Why a DC power supply doesn't have a voltage with respect to ground?

Summary
I cant understand why DC Power supply doesn't have voltage with ground
I see that DC power supply have voltage between it's + & - and its 24V.
However, there is no voltage with ground.
I don't understand - if device's "point" has some potential, why doesn't it give some voltage with ground (which has ~0 potential)

I tried this with phoenix contact mini-ps-100-240ac/24dc/1
 
485
211
“If a device’s point has some potential...” You are talking about absolute potential, ie the energy required to bring a unit charge from infinity to that point.

“ground... has 0 potential” Not necessarily. We use that convention for convenience. You haven’t measured the energy required to bring unit charge from infinity to that point either.


Your voltmeter does not measure the absolute potentials of each probe and display the difference. It takes a very small current through its internal resistance and calculates voltage by V=IR. For that to happen, a circuit must be completed and since there is no appreciable circuit between a power supply terminal and ground (the PSU output is floating with respect to ground), no voltage is displayed.
 
First of all, Thank you for you reply.
Based on your reply - I have some questions or statements which needs clarification:

circuit must be completed
Correct.
As I understand, if we have 2 points with different potentials and they are connected - the circuit is complete (which is my situation).

“ground... has 0 potential” Not necessarily.
Agreed.
However, I don't have voltage neither: with + or -.
Yet, I know, they have different potentials. So, If at least one point must give me Voltage...


since there is no appreciable circuit between a power supply terminal and ground
Can you explain what do you mean?
 
1,283
651
The linked PSU is an 'isolated power supply'. This means it has no galvanic connection between the input and the output side. Compared to the ground (input side) the output side '-' terminal can be anywhere between ~-3kV and +3kV (based on device datasheet), depending on the environment it is actually used - it is the environment which will determine this potential difference, not the PSU.

When you connect input ground and output '-' terminal with your voltmeter you are actually not measuring but setting the voltage of the output terminals through the internal resistance of your voltmeter.
 
Last edited:
485
211
However, I don't have voltage neither: with + or -.
Yet, I know, they have different potentials. So, If at least one point must give me Voltage...
Remember how your voltmeter works - it allows a small current to flow through its known resistance. To measure any voltage between +ve and ground, or -ve and ground, current must be able to flow from the terminal, through the meter, to ground, and then back TO THE OTHER TERMINAL. This is a very high resistance path, and that is what I meant by “no appreciable circuit...”.

Thinking in terms of potentials is confusing you. If there were a difference in absolute potential between a PSU terminal and ground, connecting a wire between them would equalise that by allowing a possibly minuscule current to flow. Then it would stop. The terminal and local ground would be equipotential.

The above is not measurable by a normal voltmeter. And it’s meaningless for the purposes of understanding electronics, as meaningless as asking how high your head is in relation to Mars.

The PSU outputs have a potential DIFFERENCE of 24 J/C between them, and the -ve is the lower one. The PSU has a low enough impedance to drive the tiny current through the meter and allow it to measure this difference. Whatever absolute potential difference there is between either terminal and ground is negligible and not measurable with your voltmeter.
 

sophiecentaur

Science Advisor
Gold Member
23,151
3,716
Remember how your voltmeter works - it allows a small current to flow through its known resistance.
However true this happens to be (and the resistance of the best Voltmeters is extremely high), the way that the PD happens to be measured is really nothing to do with the situation. It is possible, with suitable feedback, to have Zero Current flowing. If you want to modify the question to include a resistance of value R in parallel with an ideal / perfect Voltmeter then the question is properly defined.
In practical terms, PSUs tend to be designed with an unspecified PD ground (i.e. 'floating') so that any number of them can be connected in a circuit (usually only two aamof) and the PSUs behave quite independently. You may notice that schematic diagrams of many theoretical circuits use Batteries, to eliminate any confusion of this sort.
 
208
19
This is a power supply to be used with other devices. It can be used 3 ways.
1) by connecting negative to circuit ground the output is + 24 volts

2) By connecting the + to ground the output is - 24 volts.

3) by leaving both + and - unconnected the output is 24 volta floating with respect to ground.

Different applications may require one of these three configurations.
 

davenn

Science Advisor
Gold Member
8,712
5,689
First of all, Thank you for you reply.
Based on your reply - I have some questions or statements which needs clarification:

Correct.
As I understand, if we have 2 points with different potentials and they are connected - the circuit is complete (which is my situation).
Only when connecting something between the + and - terminals, otherwise, no it isn't,

I see that DC power supply have voltage between it's + & - and its 24V.
However, there is no voltage with ground.
I don't understand - if device's "point" has some potential, why doesn't it give some voltage with ground (which has ~0 potential)
Again ... there is no circuit !
the negative, 0V and positive terminals of the DC output are isolated from ground

A 0V point doesn't have to be a grounded ( as in earthed point) it can still be isolated

eg ......

242266


Note the 0V rail s not earthed. This is pretty much the normal.
Your PSU will be the same except it uses a switching supply so that it can be made physically smaller


Dave
 
Last edited:

anorlunda

Mentor
Insights Author
Gold Member
6,854
3,779
We may have a case of confusion between voltage and potential. I'll repeat a post from an earlier thread.

See if this helps.

In the pictures below, I show a simple circuit with 6 variations, moving the ground symbol around. In case 5, there is no ground; the whole circuit is floating. In the table of results, I show "potentials" each at a single point, and voltages each s measured between two points. The potentials are expressed relative to ground, but they could be expressed relative to any other reference.

