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Design a circuit diagram

  1. Sep 17, 2012 #1
    1. The problem statement, all variables and given/known data
    Given =
    5 - light bulbs
    Light bulbs 1 and 2 each dissipate 180 mW at 12 V
    Light bulbs 3, 4, and 5 each dissipate 45 mW at 9V.

    1 - 12 V supply

    2 - Resistors (Range 10-1000 Ω)

    5- switches

    The design must meet the criteria:
    Each light bulb can be independently turned on and off
    Light bulbs 1 and 2 must get 12 V from the power supply when on.
    The other light bulbs must get 9V (+/- 5%) from the power supply when on.

    2. Relevant equations
    V = IR
    P = I2R

    3. The attempt at a solution

    Light bulbs 1 and 2 have 800 Ω each.
    Light bulbs 3, 4, and 5 have 1800 Ω each.

    I can solve the diagram easily when I have only one light bulb on each time. It is when I have multiple light bulbs on that the voltages change and no longer fit the criteria.
     
  2. jcsd
  3. Sep 17, 2012 #2

    NascentOxygen

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    Cisneros778, what could be the idea behind allowing the use of resistors? http://imageshack.us/a/img706/5864/hidden14.gif [Broken]
     
    Last edited by a moderator: May 6, 2017
  4. Sep 17, 2012 #3
    The 2 resistors will keep all light bulbs within their allowed voltages.
     
  5. Sep 17, 2012 #4

    NascentOxygen

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    "2 resistors"http://physicsforums.bernhardtmediall.netdna-cdn.com/images/icons/icon5.gif [Broken] There is no limit of 2; you may use as many as you like. :wink:

    Sketch a diagram to illustrate your idea.
     
    Last edited by a moderator: May 6, 2017
  6. Sep 17, 2012 #5
    I actually have only two resistors to work with. I mean, the light bulbs are resistors as well, but apart from that I only get to use two resistors for my diagram. :-/
     

    Attached Files:

  7. Sep 18, 2012 #6
    https://mail-attachment.googleusercontent.com/attachment/u/2/?ui=2&ik=c0da1462ed&view=att&th=139d7c80e1de131f&attid=0.1&disp=inline&realattid=f_h78jmhci0&safe=1&zw&saduie=AG9B_P_MKhT_3deMhgRG8lpbZ3yh&sadet=1347945000944&sads=cscxVT049sxfN8g2eBCSKNZUd3k&sadssc=1

    Okay, I figured out the first two light bulbs, having them parallel will give them 12 V. For light bulbs 3,4 and 5 I need to have 9V running through each one of them. So, I'm guessing to use the same strategy by having them parallel but I only have the 12 V source. So I believe here is where the 2 resistors come into play. How to include them, is what puzzles me.
     
  8. Sep 18, 2012 #7
    any suggestions?
     
  9. Sep 18, 2012 #8

    NascentOxygen

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    Are the five switches all SPST? :smile:
     
  10. Sep 18, 2012 #9
    Yes, just on or off
     
  11. Sep 18, 2012 #10
    Re: Building an electrical circuit with given criteria

    If I understand you correctly, you just need to know how to reduce the voltage using two resistors? The simplest solution would be a voltage divider. I'll leave it to you to look that up and figure out exactly how to implement it in this situation, but it shouldn't be too difficult. (Maybe steer clear of the Wikipedia page and look for a simpler explanation if you're new to circuit analysis.)
     
  12. Sep 18, 2012 #11

    CWatters

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    Re: Building an electrical circuit with given criteria

    For bulbs 3 and 4. Use..

    Power = Voltage * current

    to work out the current going through the bulb.

    Then use one resistor in series with each bulb to drop the 12V down to 9V.

    V=I*R
    so
    R=V/I

    where the drop across the resistor V = 12-9 and I is the current you calculated above.

    However the answer is not a standard resistor value in the 5% range.

    http://ecee.colorado.edu/~mcclurel/resistorsandcaps.pdf

    Try nearest value up. Recalculate the voltage to check it's within 5%. It should be according to my calc.
     
    Last edited: Sep 18, 2012
  13. Sep 18, 2012 #12

    CWatters

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    Re: Building an electrical circuit with given criteria

    Humm Ok I missed that there are THREE 9V bulbs not TWO. Will have a rethink.

    Does the problem really say that ONLY two resistors can be used for all three bulbs?
     
  14. Sep 18, 2012 #13

    CWatters

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    Re: Building an electrical circuit with given criteria

    thegreenlaser is correct. Look at a voltage divider. Bit of maths to do to ensure the voltage does not go out of tollerance with one or three bulbs connected.
     
  15. Sep 18, 2012 #14

    NascentOxygen

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    Re: Building an electrical circuit with given criteria

    That is certainly the simplest solution! This makes for a good challenge question, too.
     
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