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I'm designing a USB device (a 3D printer board) which I want to power from an old PC power supply and use the USB only for communication. My understanding is that both the power supply and my PC are connected to earth ground, therefore form a ground loop.

My questions are:

  1. Can it cause some problem in this application?

  2. If yes, how can I make a sufficient (and cheap) isolation?

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  • \$\begingroup\$ What will be the interface to USB? IE like a FTDI chip or a PIC with built in USB> \$\endgroup\$
    – vini_i
    Commented Sep 29, 2015 at 16:30
  • \$\begingroup\$ Probably not very cheap as it's from Linear but: linear.com/solutions/5123 \$\endgroup\$
    – Arsenal
    Commented Sep 29, 2015 at 16:31
  • \$\begingroup\$ I planned to use MCP2200 \$\endgroup\$
    – random8882
    Commented Sep 29, 2015 at 16:38

2 Answers 2

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An ATX power supply should be isolated in conformance with IEC60950, 1.5kV if I remember correctly. USB's ground reference is consistently inconsistent. On a laptop, it will probably be referenced to the laptop circuit ground, which is usually isolated. On every desktop I've ever checked, it's referenced to earth. As long as your ATX power supply is isolated (get your DMM out and double check it), neither scenario is a problem. If both devices are ground referenced, there is no problem.

It's not necessary, but if you really insist on adding isolation, power the MCP2200 from USB power referenced to the USB ground. Put an ADUM1201 or equivalent between the MCP2200 and your micro.

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  • \$\begingroup\$ The ATX output rails should be isolated from the mains (throw it away if not) but the common rail should still be connected to ground. This is where the potential ground loop comes from. \$\endgroup\$
    – AaronD
    Commented Sep 29, 2015 at 20:09
  • \$\begingroup\$ @AaronD Not sure what your point is? \$\endgroup\$
    – Matt Young
    Commented Sep 29, 2015 at 20:15
  • \$\begingroup\$ The ATX supply is not completely isolated like a wall-wart is. It still has a fixed ground connection when wired correctly. \$\endgroup\$
    – AaronD
    Commented Sep 29, 2015 at 20:19
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I have used the ADuM4160 for isolating full-speed USB. It doesn't work for high-speed. This was used in a case where the device being connected to could produce kV-sized transients. This shouldn't be needed just because of ground issues though.

PC (I assume you mean ATX) power supply outputs are generally not connected to earth ground. Neither is the USB ground on a PC (since it is tied to the power supply ground). So, there should be no ground loop.

Note that the different outputs (+12V, +5V etc) on a PC power supply do use the same ground.

If both supplies were grounded instead of floating, they would be grounded to the same ground, so there still wouldn't be a ground loop. There also wouldn't be a loop if one of the supplies was floating and the other was grounded.

The only way to create a loop is to have two supplies which are both tied together but one uses a virtual ground which is different from the real ground; for example, if you had a single PC power supply and wanted to generate +7V as 12V-5V, then the virtual ground would be at 5V relative to supply ground for that circuit. If you then tried to power something in that circuit from +3V as 3V-0V from the same supply, you would have a loop.

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  • \$\begingroup\$ I recently fixed an audible ground loop in a sound system that used a USB sound card to feed an analog mixer board. Both the board and the laptop used the same grounded power strip, but there was still a hard-drive-like noise that would quit when the laptop was running on battery. Turns out it was a ground loop through the laptop's internal power supply. I cut the USB cord and soldered in a '4160 along with an external wall-wart to power the sound card and its side of the isolator, and now it's quiet. \$\endgroup\$
    – AaronD
    Commented Sep 29, 2015 at 20:21
  • \$\begingroup\$ The amount of voltage noise required to be audible in a sound system might be less than required to mess up a digital CNC device, but it's still there. \$\endgroup\$
    – AaronD
    Commented Sep 29, 2015 at 20:21

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