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Voltages - continuation of the discussion

Discussion in 'General Electronics Discussion' started by duke37, Jan 5, 2018.

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  1. duke37

    duke37

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    Jan 9, 2011
    I do not understand the question. Why should there be any voltage when the DC is blocked by C1?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    At first I ran into the same issue. Considered statically you're right. But considering that the circuit is built from the capacitors and the voltage source is applied as the last component, a charge current will flow into the capacitors and a DC voltage will develop accordingly across the different capacitors.
    Alternatively one may replace the DC voltage source by an AC voltage source. Then it is obvious that AC current will flow. The voltages across the capacitors will be the same as in the DC case.

    It is unfortunate to put this question in this form to beginners, I think.
     
  3. dorke

    dorke

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    It has nothing to do with DC blocking.
    All the capacitors will conduct current till they are "charged-up" to a final DC voltage.
    In the steady state (t--> infinity) there will be no more current in the circuit and capacitors ,
    each capacitor will have a voltage depending on it's value and the way the capacitors are connected in the circuit.
    It is sometimes called a capacitor voltage divider.
     
  4. Ratch

    Ratch

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    Without any resistance in the circuit, the current will be at an infinite value for an infinitesimal amount of time to fully energize the capacitors. I would include a little resistance in the voltage source so that the calculations don't "blow up".

    Ratch
     
  5. dorke

    dorke

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    Jun 20, 2015
    @Ratch,
    That is true,but for beginners it is a simple steady state homework question,no need to complicate the issue.
    And since it is homework,we only need to guide, not solve it for the OP.

    @duke37,
    here is a simpler question,solvable as a capacitor voltage divider:
    Find the voltages and charges on caps(in Farads),try it out.

    untitled-3.JPG
     
  6. Ratch

    Ratch

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    After an infinitesimal time and an infinite current spike, the voltage at the junction of c2 and c4 will be 4 volts. The voltage at the junction of c3 and c4 will be 2 volts. The charge imbalance is Q = C E for every capacitor. And the point is?

    Ratch
     
  7. duke37

    duke37

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    Jan 9, 2011
    So, you would start with no voltage on any capacitor and then input a step voltage change. In other words, an AC component would be added to the voltage source. This is not shown in the diagram.

    In practise, the voltages will be dependant on resistance leakages and the ionisation of the air.
     
  8. Ratch

    Ratch

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    A step voltage is what a switch does. I don't see any resistance leakages documented in the problem or schematic. Not much ionization at 12 volts. I am just doing what the problem specifies.

    Ratch
     
  9. Ratch

    Ratch

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    Yes, that will work if there is no real resistance in the problem and the only interest is the voltage after a long time. But if RC is present in the circuit, that method breaks down.

    Ratch
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    It works for an idealised problem where the voltage source can supply the infinite current for zero time to capacitors with zero leakage and series resistance.

    In short, it will work in the theoretical case which is what these problems represent.
     
    Harald Kapp likes this.
  11. duke37

    duke37

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    Jan 9, 2011
    The problem is not properly specified.

    The voltage source is 20V not 12V, not that it makes any difference to the problem.
    No switch is shown so it is not clear what the initial conditions are.
    There is plenty of ionisation here if I skuff my feet over the nylon carpet.
     
  12. Ratch

    Ratch

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    Agreed. That is what I more or less said. It will also work if only the voltage source has some resistance and the time after activation is long.

    Ratch
     
  13. dorke

    dorke

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    No one asked or is interested in the current(at any time/state of the circuit),
    it is totally irrelevant to the question.period!
    Why are you needlessly complicating things?

    It is a simple Voltage Capacitor Divider question for a novice in the stage of learning DC circuits(who probably didn't even start learning about AC at all) ,the answer is in voltage on the caps,that is all !
     
    davenn likes this.
  14. Ratch

    Ratch

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    Yes, I was thinking of the 12 volt problem Dorke submitted to me. Well, the connection or switch on had to be made at some specific time. Your feet are not in the circuit.

    Ratch
     
  15. Ratch

    Ratch

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    It is a DC problem, not an AC problem. The time for final values of voltage will depend on the resistance in the circuit. That is a fact, not a complication.

    Ratch
     
  16. dorke

    dorke

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    Jun 20, 2015
    Duke,
    I have changed the values intentionally, (assume steady state and ideal battery and caps which is obvious for these questions).
    If you can't answer the simple question at #14 ,it is back to school amigo ;)
     
  17. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    ... but we can make reasonable assumptions - and we did - to make the problem solvable.
     
  18. duke37

    duke37

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    Jan 9, 2011
    I can solve the problem in #14 with an AC input but cannot do it without initial conditions, resistance or other current input.
    LTspice is much cleverer than I but it cannot solve it, it just goes into a sulk.
    Ratch
    My feet are not shown in the circuit but I often put my foot in it.:)

    What were the assumptions made?
    Trevor
     
  19. Ratch

    Ratch

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    My assumptions were that there was no initial voltage on any of the capacitors, and only a DC voltage was applied. I also assumed the circuit did not contain any resistance. If resistance was present, then a full solution would have to show the voltage varying according to time. Resistance or not, the final voltage will converge to the same value after a long interval.

    Ratch
     
  20. dorke

    dorke

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    Jun 20, 2015
    It is astonishing,truly so!o_O
    What the hack do you need LTspice for?
    If it were resistors only ,not capacitors, would you use LTspice?

    Assumptions?trivial really.
    All components (and wires) are ideal.
    If you must, then initial conditions :all voltages are 0 volts.
     
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