| Electronics > Repair |
| Weird USB oscilloscope ground issue? |
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| Alex_Baker:
I put a hvac style current clamp meter around the usb cable, nothing. I live in the US where GFCIs are not standard equipment, only bathrooms and outdoor outlets have them. Does this mean that my PC's power supply is at fault? I have several other ATX PSUs sitting around and they all measure a couple of ohms between black and earth, I did the same test on my PC with it turned off and got 2 ish ohms. So it seems that whatever bonds black to earth is ok. |
| wraper:
--- Quote from: Alex_Baker on June 19, 2024, 03:19:45 pm ---I put a hvac style current clamp meter around the usb cable, nothing. --- End quote --- Most of the clamp meters only measure AC current. --- Quote ---Does this mean that my PC's power supply is at fault? --- End quote --- No. As I said measure voltage between oscilloscope and computer GND. Also if PSU has GND and earth shorted, it cannot be at fault. |
| Alex_Baker:
--- Quote from: wraper on June 19, 2024, 03:21:44 pm ---Most of the clamp meters only measure AC current. --- End quote --- This was in response to Jeroen3 --- Quote from: wraper on June 19, 2024, 03:21:44 pm ---No. As I said measure voltage between oscilloscope and computer GND. Also if PSU has GND and earth shorted, it cannot be at fault. --- End quote --- I missed your earlier response, I apologize. This issue only occurs with my PC tower, not the laptop. I measured 19mV between scope ground and the shield on another usb port on the front of the computer, the scope is on a very long cable. I measured 100mV between earth and scope ground today, which is interestingly not as bad as yesterday. This ground "leakage" problem has nothing to do with the scope, I can replicate this stray current flow from any GND point on the computer, not just the scope. |
| wraper:
I have another thought. Plug everything in the same mains extension cord and see what happens. It could be something funny is going on with mains earth wiring and you connect devices in question into different "earths". |
| shakalnokturn:
--- Quote from: 2N3055 on June 19, 2024, 09:38:08 am ---This is not ground loop question. Something is wrong. 240mV voltage drop with 180mA current suggest 1.3 Ω resistance. --- End quote --- I don't often disagree with you but here I'll have to, not on the fact that the resistance is rounded down from 4/3 \$\Omega\$ ;D There is a ground loop sketch it out, you'll see... Probe ground, USB ground and PC ground are all common now if you connect the probe ground to another length of wire leading to the PC's PSU inlet ground connection you create a loop which is also an additional return path for the supply current to the Picoscope. This alternate return path runs through the Pico's frontend ground tracks and probe ground wire which also have resistance (say approx. 1.3 \$\Omega\$) so measuring anything is offset by the voltage drop across the probe's ground wire. Edit: OTOH you weren't saying there wasn't a ground loop, rather it wasn't the problem. It still looks the most plausible problem to me considering it's DC. |
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