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Traumflug
This ground loop thing is right, but ignoring the shield doesn't help, you get the loops through the data lines anyways. In fact, I found connecting the housing to GND to be neccessary even in my low EMF environment.
Depends on where the link is.
On some PC's, there is no link between AC earth and DC ground, and the PSU expects them never to be in contact. If the USB shield gets connected to AC earth on the PC side (instead of DC ground), then the dev board actually provides the short between DC ground and AC earth. As long as all parts of this setup (which includes the PC) are not going to cause an issue, then it's all ok.
The problem with PC supplies (and those generic brick PSU's a lot of people use to power printers) is that they are NOT all the same, and there is absolutely no spec for whether a DC offset between the DC ground and AC earth can exist or not. Especially since people have a penchant for saving every last penny/cent on things, the chances of issues like this being present is higher.
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Mallgan.O
Cefiar:
Can you elaborate or point to a thread where this problem has been investigated? I am curious to see under what circumstances these problem shows up and if it is absolutely determined that it is a ground loop problem and not interference that causes this behavior. Many, if not most, dev boards are also missing filtering that is commonly used in commercial products to pass EMC compliance tests. I want to try making this board as reliable as I can and that is why it is going to be limited in functionality so I can put more effort in verifying and testing, hopefully. I am aware it is not a trivial task to make a board with an MCU working at its limits, communicating over USB while switching the currents involved here on the same board so I hope I am not percieved as critical, I just want to try to understand and learn.
It's not so much interference. In this case, it results in possibly high current flow (the USB cable gets warm or hot to the touch) and damage could happen to either the PC's USB port, the dev board, the cable or any combination of them.
We discussed this issue with RAMPS-FD a bit, because one user had issues where the extra current draw through certain tracks (on an old revision) was actually affecting the DC ground bias for the thermistors (the ground value in the voltage divider wasn't ground any more, and this shifted the thermistor readings). As a result we beefed up the thickness of the ground track in this area (that was a fault in the design), but there shouldn't be any current flowing through the USB shield IMO. Originally we thought it was due to the hot end/heated bed current draw, but it turned out disconnecting the USB cable removed the ground loop. The Arduino Due (and the Mega) has the USB shield and DC ground connected (with no way to disconnect it).
Note: I'm not saying leave it disconnected permanently. Just make it separate and put a physical jumper there (or a cuttable track). This provides a place to:
1. Isolate the USB shield from the rest of the DC ground circuit.
2. Measure any voltage difference between USB shield and DC ground.