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I do power the Raspberry via a 5V USB charger. The Arduino is soldered into a different circuit and is powered from a 24 V circuit via a 5V charge regulator. Both will be connected to the same 230 V power circuit. Is it safe to connect both serial grounds together ? Or is it possible to create a ground loop ? As far as I understood, both devices have to share common ground if serial connection should work (without optical isolation) ?

This is the circuit with the Arduino. The pins on the upper left are the output of the relais. The 6 head pin is made for the control over the relais card. It is essentially a controler for a 8 port valve (IMV-8, Pharmacia)

enter image description here

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Post your schematics, if no galvanic isolation then they must be referenced at same ground somehow, but you take the risk to burn everything that is connected. – Marko Buršič Jan 19 at 9:20
up vote 3 down vote accepted

Generally your power supplies will be isolated from the mains, and generally it's safe to connect the ground of the circuits together.

If you have a concern that they are not isolated, get new power supplies that are, or perform this test:

Connect the two grounds via 100k ohm resistor, then plug them into the mains. Using a multimeter, test the voltage drop across the resistor. If there is a drop larger than a few millivolts, then you will likely be running a ground loop that will cause problems. If there's a very, very small drop then you'll be running a ground loop, but it will probably be acceptable. If there's no drop then you will have no problem.

If you do have a small drop, you can limit the ground loop current with a resistor, and may still be able to have good communications depending on the ground loop signal, but you'd need to do more testing or use an oscilloscope to determine the nature of the ground loop.

Note that you'll want to test for both DC and AC voltage drop.

Also, by plugging into the same phase and ground you'll probably find this test works even if the supplies are not isolated, but if you later connect them to separate phases or grounds you'll have problems. Just keep this in mind if you ever extend your communications line or use these in a different way than currently used.

There are isolated communication methods that will help you if needed, but they will require additional hardware.

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It depends on your power supply. If either your USB charger or your 24V circuit is powered with an isolated power supply, this would not create a ground loop.

Nowadays devices exsist that are not isolated, but have a ground potential a diode drop higher than the neutral line (e.g. some switch mode power supplies, or PC power supplies).

An easy way to check wheater the devices are isolated, is to do a continuity check between both GND pins (or to measure the resistance between the 230V mains pins and the GND, this should be in the order of megaohms. Be careful to not measure this when connected to the 230V line!!).

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How to I find that out. I do use standard chargers. – Moritz Jan 19 at 9:48
    
Just added it to my answer ;) – Douwe66 Jan 19 at 9:50
    
By the way. The continuity check might not be the best option. Maybe you can better measure the potential difference (voltage) between both grounds when plugged in. However it can be hard to distinguish between noise (changing voltage in mV range when the GNDs are not connected, and a low potential/voltage difference between the GNDs) – Douwe66 Jan 19 at 10:16
    
Would an ADC with 10 MOhms Impedance be enough to do that ? – Moritz Jan 19 at 11:01
    
What do you mean with ADC? Also take a look at Adam Davis' answer. It's very useful! – Douwe66 Jan 19 at 11:08

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