mode noise at 50Hz (from power supply) on the 12V CC line that I'd like to reduce.
Are both solution still applicable? I think the choke coils are useless at low frequencies and maybe also the Y-Capacitor.
Looking at method 2, I understand that the metallic casing must not be connected to the circuit GND, right?
Which kind of Capacitors are suitable for this application?
Can you please suggest me other solutions or place where to find this kind of information: I found a lot on internet but in order to reduce noise from circuit to power line and not vice-versa.

Part II:
The situation it's a little bit more complicated. Actually, since my device is connected to the network, the 12 volt power is delivered through PoE in the following way:

network cable -> power injector -> network + power -> splitter -> network and 12VDC.

So near the device, where the 12V is provided, I have no earth ground. The power injector is actually connected to the earth but, I don't know why, the power over the network cable is really floating following the mains AC. I could change the PoE injector (and actually I tried with good results) but, since the client has a lot of these devices already in production, he can move my device in his network easily using this poor PoE splitter that works good for other devices and already in place.

  • \$\begingroup\$ What kind of power supply exactly you have (link?) and how exactly have you connected it (photo?)? \$\endgroup\$
    – Justme
    Jul 12, 2020 at 11:11
  • \$\begingroup\$ Just an universal power supply with floating groung. \$\endgroup\$
    – Fabio
    Jul 12, 2020 at 13:12
  • \$\begingroup\$ I connected the output cables to a fixed terminals with screws on my pcb. \$\endgroup\$
    – Fabio
    Jul 12, 2020 at 13:21
  • \$\begingroup\$ That's not very helpful. It does not matter if you connected a random power supply with screws or whatever. I mean, does the power supply input have mains earth connection or not, and if it does, is it connected to earthed mains wall socket? You say the output is floating, so does that mean the mains input is only with 2-prong unearthed plug? Is it a linear transformer supply or switch mode power supply? It's just easier if you give a photo or a link to the supply if you don't give out details. Is the powered device also floating or grounded, does it have any other connections than supply? \$\endgroup\$
    – Justme
    Jul 12, 2020 at 13:45
  • \$\begingroup\$ Hey Fabio: What makes you think you have “ common mode noise”? You are measuring voltage between your supply and what? \$\endgroup\$
    – user69795
    Jul 12, 2020 at 16:51

1 Answer 1


Just as I suspected, you have a cheap 2-prong switch mode power supply.

To prevent electromagnetic interference (EMI), inside the power supply, there will most likely be a so called Y capacitor or EMI capacitor, between mains and secondary side.

This Y capacitor will make the output to not be completely isolated from mains, but to weakly follow the mains AC.

If you want less common mode noise, try another power supply, preferably one that has 3-prong mains input, so the output will have the capacitor to earth ground instead of mains voltage.

  • \$\begingroup\$ Perfect, now it makes sense for me. But what if I couldn't use a 3-prong mains input? Is there a way to mitigate the common mode noise or to avoid that my circuit, with this noise, will not work fine ? I understand that there isn't a simple answer but is there a place where can I check to improve my knowledge. \$\endgroup\$
    – Fabio
    Jul 12, 2020 at 20:38
  • \$\begingroup\$ Why can't you use 3-prong mains input? Can you connect the output negative lead to earth ground? Can you connect output negative and positive via Y caps to earth ground? Can you get a better 2-prong power supply? \$\endgroup\$
    – Justme
    Jul 12, 2020 at 21:05
  • \$\begingroup\$ Good questions but the situation it's a little bit more complicated. \$\endgroup\$
    – Fabio
    Jul 12, 2020 at 21:35
  • \$\begingroup\$ Great, well, if you want specific answers, try to put the limits of the situation in the question so we don't suggest something that would need to break your limitations. But why is the common mode noise a problem? Why do you need to reduce it? \$\endgroup\$
    – Justme
    Jul 12, 2020 at 21:45
  • \$\begingroup\$ I added these information on my first question. Sorry if I didn't explained this at the first time but I thought that they were too much information. Your suggestion about using the Y caps is intersting: is what I asked at the first. So could I use the Y caps to the metalling casing as in the document I posted ? About why I want to reduce this noise the reason is that if I use a power supply without noise my device works fine but with common noise at 50Hz I have problem. This is the only difference also because the 12VDC is provided by the splitter (DC-DC) that is always the same \$\endgroup\$
    – Fabio
    Jul 12, 2020 at 22:05

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