What is the difference between these two connections Y-capacitor (EMI filter)?

  • high-voltage side to isolated 12V DC output
  • isolated 0V DC output rail and a "GND"

Could you tell me the reason of each connection?
And what is the right way to do that?

enter image description here

enter image description here

  • 1
    \$\begingroup\$ something is getting lost in translation. it is not clear what you are asking \$\endgroup\$
    – jsotola
    Nov 13, 2017 at 7:09
  • 1
    \$\begingroup\$ Try to form a question starting with an upper case letter and ending with a question mark. \$\endgroup\$
    – winny
    Nov 13, 2017 at 7:16
  • \$\begingroup\$ I have tried to make your question easier to read. You have cropped (removed) lots of the original circuits and SMPS are not my speciality. However I think the difference you are trying to ask about is that in the top diagram, the Y-capacitor is connected between (what seems to be) the isolated 0V DC output rail and a node called "GND" (is that really mains PE?); in the bottom diagram, the Y-capacitor is connected between (what seems to be) the isolated 12V DC output rail and something on the high-voltage side. \$\endgroup\$
    – SamGibson
    Nov 14, 2017 at 16:33
  • \$\begingroup\$ Well, given that C8 in the first circuit is shunting to GND (PE?) the noise from the secondary side, and referring that potential to GND as well, the second circuit looks like part of a mess. That C5 refers the secondary side to an unspecified (in the cropped picture) input line, which I just suspect isn't GND/PE. Moreover, the connection happens before L3, then is ineffective against conducted EMI from load... \$\endgroup\$
    – LuC
    Oct 17, 2023 at 10:14

1 Answer 1


The bottom connection shows also some signal ground, is this an earthed connection? If so, that's the reason why it's better to be on the low side. In case of a floating or reversing voltage (phone chargers can be plugged in both ways), for the mains voltage side there's no difference where the capacitor is mounted. For the low voltage side: Note that the cap is mounted on the pole of the trafo that's directly connected to the DC capacitor and not to the pole with the Shottkey diode in it. This ensures correct EMI for the trafo and less fluctuation in absolute potential of the DC output.


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