Confusion about circuit schematic notations

  • #1
Selection_152.png


I'm confused about the terminals in the picture above simply because I've seen the exact same thing mean 2 different things in schematics, namely:

(1) it is an open circuit with voltage Vi(t)
(2) The terminals are connected to some other circuit and thus have a voltage/current source, and the circuit is closed.

This further confuses me, because it would be so much clearer to just use a voltage source symbol for case (2), instead of it looking like an open circuit.

So I guess my question is, how do I know what the schematic wants this symbol to represent? An open circuit, or a voltage source that is closed so that current is coming out of it?

Why don't we just use a voltage source instead? Is it supposed to be implied that the schematic symbol means case (2) (because otherwise vi(t)=0)
 
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Answers and Replies

  • #2
LvW
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To me, this circuit makes no sense.
OK - there is a voltage source Vi between the two nodes named "+" and "-".
But what is the meaning of the ground symbol, because the voltage source is floating and not referenced to a common ground?
 
  • #3
To me, this circuit makes no sense.
OK - there is a voltage source Vi between the two nodes named "+" and "-".
But what is the meaning of the ground symbol, because the voltage source is floating and not referenced to a common ground?

Ah ok, so this symbol means there is a voltage source between those two nodes, with current coming out of it right? But how do we know when the diagram wants this, and not an open circuit?

Also, I would think the voltage source is not with respect to the ground, only that the + node is vi(t) volts higher than the - node
 
  • #4
PhysicoRaj
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Case 1:
The terminals if left open with polarities mentioned, or depicted with an ideal voltage source, are the same. It just provides a potential difference between the two terminals and can theoretically provide any amount of current that the following circuit draws. Pure KVL and KCl , Ohm's law.

Case 2:
The terminals are connected to some other circuit and thus have a voltage/current source, and the circuit is closed. This would be the case if mentioned. Unless specified you don't need to assume this. This case requires Thevenin/Norton reduction of the (in this case) unknown circuit to find out how the vi(t) [in the image] was deduced from its internal/main source and also, the current that would be drawn.

For this case, since you don't know what network produces the vi(t), you can bravely assume it as a voltage source and find out vo(t).
 
  • #5
Case 1:
The terminals if left open with polarities mentioned, or depicted with an ideal voltage source, are the same. It just provides a potential difference between the two terminals and can theoretically provide any amount of current that the following circuit draws. Pure KVL and KCl , Ohm's law.

Case 2:
The terminals are connected to some other circuit and thus have a voltage/current source, and the circuit is closed. This would be the case if mentioned. Unless specified you don't need to assume this. This case requires Thevenin/Norton reduction of the (in this case) unknown circuit to find out how the vi(t) [in the image] was deduced from its internal/main source and also, the current that would be drawn.

For this case, since you don't know what network produces the vi(t), you can bravely assume it as a voltage source and find out vo(t).

This solved my question. Now something long plaguing me (that I knew at the back of my head, but had no confirmation of) has been resolved!
 
  • #6
LvW
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Ah ok, so this symbol means there is a voltage source between those two nodes, with current coming out of it right? But how do we know when the diagram wants this, and not an open circuit?

In case of an open circuit - where should the voltage comes from?
 
  • #7
In case of an open circuit - where should the voltage comes from?

Maybe they could take a bunch of electrons, put them by the - terminal, surround it by an insulating material, then doing something similar to the + terminal, except taking electrons away?
 
  • #8
PhysicoRaj
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The doubt arising from the non referenced voltage source and a confusing ground symbol is quite valid.

In case of an open circuit - where should the voltage comes from?"

If you find an open circuit anywhere, either it carries a voltage drop, or, represents an ideal voltage source.
Depends on whether it is being measured or being supplied. Just like saying.."connect those open terminals marked + and - to the supply with + and - terminals respectively." They just omit the voltage source schematic.
 
  • #9
davenn
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Maybe they could take a bunch of electrons, put them by the - terminal, surround it by an insulating material, then doing something similar to the + terminal, except taking electrons away?

no, that doesn't even make sense

when you see terminals like that is a circuit its is indicating where a power supply is connected to the circuit ... nothing more, nothing less

in the circuit you have shown where there is also an earth symbol, it is also indicating the possibility of a split rail power supply
else, as LvW said ... it doesn't make much sense


Dave
 
  • #10
LvW
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This solved my question.
Really? And what was the task connected with the circuit? To find Vo ?
 
  • #11
Baluncore
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The voltage is a floating source. The earth symbol establishes a reference terminal for voltage measurements.
The statements or questions that accompanied the circuit are needed to interpret the situation.
@ x86. Why have you not provided the essential context ?
 
  • #12
PhysicoRaj
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The statements or questions that accompanied the circuit are needed to interpret the situation.
@ x86. Why have you not provided the essential context ?
Probably the OP was not concerned about solving this particular circuit, rather the circuit was taken as an example to understand the schematic representation of voltage sources and the circumstances which require different assumptions. So the circuit didn't come with any context as it seems to me.
 
