I made the measure you suggested and I found around 20Vac between debugger GND and board GND *before* plugging them.
After adding a good wire connection between PC metallic case (near a mounting screw) and board GND, the Vac measured goes near to zero.
Could it be this the cause of issues I observed? I will continue with this connection and see.
Thank you for your suggestions.
It could not only be, it will be. Imagine you plug the connector, GND is not contacted first, then the CPU pin sees 20V-AC. That means +20V*sqrt(2) relative to CPU-GND and -20V*sqrt(2). That is far out of spec. It will kill the pin. I had something similar. A colleague had used a 2 wire extension cord for a soldering station, so no earth connection. I did not know this and killed the reset input of my controller. When I found out this I understood why we had several cases of non-functioning boards. The two wire extension cord was 2$ cheaper than a 3 wire cored. the cost of this "saving" was several 100$.
-- Reinhardt
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.
On 2022-06-27 pozz wrote in comp.arch.embedded:
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.
-- Stef TODAY IS INTERNATIONAL CAPSLOCK DAY!
Il 28/06/2022 11:23, Stef ha scritto:
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.
Yes.
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.
Yes, but I connect GND (0V of DC output) to earth.
On 2022-06-28 pozz wrote in comp.arch.embedded:
Measure voltage between PC case and earth. If that is not zero check the UPS and the power cables.
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"
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.