I've got a switch mode power supply and I have a question on the grounding of the PCB. Documentation on the PSU can be found here:


On the back of the pcb you can see 2 of the 4 screws holes with a star. I assume that these are meant to be screwed into a chassis where these screw holes will connect to chassis ground.

enter image description here

A schematic representation can be seen here :

enter image description here

M1 (the one on the AC side of the board is called "M1 - Safety Ground". The other one (on the DC side, not named but labelled M2) is not electrically connected to M1.

According to the documentation it states :

Grounding Required

M1 is safety ground. For better EMC performance, please secure an electrical connection between M1,M2 and chassis grounding.

The chassis where I will be hosting this power supply has the green/yellow wires (ground) hooked up to the chassis.

enter image description here

So the questions :

  • Do I connect 2 wires from M1 and M2 to this green/yellow wire ? I don't have any corresponding screw holes so I was thinking about soldering a wire on M1 and on M2 and hooking them up to the green/yellow wire you see in the picture above.
  • Perhaps a stupid question, but if it's always the end-goal to have these 2 grounds connected (will be the case if you screw it in), then why aren't these electrically connected in the first place ?

The PSU would be attached to the chassis like this (using a single screw as the others don't correspond with the ones in the chassis) :

enter image description here

  • 1
    \$\begingroup\$ Minor technical note: earth wire is green/yellow - not plain green. The yellow stripe is visible in the photo. \$\endgroup\$
    – Transistor
    Feb 16, 2019 at 18:30
  • \$\begingroup\$ @jsotola: It is secured using one screw at the location where HS2 is. It's the only hole that I can use to screw something in. I could glue in spacers and add screws to make it more stable but it won't be ideal. I'll update the question with how it would be attached \$\endgroup\$
    – ddewaele
    Feb 16, 2019 at 20:16
  • \$\begingroup\$ it's a repair for an old PC. The original PSU was broken and I wanted to replace it with a more modern one. As the computer is over 35 years old, I couldn't get one that matches the chassis. It's a very specific form factor. \$\endgroup\$
    – ddewaele
    Feb 16, 2019 at 20:24
  • \$\begingroup\$ manufacture a metal plate with 4 standoffs to hold the board ..... make it big enough to cover the two existing standoffs and have it reach all way under the plugs so that it touches the vertical part of the enclosure ..... looks like the enclosure has "stops" punched that would keep the metal plate from lifting .... if there are no standoffs near the plugs, then put some stickon rubber feet under the metal plate (round or square like you see on DVD players) ..... use the existing insulating sheet under the board ....... attach the resulting assembly to the enclosure \$\endgroup\$
    – jsotola
    Feb 16, 2019 at 20:33
  • \$\begingroup\$ you could also remove all components from the old power supply board and use it for a carrier \$\endgroup\$
    – jsotola
    Feb 16, 2019 at 20:37

1 Answer 1


If you don't care for EMC, you can solder the green/yellow earth wire to both the holes. Because an electrical connection may not serve as mechanical connection, you must secure them mechanically, so they don't come loose.

Why they didn't connect M1 and M2 on the PCB?
Likely for space: a chassis trace on the primary side of the PCB (having live parts) requires clearance, double isolation, which is relative much space.
And maybe for meeting the EMC requirements

  • \$\begingroup\$ ok ... but connecting both M1 and M2 to chassis ground (either mechanically or via soldering) is definitely a must ? \$\endgroup\$
    – ddewaele
    Feb 16, 2019 at 19:43
  • \$\begingroup\$ M1 is safety ground, so that one is a must \$\endgroup\$
    – Huisman
    Feb 16, 2019 at 19:46
  • \$\begingroup\$ BTW, how do you secure the PCB itself? It's likely HS1 is live to or at least NOT safe, so upside down or loose in a box in not likely to be safe (as @jsotola is pointing out) \$\endgroup\$
    – Huisman
    Feb 16, 2019 at 19:51
  • \$\begingroup\$ On the chassis I can screw it in using one single screw. The entire back of the PCB is not touching any metal. The chassis had 2 mounts that were connected to chassis ground, but unfortunately I can only use 1 of the 2 mounts, and it doesn't correspond to one of the 2 wholes in the PCB connected to ground. (hence the need to solder wires to connect it up). \$\endgroup\$
    – ddewaele
    Feb 16, 2019 at 20:01

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