  • #13
Baluncore
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So the circuit didn't come with any context as it seems to me.
Every partial circuit needs context.
 
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  • #14
LvW
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The voltage is a floating source. The earth symbol establishes a reference terminal for voltage measurements.

OK - lets consider a simple example:
A resistive voltage divider R1=1k, R2=3k connected to a floating voltage source Vi=4V..
The current will be 1mA.
Question: What is the voltage between both resistors referenced to a such an arbritrary terminal?
 
  • #15
Baluncore
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Question: What is the voltage between both resistors referenced to a such an arbritrary terminal?
What "arbitrary terminal" are you referring to ?
What do you mean by a "voltage between both resistors" ?
 
  • #16
LvW
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What "arbitrary terminal" are you referring to ?
What do you mean by a "voltage between both resistors" ?
OK - in your reply#11 you spoke about "a reference terminal for voltage measurements. "
I was referring to this sentence assuming that (a) such a "reference terminal" can arbitrarily chosen and (b) serves the purpose of "voltage measurements".
And, of course, I mean the voltage between the common node of both resistors ("between") referenced to such a "reference terminal".
This is only to understand the meaning of your reply #11.
 
  • #17
Baluncore
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"reference terminal"
All voltage measurements are differential. A single voltage must be referenced to something.
At some point you must define the reference voltage. That is what a single isolated ground symbol does.

And, of course, I mean the voltage between the common node of both resistors ("between") referenced to such a "reference terminal".
Of course that was not obvious. The “voltage between both resistors” is implicitly differential, not referenced to ground.
The “voltage at a junction or node” needs a reference. Select that reference by placing a ground symbol on the schematic.
I would make the common node of the two resistors the arbitrary reference terminal. Problem solved.
Where would you have placed the ground reference ?
 
  • #18
LvW
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All voltage measurements are differential. A single voltage must be referenced to something.
At some point you must define the reference voltage. That is what a single isolated ground symbol does.

Of course that was not obvious. The “voltage between both resistors” is implicitly differential, not referenced to ground.
The “voltage at a junction or node” needs a reference. Select that reference by placing a ground symbol on the schematic.
I would make the common node of the two resistors the arbitrary reference terminal. Problem solved.
Where would you have placed the ground reference ?

Baluncore, you can believe me that I know how a voltage is defined and how it is measured.
And if you would read my replies again you will notice that I ALWAYS have mentioned "... referenced to ...".
So - it is really not necessary to spend your time for telling me that a "single voltage must be referenced to something"
(By the way: What is a "single voltage"? Perhaps you mean the potential of a single node - referenced to something?)

But that is not the point! Perhaps there is a misunderstanding between us.
Let me summarize: The OP x86 has shown a circuit with an input voltage Vi and an output voltage Vo.
However, because the voltage Vi is floating my comment was that it would not be possible to calculate Vo.
Now - from your statement in post#11 ("The earth symbol establishes a reference terminal for voltage measurements.")
I`ve got the impression that you feel able to perform this "measurement" with the help of the "earth symbol" .
And my question (using another simplified example) was if my impression was correct or not.

With other words (to make my position clear): Without the shown symbol (ground, earth) the circuits equations could be solved , but I don`t know the meaning of the ground sign. Perhaps it was only forgotten to connect the input voltage also to ground? Do you know what I mean?
 
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  • #19
Baluncore
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I`ve got the impression that you feel able to perform this "measurement" with the help of the "earth symbol" .
I can only suggest that you enter the OP circuit into a Spice simulator such as LTspice and try to solve it without the ground symbol. Then once you insert a ground symbol somewhere, the numerical solution becomes possible.
 
  • #20
Averagesupernova
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The whole thing looks a bit cryptic to me. The voltage across R7 which is Vo(t) is irrelevant to whether there is a ground symbol attached to the node or not assuming that we hook a voltage/current source on Vi(t) that does not have the same ground symbol attached to one of its nodes. I guess we are to assume a completely isolated source. Seems this is an exercise in analyzing schematic symbols correct? If I were designing a test for just such a purpose this would be a good example.
 
  • #21
CWatters
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Vi is not completely floating. Both terminals are connected to earth via resistors of equal value. If Vi is positive then Vo=0.
 
  • #22
Baluncore
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For positive Vin it is simple, Vout will be zero.
For negative Vin, with a perfect diode, the (7k in parallel with upper 3k), forms a voltage divider with the lower 3k. Then Vout = –41.2% of Vin.

Here is the simulation.
floating (2).png
 
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  • #23
Thanks for all the replies, I was just confused about the symbols. I've just always assumed that symbol meant an ideal voltage source. The solution is rather trivial if we assume an ideal voltage source and ideal diodes, namely

Vo(t) = 0 for Vi(t) > 0
Vo(t) = -0.41Vi(t) otherwise

To earlier statements about the circuti being wrong since Vi(t) isn't with respect to ground, I think the circuit is correct. Vi(t) is just the difference between the (-) and (+) terminal.. Vo(t) is the drop across the resistor. We can just think of them as ideal voltage sources, and then they don't have to be with respect to ground.
 
