superposition and nodal analysis

Discussion in 'Homework Help' started by studentofcomputerscience, Nov 20, 2009.

  1. studentofcomputerscience

    Thread Starter New Member

    Oct 21, 2009
    6
    0
    Hello :)!

    I've got simple exercise about nodal analysis and superposition. I need to use these two methods in order to calculate current i1. My attempt is here: http://i50.tinypic.com/nq3e45.jpg . Unfortunately results from both methods are different. I guess I took current source into account in superposition method improperly. Can anybody tell me, please, what is wrong with my solution? (The second required thing is to calculate powers in this circuit).

    Thanks for help in advance!
    Greetings!
     
  2. AdamPrulhiere

    New Member

    Nov 12, 2009
    14
    0
    I'm on my way back to class right now and did not have time to look at your work in detail but in my circuits class the common mistake people make is when turning off sources. First of all, you can only use superposition with independent sources. The other big mistake people make is when turning sources off, a voltage source becomes a short circuit and a current source becomes an open circuit. I will be back home in a few hours and I will look at your work and see if I can find the problem if that does not help.
     
  3. syed_husain

    Active Member

    Aug 24, 2009
    61
    5
    could u be more specific what u need to find from node and superposition? is it voltage and current in the resistor R2?
     
  4. studentofcomputerscience

    Thread Starter New Member

    Oct 21, 2009
    6
    0
    Thanks for answers :)!

    What exactly do you mean that I can only use superposition with independent sources? I've got two schemes for my superposition, one is when I've got only voltage source, the other when I've got only current source.
    Yes, I know that voltage source is replaced by short and current source by open.

    I have written in my first post that "I need to use these two methods in order to calculate current i1". So it is current through R2, as shown on the picture.

    I still don't know what is wrong with my solution. Probably it is with i1'', but how to fix this problem? I guess calculating powers in the circuit should be rather easy. If I would have i1, there shouldn't be big problems with calculating all currents and voltages in the circuit. And with those the only what I need is to apply P=U*I for all the elements, am I right? However first of all I need to have this i1, with which I've got problem.

    Greetings!
     
  5. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    2,281
    326
    The resistor R3 has no effect on the currents in R1 and R2; you can just delete it in your calculations using superposition. Resistors in series with current sources have no effect on the current from the source. They do drop a voltage, but you don't care about that when calculating currents in other parts of the circuit.

    All you need to do is determine how the current from the current source divides between R1 and R2.

    If, however, you need to calculate the power in R3, then you will need to take R3 into account.
     
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