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Cells with different voltage in parallel????

Discussion in 'Electronics Homework Help' started by Ian Wright, Nov 30, 2017.

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  1. Ian Wright

    Ian Wright

    6
    0
    Nov 21, 2017
    I can't find any reading material on this because I know you are never supposed to do it but I have a question asking for the Vout when I have the following 3 cells on their own in a parallel circuit:
    V1 - 5V
    V2 - 5V
    V3- 4V

    Even though you are never supposed to do this is there any formula that relates to calculating the Vout of such a circuit? I know that if it was in series the 5V batteries would slowly drain and the 4V battery would slowly gain but I'm lost at what happens when they are in parallel?
     
  2. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    Is this a theoretical circuit or a real physical problem? The assumptions made about theoretical cells make it difficult to determine what might happen in a parallel situation, unless you can calculate the voltage drop caused by an infinite current flowing through a zero resistance. But real physical cells all have some internal resistance greater than zero, thereby making the solution approachable. However, the behavior of a real cell under reverse current flow would depend on the particular chemistry involved, and who knows what else.
     
  3. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    Do not do it, the 5V battery will charge the 4V battery and perhaps melt the connecting wire.
    Where do you get a 4V or 5V battery? Is this describing batteries past their best.
     
  4. Ian Wright

    Ian Wright

    6
    0
    Nov 21, 2017
    It is an entirely theoretical problem. The question is to try calculate the resulting Vout from such a circuit. I have worked out the internal resistance of each cell but struggling to come up with anything else simply because I have no idea what formulas to consider. I can find no reading material as all I can seem to find is on the matter is "Do Not Do This!" - but that doesn't help for my homework question as I am expecting to show workings etc and come up with some figures...
     
  5. Ian Wright

    Ian Wright

    6
    0
    Nov 21, 2017
    It is an entirely theoretical problem. The question is to try calculate the resulting Vout from such a circuit. I have worked out the internal resistance of each cell but struggling to come up with anything else simply because I have no idea what formulas to consider. I can find no reading material as all I can seem to find is on the matter is "Do Not Do This!" - but that doesn't help for my homework question as I am expecting to show workings etc and come up with some figures...
     
  6. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    Show us your circuit with resistances. How did you determine the internal cell resistances?
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    11,443
    2,628
    Nov 17, 2011
    With an ideal voltage source this "entirely theoretical" problem cannot be solved. The source with the higher voltage will force an infinite current into the sources with lower voltage in its fruitless attempt to raise the voltage of the combined circuit to the higher voltage.

    With real voltage sources having an internal resistance > 0 Ω this can be calculated by solving the node equations for this equivalent circuit:
    upload_2017-12-4_12-9-7.png
    Set up the node equations using Kirchhoff's laws, then solve for Vout.
     
    duke37 likes this.
  8. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Not so. Why should the 4 V source be charged when the 5 V sources are discharged? Consider the current flow in such a series conenction to come to the right conclusion.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,490
    2,831
    Jan 21, 2010
    That is true IF you are confusing series with parallel.
     
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