Safety of common grounds

Other Hardware General Electronics
1

Hi,
I learned about how do the common grounds work, yet I still have a question about safety in a specific case.

Let's suppose this schematic (EDITED):

problem.png
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Both PC and the other supply are connected to the same wiring in a house -> they're sharing the ground.
Now, what if the wire marked as X was poorly connected or disconnected at all by some accident. Wouldn't the electricity from high power supply want to flow to the GND pin on Arduino, since it then goes to the common ground from there, and thus frying the Arduino ground pin? (Or even the USB on my PC)

In my project I would like to control multiple Solenoids at once like this, so the Amp flow on the right side would be even higher. I want to be sure I plan it well and know what I'll be doing.

problem.png
problem.png636×568 6.53 KB

2

You had better HOPE the wire marked "X" is not connected the way you have shown. You have a direct SHORT between the power supply + terminal and the power supply - Terminal.

LEarn a bit more about electricity.

Paul

3

Your "High power supply" has 4 connections and none of them are marked. So I have no real idea what you're talking about. But if by any chance the wire with the X is the negative of a DC supply of some unknown voltage then if it is disconnected that supply isn't delivering any power. So not a problem.

Steve

4

I understand your diagram, but it's so wrong that people are unable to get it. Experienced people have looked at so many schematics that a mistake just looks like a mistake. But in this case, I suspect the diagram doesn't show what you really mean.

The answer is, yes. If the "x" is broken, current from the solenoid circuit will flow through the protective ground of your computer, your project, and the house wiring. Simple do not allow it to happen!

5

The digital ground (DC -) is not always connected to the third pin of a 3-wire outlet (aka "earth"). Many power bricks and wallwarts are only 2-pin. Three pin ones don't always tie the DC negative to earth, either. You generally want it this way, too.

The - side of all DC power supplies must be tied together in order for circuits to function. Otherwise, you need things like "opto isolators" to communicate between them, which is generally neither necessary nor appropriate.

6

DrAzzy:
The digital ground (DC -) is not always connected to the third pin of a 3-wire outlet (aka "earth"). Many power bricks and wallwarts are only 2-pin. Three pin ones don't always tie the DC negative to earth, either. You generally want it this way, too.

True, it allows you to configure it as a negative supply if needed. Sometimes the ground is positioned adjacent to the negative on the DC screw terminal block, so a jumper clip can join the two, or be removed if not wanted.

7

Thank you @aarg and @DrAzzy for your constructive answers! Im sorry for so vague schematic (in Paint...), it was a fast sketch for the idea - I was focused more on the upper part, so i just drew the bottom to show the grounds I meant and even forgot to write down the amps and volts of the supply. I edited it a bit for (a bit) more clarity.

I will both check whether the supply is actually connected to ground or not (yeah that didnt pop up to my mind at first), and also that the wire X is properly connected in any case, so that Im twice sure the current doesnt flow to the wrong side.

I have a follow-up question if I still havent discouraged you - the idea of this project was to allow control of at least 8 of the solenoids like this. Being connected in parallel, that makes a desire for a strong supply (thats why I wrote "high power"). Yes, it's not a small project as it may have been looking like, it's kinda meant to control a loom...
Anyway, assume eg. 24 solenoids - that is 24A at 12V when all are on and that is a lot. I looked up the supplies and they even begin to have active cooling. I assume breadboarding falls of in this case and that I should plan it well and then solder it directly and use a PCB or something for the mosfet connections. Also, I wonder whether I should rather divide the project into eg. 3 parts, so that Ill rather have three 8A supplies than one so strong one, so I dont have 24A running in one cable. Though the grounds would need to be connected anyway, so Im not sure whether this is just a stupid idea and would be a waste.

8

You most certainly do not use "solderless breadboards" for any significant power - more than an Amp at most, so that is instantly answered.

Beyond that, you use the proper wire and construction techniques for the amount of current involved, whatever that is.

It could be said that if you do not know what you are doing, you should not be doing it, but of course, we are here to help, aren't we? It may be tedious, but if you cite what bits and pieces you propose to use, down to the wire gauge and connectors themselves, you will get good advice. :grin:

9

I will both check whether the supply is actually connected to ground or not (yeah that didnt pop up to my mind at first), and also that the wire X is properly connected in any case, so that Im twice sure the current doesnt flow to the wrong side.

The "ground" on an Arduino is simply a common "reference connection" which may, or may not, be connected to earth ground. For example, if you have a USB-powered Arduino plugged into your laptop there is no earth ground. Even with the charger plugged-in there usually is no connection to the AC (earth) ground. But, it doesn't hurt anything to earth-ground the Arduino. And, nothing will burn-up if you disconnect a ground.

The "ground" in your car is simply a common-chassis connection, insulated from earth by the tires. :wink:

In fact, if the Arduino isn't earth-grounded you could connect +5V to earth-ground and everything would be OK. But, that's generally a bad idea because if somehow the grounds are connected somewhere in the overall circuit, you'd have a short.

The power-line ground (earth ground) is for human-safety in case something "goes wrong". The chassis of your TV is (usually) earth grounded so something if goes-wrong inside the TV and the AC voltage somehow gets connected to the chassis, the power is shorted-out to ground, killing the voltage and blowing the circuit breaker before anybody gets hurt.

I assume breadboarding falls of in this case

I think the rule-of-thumb is about 1A max through a breadboard. One option is to solder (and heatshrink) directly the the MOSFET. But when you mount it (maybe to a heatsink) the tab is the drain connection so they can't all be mounted to the same heatsink.

Also, I wonder whether I should rather divide the project into eg. 3 parts, so that Ill rather have three 8A supplies than one so strong one

That's up to you, depending on cost & space, etc.

so I dont have 24A running in one cable.

With one supply or multiple supplies it's usually better if each MOSFET & solenoid has it's own separate wires (power and ground) back to the power supply. [u]Here is a wire gauge chart[/u] showing recommended maximum current.