  • #24
LvW
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For positive Vin it is simple, Vout will be zero.
For negative Vin, with a perfect diode, the (7k in parallel with upper 3k), forms a voltage divider with the lower 3k. Then Vout = –41.2% of Vin.
Yes - that`s true. What does this mean? The ground symbol has no effect at all. For positive Vi there is no current through R3 (open circuit).
That`s what I wanted to say about this matter in my previous posts.
 
  • #25
Baluncore
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With other words (to make my position clear): Without the shown symbol (ground, earth) the circuits equations could be solved , but I don`t know the meaning of the ground sign. Perhaps it was only forgotten to connect the input voltage also to ground? Do you know what I mean?
The ground symbol has no effect at all. For positive Vi there is no current through R3 (open circuit).

That`s what I wanted to say about this matter in my previous posts.
For some reason your posts increased the confusion, maybe because you did not say what you wanted to say.

Yes - that`s true. What does this mean?
It actually means that by selecting an appropriate reference ground you can produce a counter intuitive, half wave inverter, all without needing an active device. The circuit was clearly designed to be sufficient to confuse most students.

The floating voltage source, Vin, is not a problem at all. Vin could be replaced by a transformer secondary. By selecting the appropriate reference terminal, a transformer secondary by itself, can make a full wave inverter.

The ground reference on a circuit schematic is not always there to provide a common ground node for circuits, signals or power currents. It may be there simply to protect the circuit from breakdown of the insulation. For me, that is a good enough reason for having one ground symbol somewhere on every circuit schematic. It is an essential engineering component.
 
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  • #26
CWatters
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The ground symbol has no effect at all.

It means Vo is referenced to ground not floating.
 
  • #27
LvW
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For some reason your posts increased the confusion, maybe because you did not say what you wanted to say.
Baluncore, perhaps it makes no sense to further discuss who has caused misunderstandings - nevertheless may I repeat my first comment (post#2):
But what is the meaning of the ground symbol?

On my side, the confusion has started with your statement in post#11: The earth symbol establishes a reference terminal for voltage measurements.
I think, meanwhile it is clear that this does not apply to the shown circuit .
(Did you always say what you wanted to say?)
 
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  • #28
jim hardy
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But what is the meaning of the ground symbol?

My soapbox again..... <BORING LECTURE ALERT>

"Ground" is the most abused term in EE.

It is confused with term "Earth" because of water analogies and our childhood experience playing in the yard with a garden hose..
The water meter is in the ground and water from the hose falls to ground(earth) and soaks in.
We mistakenly transfer that same idea to electricity because at that age we don't understand it's not gravity pulling lightning down from the sky.

Ground symbol is nothing more than a convenient place to hook your voltmeter's black wire.
Usually it's placed on the node where current from all parts of the circuit join up for return to power supply, "Power Supply Return" or "Circuit Common"..
That node may or may not be "earthed" through a power cord or bonding conductor. In airplanes and missiles it's 'vehicle skin', obviously not earthed. In cars that aren't fiberglass it's the body sheet metal, insulated from earth by the tires.
In US 120 volt household appliances the metal chassis is called "ground" and is "earthed" by a green wire,. "Power supply return" is called "Neutral" and gets a white wire.

Awareness of those distinctions helps one keep his thinking straight.

<end rant>

old jim
 
  • #29
LvW
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"Ground" is the most abused term in EE.
Hi Jim - just one question:
Taking your explanations into consideration (and, of course, I agree with everything) wouldn`t it be more appropriate to say that "Earth is the most abused term in EE"?
 
  • #30
jim hardy
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wouldn`t it be more appropriate to say that "Earth is the most abused term in EE"?


Maybe.
I first encountered "Earth" in a British motorcycle manual, my 1956 Royal Enfield.
It seemed an oxymoron because of the tires but still it was clear they meant motorcycle frame, "Circuit Common", where current from the lights gathered for return to alternator.

I don't recall seeing "Earth" used much in US electrical code , they use "Ground"
"Grounding" conductor is green here and i think brown in EU.. (see next post for correction... thanks baluncore jh )
Because our white "Neutral" wire is also "Grounded" at the service entrance it is all too often mistakenly called "Ground" instead of "Neutral" or "Return".

So i do try to call them by names that are unambiguous

When talking electronics i usually use "Earth" for the node that's hardwired into the ground for safety
and "Circuit Common" or sometimes "Power Supply Return" for the node that collects power supply current for return back to its source. .
Even though i used 'ground' in this thread it was a departure for me,

When talking house wiring i use "Neutral" for the white wire that collects current for return to your power company transformer. Observe that's the function of circuit common. Even though the neutral node is connected to earth its purpose is not to transport current to earth but back to its source, the transformer winding.
And i use "Earth" for the green wire intead of Ground" so nobody will think i meant "Neutral" .


So, to your question ---- were i to declare "Earth" the most abused term i'd be criticizing our British friends without having walked that "mile in their shoes"..

ps i'm still trying to sort out a concise summary for NEC "Bonding" article 250
http://www.necconnect.org/resources/2014nec_changes_article250/ midpage
 
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