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| Weird USB oscilloscope ground issue? |
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| Alex_Baker:
I did a bit more testing but came up inconclusive. The resistance between GND and earth seems fine, a couple of ohms on my crappy meter that reads 1 ohm with the probes shorted. Something odd that I noticed with an analog ammeter is that the needle will drift cyclically up and down a few milliamps. As a point of comparison I checked three other ATX PSUs, all from different decades and none showed current from GND to earth. I did the same test to the one in my PC with everything disconnected and got no stray current, so this phenomenon only occurs with the whole system hooked up. Another more concerning and puzzling thing that I found is that with the ammeter attached to GND in the PC, and earth, the needle jumps around in tune with a new squealing sound that the PSU is making. I don't know when this noise started, but I only noticed it when I went snooping around my computer. Remind me to never buy a corsair power supply again, because every one I have owned, new or otherwise, has squealed like a stuck pig. |
| magic:
As much as it sucks, this is absolutely normal and not a sign of any mains wiring issue. Your ATX PSU is internally grounded to mains protective earth and there is no current flowing through this connection until you connect your USB gadget. The gadget is grounded to the motherboard, which is grounded to the PSU through a bunch of black cables, as you know. Those cables carry many amperes of current, hence motherboard ground is some millivolts above true earth and its exact potential jumps around in sync with varying power consumption of the CPU. If you ground your USB gadget elsewhere, you provide a new path for the motherboard ground current to return to the PSU (through the USB cable and mains protective earth), so part of it diverts there. About the only solution is to reduce resistance between the PSU and the motherboard, and particularly between the earth connection inside the PSU and the USB port ground pin. |
| Alex_Baker:
--- Quote from: magic on June 22, 2024, 09:15:29 pm ---As much as it sucks, this is absolutely normal and not a sign of any mains wiring issue. Your ATX PSU is internally grounded to mains protective earth and there is no current flowing through this connection until you connect your USB gadget. The gadget is grounded to the motherboard, which is grounded to the PSU through a bunch of black cables, as you know. Those cables carry many amperes of current, hence motherboard ground is some millivolts above true earth and its exact potential jumps around in sync with varying power consumption of the CPU. If you ground your USB gadget elsewhere, you provide a new path for the motherboard ground current to return to the PSU (through the USB cable and mains protective earth), so part of it diverts there. About the only solution is to reduce resistance between the PSU and the motherboard, and particularly between the earth connection inside the PSU and the USB port ground pin. --- End quote --- This makes sense, thanks! I am still wondering how I have never noticed this before when using my scope, perhaps I had something hooked up weirdly that one time. It sounds like the proper way to avoid this issue is a USB isolator and external power supply(or some devices have built in isolated DC-DC). |
| magic:
Yes, USB isolation will do it. A dedicated low power SBC for running the scope also should. I don't know if USB has enough common mode noise immunity for this to work, but one could also try to star-ground the USB port in the PC. Say, buy one of those rear brackets with USB ports connected to pin headers on the motherboard, cut the GND cable and connect it to a free Molex plug going straight to the PSU. I discovered this issue long ago with audio equipment, because analog outputs have the same offset and CPU noise problem. It is a widely known problem, although few people appear to know the reason. |
| wraper:
--- Quote from: magic on June 22, 2024, 09:15:29 pm ---As much as it sucks, this is absolutely normal and not a sign of any mains wiring issue. Your ATX PSU is internally grounded to mains protective earth and there is no current flowing through this connection until you connect your USB gadget. The gadget is grounded to the motherboard, which is grounded to the PSU through a bunch of black cables, as you know. Those cables carry many amperes of current, hence motherboard ground is some millivolts above true earth and its exact potential jumps around in sync with varying power consumption of the CPU. If you ground your USB gadget elsewhere, you provide a new path for the motherboard ground current to return to the PSU (through the USB cable and mains protective earth), so part of it diverts there. About the only solution is to reduce resistance between the PSU and the motherboard, and particularly between the earth connection inside the PSU and the USB port ground pin. --- End quote --- To the level of what he'd seen it's not normal. |
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