Why negative terminal of source is marked as ground?

[electro]

Dear all,
I am beginner in electronics and I am wondering about one question. I attached some pictures. Could you please explain me why on this pictures is negative terminal of DC source marked as ground?
I have also another circuit where on connector there are just pins marked as "Vin" and "GND".

Thanks very much

[Thor-Arne]

It's the common reference for the circuit, this is usually referenced to as GND. It might or might not have anything to to with the mains-ground.

Think of it as 0V.

[Kremmen]

By convention most electrical circuits have a reference potential that is nominated as the "ground". That is just a convenient way to indicate a (usually/often) neutral potential in reference to planet Earth. A source has 2 nodes and a voltage or current between them. It is just a matter of choice how to name those nodes. Say you have a small circuit powered by a 9V battery. The battery terminals have nominal 9V between them, and usually the positive terminal is referenced to as the +9V terminal. Now the other one cannot be a minus terminal because counting down from the +9V terminal we arrive at 0 volts for the negative terminal. Now we are at earth potential, i.e. GND. You _could call the positive terminal +100V and the negative one +91V but how much sense would that make? On the other hand, connect another 9V battery in series with the first one, and you got instant -9V as well, assuming the connection betwen batteries is still called 0V.

A bit messy explanation but hopefully it made some sense.

[IanB]

In your picture all of the ground points marked with a triangle could be removed and connected with a line and the circuit would be just the same. Very often the "ground" convention is used to make the drawing appear tidier, although it is a matter of opinion whether it makes the drawing easier to read. Personally I don't find it so and I prefer to see all the lines drawn in.

[electro]

Thanks for answers, so it means that ground can be used for negative terminal marking. Because I have one circuit layout, where connector has input marked as Vin and output marked as GND. And for example balancing capacitors are connected to GND, too. So it means, that it is connected to negative terminal of voltage source, yes? It is DC circuit. 

[IanB]

Just think of GND as a "wire" or common connection point in the circuit. Everything connected to ground is actually connected to that common wire. That is all it means.

[electro]

Sorry for stupid questions. This is my last.
This common wire is connected to negative terminal, because in other case, circuit is not closed. But in this circuit is also Zener diode, which is connected to this ground. So if in this circuit is higher current it is sent back to source?

Thanks a lot for your time.

[IonizedGears]

The negative side is usually marked for ground due to the elimination of noise. With coax cables the negative wire or shielding is grounded to send all noise to ground thus protecting the signal from noise. Also, it works very nicely when constructing very complex circuits when you want to have a connection made with little effort and less spaghetti type wiring.

[IonizedGears]

I'm sorry but I couldn't understand what you where trying to say about this zener diode so if you could explain more or send a photo of what you are referring to, thanks.

[electro]

Dear IonizedGears,
I am attaching one picture, on which is circuit I talked about. I want to know what will happen when current is so high, that it pass trough diode. Because this ground is negative terminal in fact.

[LyTeo]

You will have some source (Vin) that you will connect to the left side of this circuit. This source will have its own ground wire. Connect that wire to the wire marked in your circuit as the ground(the one below the diode...).

[snowl]

I assume your Powersupply "+" is attached to Vin and Powersupply "-" is attached to the ground. When the voltage is high enough (not the current), the zener diode will start to clip the voltage of the power supply by starting to be conductive. It must then be able to handle the current which now flows through it.
If the zener diode is meant to protect the circuit from overvoltage, it would be better placed behind the fuse (in the picture on the right side of the fuse), so the fuse will also protect the zener diode in case of overvoltage (So the zener diode must be able to handle at least that much current).

[moemoe]

You're right, this thing is not ground like "earth", but just one (floating) voltage level defined as ground as reference for all the other voltages. You could main reference this by connecting it to your pe, but this is highly not recommended.

Without GND you wouldn't know where to connect the black cable when measuring, because voltage is defined as the potential difference between two points, so one point has to be defined as zero.

[electro]

Ok, so this GND is just name for negative terminal of source as I thought?. It is logical also due to connector has just two pins Vin and GND. So input to source can be just GND. And is it important to have in DC circuit real ground?
Thanks.