In these examples, you can consider the ground symbol as either an imaginary reference point, or an actual physical connection to Earth.

The whole point is to show that the potentials and the zero reference point, are arbitrary and make no difference when calculating the voltage and current. Therefore, potential relative to infinity is a useful concept in physics teaching, when we get to actual circuits it is useless and we typically don't discuss it at all. There are exceptions, but those are advanced cases, not basic cases. We always try to understand the basic rule before the advanced exceptions.

Unfortunately, we sometimes get lazy and sloppy in our speech. Once the ground point in a circuit is decided and we start talking about voltages, we presume that the second wire of the voltmeter is attached to ground and start talking about voltage "VA" rather than "VA with respect to ground." In other words, using the language of potentials when discussing voltages. It is wrong but it happens every day.

Does that help or does it confuse more?


6-cases-jpg.jpg



6-cases-png.png
 

sophiecentaur

Science Advisor
Gold Member
23,151
3,716
However, there is no voltage with ground.
There is no specified voltage to ground but there has to be a PD between the supply and ground. That PD will be either unknown (you just took it out of its box and you were in a dry room with nylon carpets etc..) or defined by the circuit it's connected to.
 
485
211
This is how I interpreted the OP’s question:

1. A ground point has some absolute potential value.
2. The PSU terminals have too.
3. There is a potential difference between the terminals of 24V.

When you measure from ground to one terminal, then to the other, why don’t you see the 24V PD? Taking ground as a reference, one terminal must be 24 J/C different from the other.

[EDIT: I’ve used “ground’ here to mean the physical soil, earth and everything connected to it. Perhaps the UK convention of “earth” for this, and “ground” for an arbitrary reference would be clearer ;) ]


I don’t think this is about moving the arbitrary circuit ground reference around to make split supplies, point voltages and so on. This is where the abstract (infinity and absolute potential) meets the pragmatic (how a DMM measures voltage).

Perhaps @EEristavi could clarify? We’re getting two answer streams at the moment.
 
This is how I interpreted the OP’s question:

1. A ground point has some absolute potential value.
2. The PSU terminals have too.
3. There is a potential difference between the terminals of 24V.

When you measure from ground to one terminal, then to the other, why don’t you see the 24V PD? Taking ground as a reference, one terminal must be 24 J/C different from the other.

I don’t think this is about moving the arbitrary circuit ground reference around to make split supplies, point voltages and so on. This is where the abstract (infinity and absolute potential) meets the pragmatic (how a DMM measures voltage).

Exactly my point!

I didn't write any reply, because I'm analyzing all the answers.
I don't want to ask silly question.. :/
 
current must be able to flow from the terminal, through the meter, to ground, and then back TO THE OTHER TERMINAL
Can you explain: why "TO THE OTHER TERMINAL" also


connecting a wire between them would equalize that by allowing a possibly minuscule current to flow. Then it would stop. The terminal and local ground would be equipotential.
I think, this will be correct if we'd have "chemical" potential.
However, in this situation we have "unlimited" charges(from AC source).
Moreover, Earth can take as many charges as possible without changing it's potential (with some approximations)
So I don't agree with this, for now :biggrin:
 
Then the best answer is to stop trying to think in absolutes. Only relative voltages are relevant. See post #9.

I will return to this post.
However, now I'm thinking in "absolute" point of view. I want to understand where I'm making mistake
 
485
211
Can you explain: why "TO THE OTHER TERMINAL" also
I think, this will be correct if we'd have "chemical" potential.
However, in this situation we have "unlimited" charges(from AC source).
Chemical potential? Not relevant here. The AC source is also not relevant, since the PSU output is isolated from the input. It is an independent source. Forget it’s a mains-powered PSU, and imagine it’s a 24V battery. The EMF will push charge around a circuit and back to the source, hence “to the other terminal.”

Similarly, if you measure from a battery +ve to another, unconnected battery -ve, you will see no voltage. See the previous sentence.

Moreover, Earth can take as many charges as possible without changing it's potential (with some approximations)
My Physics teacher said the Earth had a capacitance of about 1 farad, if we needed a ballpark figure for exam questions. That would be 1 coulomb per volt.
 

anorlunda

Mentor
Insights Author
Gold Member
6,854
3,779
Current is the flow of charge. One fundamental of circuit analysis is that charges never bunch up in a circuit. Neither a surplus nor a deficit of charges will build up at any one point.

Think about that for a while, and you will see why current must return to the source.
 
OK, Agreed on what you've said.

lets forget about the circuit once more.

Now I have a feeling, that if I put my finger inside the electric socket, nothing's gonna happen (Joke of course - not planning to), or it will just "bite" me, for a very few time period
 
Last edited:
485
211
OK, Agreed on what you've said.

lets forget about the circuit once more.

Now I have a feeling, that if I put my finger inside the electric socket, nothing will happen (Joke of course - not planning to), or it will just "bite" me, for a very few time period
Certainly not a good idea. Mains power is usually referenced (ie connected) to earth ground, of which you are a part, and hence able to complete a circuit.
 

davenn

Science Advisor
Gold Member
8,712
5,689

Want to reply to this thread?

"Why a DC power supply doesn't have a voltage with respect to ground?" You must log in or register to reply here.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top