I've been wondering about the cause of this phenomenon for a long time and since I haven't managed to find a good explanation, I thought I'd post it here.
My desk is in a corner, there's a radiator running along the wall. I have a desktop computer next to the desk which is connected to a monitor. I also have a laptop with a power brick. The monitor, the computer and the laptop are plugged into the same power strip which is plugged into a non-earthed outlet. The power strip has a switch on it so I can turn everything off at night.
Here's the deal: I have a headset connected to the laptop but I'm only using the microphone. If I hold the top part of the headphone's jack plug to the radiator, I get static noise (ground loop) coming from the left speaker. The middle part gives me static coming from the right speaker. But if I hold the bottom part to the radiator (the ground), the image on the monitor that's connected to the desktop via HDMI disappears.
The same thing happens if I use USB-powered speakers. If I connect the USB cable to the laptop and hold the audio jack to the radiator, I get the same effect.
This only happens if the power brick is plugged in. If I'm running off the battery, it doesn't do this.
Why does this happen and how could it affect a completely separate device?
答案1
Get yourself a multimeter and measure the impedance and voltage between the interesting points. Is the ground pin of the headphone jack connected to the ground in the power strip? Is there voltage between the ground of the power strip and the radiator? Are the monitor and the PC both connected to the ground in the power strip? My guess is that the monitor is not and is only weakly grounded via the HDMI cable, and is leaking current to ground. When the power strip has no ground, the monitor and desktop are sufficiently coupled by that HDMI cable that both of their grounds have the same float voltage. When you touch the headphone jack to the radiator though, you ground the desktop through the laptop, allowing current to flow from the monitor through the HDMI cable, to the desktop, then through the power strip and out to the radiator. The ground in that HDMI cable though isn't meant to carry current, and so some voltage builds up between the ground of the monitor and the ground of the desktop.
答案2
First off, why would you do this? You're creating a path to ground that wasn't meant to be. Chances are the radiator is earth grounded, so touching an ungrounded electrical device to it is asking for trouble (difference in potential). Grounds loops can be extremely confusing and difficult to troubleshoot. They can also damage equipment and people.