[moemoe]

Yes, it's just the negative source of your power supply.

Or, more technically speaking, you just have to feed 5V more on Vin than on GND.

E.g. my lab supply has also an output with -5V/0V/+5V, I could attach -5V to GND and 0V to Vin – and it would work fine.

You normally don't even want it mains referenced (real ground on GND), as there are no advantages. This can even lead to problems under certain circumstances.

[ignator]

<snip>

E.g. my lab supply has also an output with -5V/0V/+5V, I could attach -5V to GND and 0V to Vin – and it would work fine.

<snip>

moemoe: I think you mean connecting the GND to Vin+, as it is positive relative to -5v.  Unless I'm more confused then I thought.
electro: not to add confusion, but you can connect the + power to what is called ground, and use the negative as Vin.  My 1965 Datsun Fairlady SPL310 (attempted copy of a MG) has the 12Volt + battery terminal connected to the vehicle "ground", which is the metal chassis.  There are other cars that also have done this.  I think the mentality is the high voltage ignition system is positive ground, as the hot electrons are much more happy jumping off the thermally hot electrode to the positive cooler engine positive ground.  It makes for a "hotter" (i.e. more energetic spark).  This system was a PITA if you ever tried to add an off the shelf 8 track tape, that used a negative ground, with the +12 brought in on a wire.
Hope this helps, I understand this is a difficult concept, that of ground, which also carries electrons.  In your home wiring is the neutral, this carries AC current, but as it's physically tied to a ground rod at the service entrance (the main circuit breaker panel down stream from the power meter).  Also (at least state side) is the safety ground, this does not carry current, and is there to protect against faults where the Line (hot wire) could short to this, and protects the user, by not letting the chassis become electrified, as in this case the circuit breaker will trip.   Just because ground carries current should not cause confusion.  As in the electric home power case, ground does not carry current.  In DC low voltage system, ground will generally carry current.  But since it's at the same potential as you (you are grounded), there is no current flow though you, unless you are the return path because the ground connection broke. 

[moemoe]

E.g. my lab supply has also an output with -5V/0V/+5V, I could attach -5V to GND and 0V to Vin – and it would work fine.

moemoe: I think you mean connecting the GND to Vin+, as it is positive relative to -5v.  Unless I'm more confused then I thought.

I connect "-5V PSU" to GND@Circuit and "0V PSU" to Vin@Circuit.

I think you missinterpreted the hyphen "–" as a minus. But there is a difference: –- (hyphen, minus)

[ignator]

moemoe: I think you mean connecting the GND to Vin+, as it is positive relative to -5v.  Unless I'm more confused then I thought.

I think you missinterpreted the hyphen "–" as a minus. But there is a difference: –- (hyphen, minus)
[/quote]
That's exactly what I did.  I was confused.

[electro]

Thanks everyone for advices. I hope I understand this problem better.

[dentaku]

I like this explanation. I never thought of that (I'm very new to this kind of stuff) but it's logical that a 5V DC device would work fine with -5V connected to GND and 0V connected to Vin because it's the difference between the 2 points that matters.
While all of this is true I would assume that people should not use this method when powering stuff from ATX power supplies because the -5V on those is usually much less capable than the +5V?

I'm glad I signed up to this forum

Yes, it's just the negative source of your power supply.

Or, more technically speaking, you just have to feed 5V more on Vin than on GND.

E.g. my lab supply has also an output with -5V/0V/+5V, I could attach -5V to GND and 0V to Vin – and it would work fine.

You normally don't even want it mains referenced (real ground on GND), as there are no advantages. This can even lead to problems under certain circumstances.

[moemoe]

While all of this is true I would assume that people should not use this method when powering stuff from ATX power supplies because the -5V on those is usually much less capable than the +5V?

ATX is no proper power supply at all

But of course you always have to look at the current capabilities of your power rails. But this run for large PSUs is mostly also just ridiculous, most circuits don't need that much. I went fine for the first 10 years with a simple 12V max 1A power supply.