I am laying out the schematics of a new circuit and before starting with the board I’ve found an error in the ERC I don’t know how to solve.
The problem is that I have a TMLM AC to DC power supply with GND labeled as a power output pin and also a micro USB B that has also GND marked as power output. I am not powering my board through the USB but communications and the power supply need to share grounding. As one my expect, wiring those two pins produce the following ERC error:
Any suggestions on how to avoid the problem? Should I modify the components because there is a better pin type definition for any of them? Is there any other way like for example a net tie or similar?
I agree but again it is nice to note that USB connections can ground your circuit at a different level thus ERC is doing a good job warning me about posible conflicting voltage levels. Don’t you agree?
No. USB B connectors are usually used to supply power to non-host devices, therefore the power pins are defined as power outputs eliminating the need for power flags. In your case the connector will not be supplying power to your device as you have an internal power supply. ERC is simply complaining that you have two power sources driving the same net, if they were +5V and +12V then yes you would have a problem, but in this case they are both GND and ERC can be ignored. Of course it would be a good idea to make adjustments to satisfy ERC, one would be a net-tie, another would be to change the type of the connector pin to reflect it’s actual use.
Usually devices provide ground isolation (given that it is now mandatory in many countries) but there is still some that dont. One example: While working in a non isolated project (someone was measuring current with a shunt) this person plugged the USB uC programmer to the computer to find out that the screen being used was shorting USB ground with some voltage we suspect was mains neutral wire. We don’t know if it was defective or if it was on purpose.
If you really believe there should be another version of the connector without the power output flag I am willing to create a pull request to the KiCad library . It may have a good acceptance. What do you think?
Well… we see a lot of 3D printer owners frying their USB when they plug their controller into the PC, or “USB cable gets hot” is another reported issue. The 3D printer controller has 5V provided by the PC, and 12V (or 24V) from external PSU for motors and heaters. The 5V and 12V are not connected, but the USB ground is shared with external PSU ground. I believe if those grounds are at a significant potential difference, it fries the USB.
It seems to happen with cheap PSUs designed for LEDs, I think +12V is actually referenced to ground (so 0V would be -12V). I’ve also seen it happen with pukka kit, except that the USB device was plugged into a separate extension to the PC, and the extension socket was miswired with ground/neutral swapped.
Hence why I am interested in isolating USB, since management are cheapskates we look for a cheap solution. We tried USB isolators but they seem to cause issues. The alternative I suggested is to have the USB interface connected to UART which can be isolated. That only works for CDC devices of course.