On 2022-06-28 pozz wrote in comp.arch.embedded:
> Il 28/06/2022 11:23, Stef ha scritto:
>> On 2022-06-27 pozz wrote in comp.arch.embedded:
>>>
>>> Anyway in my case the power cord of my PC is connected to a UPS and the
>>> AC/DC that powers the board is connected to earth connection of the grid.
>>>
>>> Working with two wires power cords is risky and I learned that in the past.
>>>
>>> However this isn't the case now, both PC and board have a good 3-wires
>>> power connection, but there's a 20Vac voltage between the grounds.
>>
>> With both grounded, there cannot be a 20VAC voltage between the grounds.
>> (unless there is an extreme amount of current ofcourse, but that will
>> not be the case)
>
> The PSU of desktop PC and AC/DC of board are connected to the earth
> conductor, i.e. I'm using 3-wires power cord.
>
> But their separate GND aren't connected.
>
>
>> Your PC is a standard desktop?
>
> Yes.
>
>
>> Then the 0V is connected to the earth
>> connection of the power plug. This is not the case if you have a laptop.
>> The supply will have an earth connection, but the DC output is floating.
>
> No, it's a desktop PC, not a laptop.
> I don't know why I have 20Vac between GND of J-Link and board.
> Maybe the problem is in the UPS that has some failure.

Measure voltage between PC case and earth. If that is not zero check the
UPS and the power cables.

>> Your AC/DC probably has a similar construction as a laptop supply: Earth
>> connection on the power plug and a floating DC output.
>
> Yes, but I connect GND (0V of DC output) to earth.

If your PC is connected to earth as well, you should measure 0V between
them. If not there is something wrong.

--
Stef

Original thought is like original sin: both happened before you were born
to people you could not have possibly met.
-- Fran Lebowitz, "Social Studies"