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Homework Help: Potential divider problem

  1. Mar 28, 2010 #1
    potentiometer question

    hey there

    i hate coming online to ask for help but i am well and truly stuck!

    attached is an image containing 2 potential divider setups.
    i have a few questions about them, and any help would be appreciated!

    a) for the first setup, A, how could the 10k pot be split into 2 resistors to give a Vout of 3v? i am confused by the +-15v source. can i treat this as if the +15v is 30v and the -15 is 0v? making the 2 resistors 9k and 1k? if not, how else is it done; i can find absolutely no examples of potential diviers which aren't grounded?

    b) how do i calculate the thevenin source impedance of the potentiometer set to give 3v? im familliar with thevenin's theorem but dont fully understand the question. as far as i understand, thevenins thereom is used to reduc circuits down to just a voltage source, a resistor and a load; here i'm unsure how this would be done.

    c)finally, how do i calculate Vout for the setup B when the effective impedance of the multi meter measuring Vout is reduced to 3.3K? my initial thought was a simple voltage division equation but this came unstuck when i realised that R1 = 0, therefore that doesnt work. im out of ideas after that!

    like i said, i'd really really appreciate some help on this!

    many thanks

    Attached Files:

    Last edited: Mar 28, 2010
  2. jcsd
  3. Mar 28, 2010 #2
    i've spent all day thinking about the above problems to no avail. can no one help me?

    thank you
  4. Mar 28, 2010 #3
    i think i might be on to something, maybe someone can confirm if this is right, and if it is eplaiin why.

    i think it goes +15v into R1 = 5k into r2 = 7.5 k and -15V out. then the output measuered between R1 and R2 = 3v. ive been messing around on a simulator and it works, but i dont know why!

  5. Mar 28, 2010 #4
    i9 realise ive been being a fool.
    think the output voltage can vary anywhere between -15V and +15V; a range of 30V.
    An output of +3V is therefore 18V above the lower -15 level and 12V below the +15 level. In other words, the potential divider must divide the 30V total in the ratio 12/18. The resistance ratio must be the same; 4 ohm and 6 ohm

    part a) done!

    some help on b and c would be great still though
    Last edited: Mar 28, 2010
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