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In my specific case, I have an arduino and a servo motor.

  • Arduino is powered from PC via USB (5V).
  • Servo is powered from a separate AC-DC adapter because it needs more current.

In order to control the servo from Arduino, they must have common 0V, so grounds have to be connected. That part is clear.

My question is should I worry about current going through this ground-to-ground connection? In other words, can I consider those two voltage sources to be completely decoupled or is there a situation in which a voltage will be maintained between them by some external source?

Would it be a good idea or an extremely bad idea to place a resistor between the two grounds?

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    \$\begingroup\$ Always OK? Hell no! We had someone connecting car batteries in series, then planning to connect their -ve terminals together! But in your specific case, with two isolated supplies, yes. The only current in that ground connection will be the return from the Arduino -> servo signal - a tiny current. \$\endgroup\$ – Brian Drummond Jan 15 '16 at 22:44
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It's OK if the two supplies are isolated from each other, ie. have no other common connection.

Most AC/DC adapters are 'double-insulated' and not connected to mains Ground. Provided that you only use such an adapter to power the servo it will should be OK. If your adapter has a ground pin connected to the mains socket then its output might be grounded. This could be a problem in a circuit that is sensitive to small differences in ground potential.

Most servos can handle a fair bit of noise on the power lines, so unless you have some high current equipment operating on the same circuit it shouldn't be affected by common ground noise.

Putting a small value resistor between grounds may help to reduce 'ground loop' current flowing through a sensitive device. The servo should still work with 200Ω or more of resistance in the signal path.

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  • \$\begingroup\$ Thanks. I tried a 220Ω and servo stopped working, but with 100Ω it was fine. I guess I'll leave the 100Ω there. I feel better with it than without it :) \$\endgroup\$ – zvone Jan 16 '16 at 9:56

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