Multiple power supply potentials

I am making a Project with an Arduino driving a stepper motor with a driver board. USB will still be connected to send commands over serial. EDIT: I am building an EggBot driver board for my Arduino Uno which needs to be connected to my pc by USB for sending commands with the eggbot-software. Here is my schematic https://imgur.com/a/nrF8RZ7)

Will I destroy my Laptops USB by having another power source connected to the same ground?

(I felt that I lacked general knowledge about DC power supplies and the resulting potentials. Therefore I tried to ask a more general question about AC to DC PSUs)

1. How different are the potentials between the neutral wire, the GND USB** and the GND PSU?

2. The power supply is a cheap one from amazon. So I assume there is no transformer and the 12V are not "floating". Is this safe to assume?

3. If both supplies are not floating, can there be a difference in potential between the GNDs?

4. If not, is there any workaround besides opto-isolation of logic?)

simulate this circuit – Schematic created using CircuitLab

• I don't think a non-isolated supply would be legal to sell most places, considering how common it is for live and neutral wires to be swapped. – Hearth Apr 2 '19 at 13:25
• Use an LCD display on Arduino for serial print debugging. Connecting external DC power supply while USB connected is not recommended particularly in case of motors where sudden current requirement is very high. – Prasan Dutt Apr 2 '19 at 13:53
• @seeknecht It's not yet clear what you're asking. What are you trying to accomplish? What's the purpose of your entire device? – Nick Alexeev Apr 3 '19 at 2:40
• It is not an option for me to disconnect USB, because I need the Eggbot software on my laptop. The question is if current will flow between the grounds. – seeknecht Apr 3 '19 at 8:41
• @Hearth actually in Germany you can always put a plug in both ways. So you are implying that even cheap power supplies will be isolated by transformer? – seeknecht Apr 3 '19 at 8:49

Here is what I have learned:

Every decent AC to DC Power supply uses a Transformer of some kind. This means that the Difference in electric potential (Voltage) to a known reference point e.g. earth will be undefined. See diagram 1)

As soon as a reference point is set, the Voltage to a known point is defined. See diagram 2) and 3)

There are also other unisolated ways to make DC-Voltage from AC. For example by usind a capacitive dropper (See this Video by DiodeGoneWild). Or directly using a rectifier of some sort.

Also as Unknown123 has mentioned:

One more thing is that it doesn't have to be a crappy capacitive dropper circuit, take a look at here and here. They have a transformer, but the lack of: PCB isolation length, transformer winding insulation between primary and secondary, safety capacitor interference class, inductive spiking protection, et cetera, making it to be the worst of the worst. So don't assume every supply that use transformer are 100% safe.

This is problematic and possibly dangerous because the potential of one DC output pins will be either at 0V (Neutral) to earth or 230V to earth. Danger of electrocution!

simulate this circuit – Schematic created using CircuitLab

So to answer my own questions:

• Connection the grounds of two DC power supplies is generally not problematic if at least on of the supplies is floating.

1. The Voltage between the Ground is the diagram is undefined.
2. Any proper power supply will have a "floating" DC Output. (Any thing else is a serious hazard)
3. Yes. As there is no reference.
4. probably not

Thanks to everyone commenting on my post and taking the time to explain! Especially Unknown123.

• 4. probably not You could theoretically put a legit dc-dc isolated converter at its output, but it will probably shocked you earlier when you connect it, so the answer is throw it to the bin if the supply has no isolation, your safety is worth more. – Unknown123 Apr 4 '19 at 23:26