I'm using IRM-30-12 for power supply for my PCB. This SMPS doesn't have an earthing connection pin as it has Class 2 isolation protection.There is an AC potential is generated at the circuit GND which is initially found out by using a tester. When I'm connecting the earthing pin(The earthing of the supply is left open as I'm using a plastic enclosure)directly to the circuit GND the AC potential is no longer exists. Is it the correct approach?

My question is 1. What might be the cause of the generated AC potential? 2. As I have gone through some possible solutions I have found the following LINK; enter image description here

I have connected the circuit ground to a metal and again connected the earthing wire to the metal. What impact will it have on my PCB?

  • \$\begingroup\$ I don't recognise IRM-30-12 as a part number and I don't want to google it in case this number referrers to more than one can you provide a link to a data sheet or circuit? Please add this to your question, not as a comment. Note: testers with a high input impedance such as a DVM or Oscilloscope will often pick up a signal from radiated fields if the return path is not defined. \$\endgroup\$ Commented Feb 22, 2020 at 12:58

1 Answer 1


What might be the cause of the generated AC potential?

To meet EMI limits on the output DC voltage of an unearthed power supply, usually a capacitor is placed between one of the DC output lines and the AC rectified bus voltage on the primary. It isn't perfect but it does the job. See this example here of the internal working of a wall-wart: -

enter image description here

Picture taken from here.

And, the impact of adding this capacitor is that it couples the input AC voltage to the output DC terminals but in a safe way so as not to risk electrocution but, if you looked at the DC output your would see a voltage waveform at the same frequency as the AC power. Of course shorting the DC output to "real" ground will remove this safely.

What impact will it have on my PCB?

Probably no impact but, the devil is in the detail and we have no detailed knowledge of what your circuit is or how you have precisely impemented the grounding.

  • \$\begingroup\$ I'm using the device for IOT gateway purpose which supports multi mode communication. Can I connect the system GND to the Earth wire directly or I have to use a metal plate for it \$\endgroup\$ Commented Feb 23, 2020 at 16:13
  • \$\begingroup\$ You may not need to do anything with this rogue voltage. It has very low ability to drive current anywhere. If it does cause problems then say what they appear to be. If you suspect it might cause problems and you can’t afford to earth your PCB, then prudent design of the PCB layout is usually a cure. Else, earth it. \$\endgroup\$
    – Andy aka
    Commented Feb 23, 2020 at 16:34
  • \$\begingroup\$ So that could be optimised in layout design, right ? Temporarily I'm connecting it to GND through a chassis/metal as in some circuits users get shock from the antenna and other exposed connectors. \$\endgroup\$ Commented Feb 23, 2020 at 17:16
  • \$\begingroup\$ I don’t know that there is anything more to add. \$\endgroup\$
    – Andy aka
    Commented Feb 23, 2020 at 17:48
  • \$\begingroup\$ Ok, this much information is helpful. For now I'm connecting to the Earth wire through metal plate to avoid the issue. \$\endgroup\$ Commented Feb 24, 2020 at 10:40

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