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)
Your PC is a standard desktop? 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.
Your AC/DC probably has a similar construction as a laptop supply: Earth
connection on the power plug and a floating DC output.
That there still is a measurable voltage is because of capacitive
coupling in the AC/DC between AC input and DC output (assuming PC is
really grounded). The voltage you measure depends on the impedance of
that coupling and the impedance of your meter.
Just measured 0V to earth on a few supplies here with a volt meter with
input impedance of > 10 MOhm / < 100 pF:
Recent 150W lab supply: 15 mVAC
Old analog 2x40W supply: CH1: 15VAC, CH2: 5 VAC
Recent 10W switching wall wart: 90VAC
Tried to measure short circuit currents as well, but that was not very
reliable as I don't have an AC uA meter here. Currents are too small.
As the current is very low, this should give no problems. There could be
a small surge if you just happen to connect at the peak of that AC
voltage. But the capacitance behind that should be very low, otherwise
the voltage/current measurements should be different. (Haven't drawn out
the resulting schematic and the consequences though).
So you could do a current measurement between the grounds. I would
expect less than 0.1 mA AC there.
But with an isolated supply there also is a chance of static (DC)
buildup. Depending an the capacity to earth this could give and ESD
discharge on connection.
So all this may not be your problem, but connecting an additional earth
between PC and your supply will not hurt anyway. And it has the (small)
possibility of solvng your problem.
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