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Circuit analysis

  1. Apr 30, 2004 #1
    :confused: I'm a beginner so be gentle.

    Trying to figure out currents in the attached image of a ciircuit I've been pulling hair out over.

    No need to fully analyze it, simply trying to determine what current (AC or DC) is flowing through each component.

    More importantly, how on earth can I tell if/where AC or DC current is flowing just by looking at the circuit, without regard to the component values?

    Thanks in advance

    Attached Files:

  2. jcsd
  3. May 2, 2004 #2


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    Staff Emeritus
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    Gold Member

    Welcome to the forums, neptunemassif!

    Your picture is really hard to read. Can you make another copy of it, only a hare larger (or size up the text)?

    Also, is this a homework assignment? If so, it should really go into the homework help forums (I can move it there, if needed).
  4. May 3, 2004 #3
    If you're a true BEGINNER.. then this circuit might be a little too hard for you.

    There is a DC Voltage source (represented by the 12V battery symbol) and an AC Source (the circuit with the sine wave in it- 120V 60Hz).

    So your network is going to have a mixture of DC currents and AC currents (AC currents riding on a DC offset).

    You will have to do SUPERPOSITION to figure out the DC currents and AC currents independently.

    For DC analysis:

    -SHORT the AC source.. which will effectively prevent DC energy from flowing into the circuitry to the right of the AC source.

    Now solve for the DC currents in the remaining circuit. This is easy because the CAPS are OPEN to DC and COILS are SHORT so the only path for DC current is through R1. Use Ohm's Law.

    Now AC analysis:

    -Short the DC source. This doesn't help you much, because all the other components in the schematic still have an effect on AC energy (no shortcuts like before). Covert all the components into impedances in the phasor domain (Zr= R< 0degrees , ZL = 2PIfL < 90 degrees, Zc= 1/(2PIfC) <-90 degrees) NOTE: f= 60Hz in your case ; PI = 3.14

    For the transformer and R3, simply treat the input to the transformer as a pure resistance that has been reflected back. I'm not sure what the transofrmation of that transformer is, whatever it is, the reflected resistance should be Zreflected = (Np/Ns)^2 RL < 0 degrees

    Then you should have a AC network with a series-parallel flavour which you can wrestle to submission.

    Once you've done all that, there will be a few branches in your schematics with both DC and AC currents through them.... and the rest should just have an AC current.

    I hope I'm right... if not I just wasted 5 min proving that I'm an idiot.. haha.
  5. May 3, 2004 #4


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    Homework Helper

    I basically agree with flexifirm. Except, you seem to want to know the general approach to "solving" a circuit. There are two basic general approaches: mesh current analysis and nodal (voltage) analysis. In some cases, you must use one or the other. In the given circuit I would use mesh currents. Do you know how to do this? It is based on KCL. Work in the frequency domain, and superpose the values from the two sources as described by flexifirm.
  6. May 3, 2004 #5
    All, thanks for the help - flexifirm, your stuff is like GOLD!
    The technique of shorting the power supplies in turn helps me a lot, but the clarification on how caps and coils affect ac/dc current is what did it for me. Perhaps that's what I should have made more clear to begin with.
    Back to enigma's question, yes, this was homework, and i *thought* i posetd it there but got no replies... The circuit came from my textbook, and had NO values attached to the components; the question was to determine what current was running through each component, ac or dc - looks like I need to brush up on L-C basics.
    Again, thanks!
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