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Designing a circuit (basic electric circuit analysis)

  1. Feb 22, 2013 #1
    The problem statement, all variables and given/known data

    Design a dc circuit utilizing a 15V voltage battery to provide the following node voltages: +10V, +5V, and -5V w.r.t. a circuit ground node. Select your resistors such that the maximum power demand on the battery does not exceed 1mA.

    The attempt at a solution

    I've designed the attached circuit, but I'm not sure how to find the resistance for each circuit. I know that the total resistance of all four circuits must add up to 225000Ω using the calculations attached.
     

    Attached Files:

  2. jcsd
  3. Feb 22, 2013 #2

    CWatters

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    Perhaps start by marking the ground node and which nodes you intend being 10V, 5V and -5V. Hint: You only need 3 resistors all the same value.
     
  4. Feb 22, 2013 #3

    gneill

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    mA is not a unit of power; it's a unit of current. Are you sure that the question states that it wants a power limit and not a current limit? Or is it just badly phrased and they really expect a current limit of 1mA? (it would make sense)
    Suppose you set a fixed current value. How many resistors do you think you'd need to provide three potential drops? Should the drops be of different or equal magnitude?
     
  5. Feb 23, 2013 #4
    Alright. I'm trying that right now.

    Hmm... I doubled checked the question and it seems that they used the wrong units. I guess I'll ask the professor or TA. I think they just wrote the wrong units, because that's an easy mistake to make.

    Resistors cause voltage drops, so for 3 voltage drops, I would need three resistors. Since the increments are in equal value (i.e. -5, 0, 5, 10), then I guess that the resistors need to be all the same value.
     
  6. Feb 23, 2013 #5
    I think I did it!

    One question: how would I make (1) the ground node and still have the other nodes have voltages of: -5V, +5V, and +10V w.r.t. (1)?
     

    Attached Files:

    Last edited: Feb 23, 2013
  7. Feb 23, 2013 #6

    gneill

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    Yes, looks fine.
    You can't. Making node (1) the reference node would put the other nodes at -5, -10, -15 volts with respect to it.
     
  8. Feb 24, 2013 #7

    CWatters

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    I think you have the battery the wrong way around.
     
  9. Feb 24, 2013 #8

    CWatters

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    This is how I would draw it.. Note how this layout has the higher voltage nodes at the top of the page and lower voltage/negative voltages at the bottom.
     

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    Last edited: Feb 24, 2013
  10. Feb 24, 2013 #9
    I'm curious. What did you use to draw that circuit?
     
  11. Feb 24, 2013 #10

    CWatters

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