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I am building a system that consists of an Arduino-type device, connected to a host computer via USB, which interfaces with some additional electronics which are driven off an isolated switching DC power supply (https://www.mouser.com/ds/2/260/lrs-100-spec-752931.pdf). I believe the GND node of the Arduino is connected to the GND pin of the USB. Can I assume that the GND pin of the USB is at the same potential as the EGC of the AC mains ? Thus if I connect the negative terminal of the power supply to EGC, then it'd be at the same potential as the Arduino's GND, and I can tie the Arduino's GND to the additional electronics' GND ?

If not, how can I best interface the Arduino to the separate electronics ?

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  • \$\begingroup\$ At some point, the Arduino will be connected to the host (or other hosts) by wifi or Bluetooth, so the question will become moot. I'll simply power the Arduino off a 5v supply derived from the Meanwell supply). \$\endgroup\$ Jan 11, 2018 at 22:23
  • \$\begingroup\$ do not put additional information into comments. ... add it to the question \$\endgroup\$
    – jsotola
    Jan 11, 2018 at 23:35
  • \$\begingroup\$ Can I assume that the GND pin of the USB is at the same potential as the EGC of the AC mains ? ... when it comes to AC mains, never assume anything. \$\endgroup\$
    – jsotola
    Jan 11, 2018 at 23:37

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You are correct, the Arduino ground and USB connector ground ARE connected. If the USB cable is connected to something grounded like A PC, then there will be a ground path through that. If your other electronics have their own ground path (you can test with a multi-meter), then you've created a ground loop. This may or may not be a problem, but it's best to avoid if you can. A simple way to fix this is to isolate your USB connection, so that the system has only one ground.

You can use a device like this: https://smile.amazon.com/Isolator-Protection-Isolation-ADUM4160-ADUM3160/dp/B06XB9YBHJ

Or an IC like this one: https://www.digikey.com/product-detail/en/analog-devices-inc/ADUM3160BRWZ-RL/ADUM3160BRWZ-RLCT-ND/3897186

Some symptoms you might experience if you have a ground loop problem are glitches and dropouts of the USB connection.

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  • \$\begingroup\$ Yeah, ground loops could surely be an issue. I could isolate at the other electronics as well (the Meanwell supply), but I think it makes more sense to do so at the USB connection, because there are much greater currents flowing from the Meanwell than from the USB, and because it'll be more similar to the isolation I'll achieve when connection to the Arduino is via wifi. I didn't know there was a USB isolation device like that available; thanks for the link ! \$\endgroup\$ Jan 12, 2018 at 18:45
  • \$\begingroup\$ I was wondering about that ADUM4160 isolator you linked @Drew (the DIYmore one at Amazon), do you know what the switch does ? One place I saw said that "on" means low-speed mode; but I wonder if it actually refers to whether it's in isolation mode, or just a normal connection. \$\endgroup\$ Jan 24, 2018 at 5:16
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    \$\begingroup\$ From an article about a similar device: "Some devices requires USB low speed (1.5Mbps), this includes mouse, keyboards and some USB to Midi devices. USB Isolators made after June 2015 can work with low speed devices. You need to open the box (while disconnected) and change the position of both the switches inside. Note: This is a quite easy task, but it's performed on your own risk. " It does not not change the isolation as far as I can tell. \$\endgroup\$
    – Drew
    Jan 24, 2018 at 8:16

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