Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Confusion about circuit schematic notations

  1. Jun 21, 2016 #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)
     
    Last edited: Jun 21, 2016
  2. jcsd
  3. Jun 21, 2016 #2

    LvW

    User Avatar

    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?
     
  4. Jun 21, 2016 #3
    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
     
  5. Jun 21, 2016 #4

    PhysicoRaj

    User Avatar
    Gold Member

    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).
     
  6. Jun 21, 2016 #5
    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!
     
  7. Jun 21, 2016 #6

    LvW

    User Avatar

    In case of an open circuit - where should the voltage comes from?
     
  8. Jun 21, 2016 #7
    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?
     
  9. Jun 21, 2016 #8

    PhysicoRaj

    User Avatar
    Gold Member

    The doubt arising from the non referenced voltage source and a confusing ground symbol is quite valid.

    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.
     
  10. Jun 21, 2016 #9

    davenn

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    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
     
  11. Jun 21, 2016 #10

    LvW

    User Avatar

    Really? And what was the task connected with the circuit? To find Vo ?
     
  12. Jun 21, 2016 #11

    Baluncore

    User Avatar
    Science Advisor

    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 ?
     
  13. Jun 21, 2016 #12

    PhysicoRaj

    User Avatar
    Gold Member

    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.
     
  14. Jun 21, 2016 #13

    Baluncore

    User Avatar
    Science Advisor

    Every partial circuit needs context.
     
  15. Jun 21, 2016 #14

    LvW

    User Avatar

    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?
     
  16. Jun 21, 2016 #15

    Baluncore

    User Avatar
    Science Advisor

    What "arbitrary terminal" are you referring to ?
    What do you mean by a "voltage between both resistors" ?
     
  17. Jun 21, 2016 #16

    LvW

    User Avatar

    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.
     
  18. Jun 21, 2016 #17

    Baluncore

    User Avatar
    Science Advisor

    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 ?
     
  19. Jun 21, 2016 #18

    LvW

    User Avatar

    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?
     
    Last edited: Jun 21, 2016
  20. Jun 21, 2016 #19

    Baluncore

    User Avatar
    Science Advisor

    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.
     
  21. Jun 21, 2016 #20

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    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.
     
  22. Jun 21, 2016 #21

    CWatters

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Vi is not completely floating. Both terminals are connected to earth via resistors of equal value. If Vi is positive then Vo=0.
     
  23. Jun 21, 2016 #22

    Baluncore

    User Avatar
    Science Advisor

    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
     
    Last edited: Jun 21, 2016
  24. Jun 22, 2016 #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.
     
  25. Jun 22, 2016 #24

    LvW

    User Avatar

    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.
     
  26. Jun 22, 2016 #25

    Baluncore

    User Avatar
    Science Advisor

    For some reason your posts increased the confusion, maybe because you did not say what you wanted to say.

    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.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted