I have a mains to 60v DV power supply that I’m using for a project. Needless to say it managed to give me a bit of a shock. This surprised me as it’s supposed to be floating.

With no load the power supply seems to show a -60v potential to earth ground from DC -ve.

With the power supply off. Measuring Resistance between the DC -ve terminal and the earth ground there seems to be some capacitance as resistance goes from 1m to 0 ohms.

Measuring for capacitance there seems to be about 400uf capacitor between earth and the -ve terminal.

Putting a 5k ohm resistor between DC -ve and ground shows about 11mA flowing...

Obviously measuring anything with my bench oscilloscope is out of the question as this power supply -vs has a current path to earth.

Is this a poorly designed power supply?

  • \$\begingroup\$ Product link please. \$\endgroup\$
    – Andy aka
    Jan 19, 2020 at 16:42
  • 1
    \$\begingroup\$ "supposed to be floating": how exactly is that specified? \$\endgroup\$ Jan 19, 2020 at 16:53
  • \$\begingroup\$ So I bought this off eBay and the seller told me its floating... Digging online I can see this; mtlcctv.com/en/s-400w%20switch%20power%20supply.pdf also found this AliExpress listing that has a block diagram and that seems to indicate there is a output gound cap. aliexpress.com/item/2028424878.html SO yes my floating assumption seems wrong... Followup question, what would happen if I plug this into an isolation transformer? \$\endgroup\$
    – NightWolf
    Jan 20, 2020 at 0:05
  • \$\begingroup\$ Isolation transformer breaks the connection from the internal circuits to mains input protective earth. I would try it. \$\endgroup\$
    – user136077
    Jan 20, 2020 at 0:26
  • \$\begingroup\$ Also I'm assuming in the block diagram from AliExpress that FG means Frame Ground or does it mean floating ground? \$\endgroup\$
    – NightWolf
    Jan 20, 2020 at 1:15

1 Answer 1


There's plenty of mains AC powered equipment which behave as yours. They have metallic shield to reduce electromagnetic interference. That shield is connected directly or with a big capacitor to the output which the designer considered to be the signal ground or the voltage ground.

In addition it's connected to the protective earth wire of mains AC input. Finally they have a LC filter in the mains AC wires which has substantial capacitors connected from the shield to both mains AC wires.

enter image description here

If you connect a device like this to unearthed mains AC socket, you will have perfect 50% of the local AC voltage at the output and the shield. A lot of people have destroyed loads of equipment when they were plugging or unplugging devices to other devices without knowing this.

Especially disastrous are attempts to get rid of noise gathering ground loops by breaking somehow the protective earth connection. I have seen several audio systems silenced with this method. Connect or disconnect a cable or cables in a wrong order = kaputt.

Is it poor design? It is poor if one has a reason to assume it will work safely without a connection to the protective earth.

  • \$\begingroup\$ "That shield is connected directly or with a big capacitor to the output which is considered to be the signal or output voltage GND. In addition it's connected to the protective earth wire of mains AC input." - The OP said his supply is supposed to be floating output though, which your explanation isn't. \$\endgroup\$
    – marcelm
    Jan 19, 2020 at 19:09
  • \$\begingroup\$ It's not floating, the questioner just wrote that it isn't. Maybe someone supposed it's floating, but the measurements told the truth. \$\endgroup\$
    – user136077
    Jan 19, 2020 at 19:12
  • \$\begingroup\$ I’m not sure why I can measure -60v between the protective earth and ground/negative/black output terminal of the power supply \$\endgroup\$
    – NightWolf
    Jan 19, 2020 at 23:42
  • \$\begingroup\$ Is it DC, not AC? check if your +output is connected to PE \$\endgroup\$
    – user136077
    Jan 19, 2020 at 23:45
  • \$\begingroup\$ Thanks @user287001 - Protective Earth in this case is properly connecting as I have a separate earth point on my bench which im measuring from... I can measure the cap between both the +ve and -ve DC outputs and PE... \$\endgroup\$
    – NightWolf
    Jan 20, 2020 at 0:08

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