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As you may know, if you have been reading some of my other posts, I am working on an Arduino project where the Arduino controls the output of a welder. This involves sending a voltage to a pin on the welder that the welder maps to output level. The welder expects a potentiometer to be on the circuit, but instead I am connecting the Arduino to the wiper pin, ignoring the Vcc pin altogether, and I have hooked the welder's ground pin to the Arduino's ground bus.

I assume that I must have the welder and the Arduino share a ground, so that the voltage put out by the Arduino will be referenced to the same level as the welder is expecting. And I assumed that, when the Arduino and the welder were hooked up together, that any difference in their ground voltage level would be resolved--that they would "split the difference," if you will and come to a common ground. But then I got to thinking about ground loops, and I wondered if I was making some incorrect assumption.

Is there some issue that I should be aware of with connecting the Arduino's ground to the welder's ground like this?
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Is there some issue that I should be aware of with connecting the Arduino's ground to the welder's ground like this?

Yes. If whatever is powering the Arduino (e.g. a PC via the USB port) has ground connected to mains ground, and so does the welder, then you will have a ground loop. Ground loops are generally a bad thing, particularly when they involve high current devices such as your welder, and could damage your Arduino.

The solution is to power the Arduino from a supply that is not connected to mains ground. Examples include a laptop computer running from battery power, a battery, or a 2-pin wall wart (in UK/Ireland, that would be a wall wart with a plastic earth pin).
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The solution is to power the Arduino from a supply that is not connected to mains ground. Examples include a laptop computer running from battery power, a battery, or a 2-pin wall wart (in UK/Ireland, that would be a wall wart with a plastic earth pin).

Fascinating. The Arduino is currently being powered either by a 9v battery or by a 9v "wall-wart" with only 2 prongs. So I guess I should be good to go there.

If such a solution was out of the question for some reason, then what would the approach be? Derive power and ground from the controlled device and use opto-isolated connections on the control lines? Something like that?
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If such a solution was out of the question for some reason, then what would the approach be? Derive power and ground from the controlled device and use opto-isolated connections on the control lines? Something like that?

Either that, or use a digital isolator chip such as the ADUM6401.
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