| Electronics > Repair |
| Weird USB oscilloscope ground issue? |
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| Alex_Baker:
Today I ran into an odd problem when adjusting two old scopes. The tools I am using are a B&K 3026 signal generator and a picoscope 2204A for sanity checking the B&K. The problem is that when I connect the pico to the generator the trace is offset by about 240mv, regardless of if the generator is on or not, it only has to be plugged in. I put an ammeter between the grounds of the pico and the generator and get nearly 180mA of current flow. I can also get this current flow between the pico and a different grounded object, like a grounded power supply case. So from what I can tell, there is a potential difference of 240mV between the GND (i.e. the black wires of the PSU) of my PC, and earth (the pin on the outlet). I need some help figuring out why this would happen. The power supply is fairly new, but I have been using the picoscope for years on this PC. Is this a failure of the PC or is it user error? :-BROKE |
| shakalnokturn:
180mA sounds an awful lot ! Is that AC or DC or a mix of ? The Pico actually works (powers on) when it isn't connected to a DUT's ground ? (Thinking USB cable ground could be cut then it powers itself finding a ground path on the DUT.) If this isn't the case I'd get really worried, start checking power outlet ground continuity and being weary of the various protagonist's power supplies safety (Y capacitors). |
| coromonadalix:
can you plug the pico on a lappy battery powered ? if this offset / current dissapear the generator could be at problem, something in the ac section, ac filter hoping its not those who get defectives or phases problems between your main computer and the generator ? |
| Alex_Baker:
--- Quote from: shakalnokturn on June 17, 2024, 11:46:14 pm ---180mA sounds an awful lot ! Is that AC or DC or a mix of ? The Pico actually works (powers on) when it isn't connected to a DUT's ground ? (Thinking USB cable ground could be cut then it powers itself finding a ground path on the DUT.) If this isn't the case I'd get really worried, start checking power outlet ground continuity and being weary of the various protagonist's power supplies safety (Y capacitors). --- End quote --- It is purely DC as far as I can tell, at least the 240mV I measured is. The pico seems to work perfectly fine, always has. --- Quote from: coromonadalix on June 18, 2024, 12:00:47 am ---can you plug the pico on a lappy battery powered ? if this offset / current dissapear the generator could be at problem, something in the ac section, ac filter hoping its not those who get defectives or phases problems between your main computer and the generator ? --- End quote --- I don't believe the signal generator is the problem. I can get current to flow between the shield of the usb cable and a grounded surface, regardless of how the scope or the generator is hooked up. I did not test with a laptop, but I would assume that the issue would go away. I will test with a laptop tomorrow just so that I can finish what I was doing before I noticed this problem. |
| Alex_Baker:
I tested with a laptop on battery, not issues at all. There is definitely a problem with my PC, power supply or otherwise. The power supply is a Corsair RM850e that I bought back in march, not super high end but not bottom of the barrel either. |
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