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Primary and Secondary Voltages In A Transformer

  1. May 4, 2005 #1
    Hi!

    I just got to my last homework question and am not quite sure how to figure it out, it goes like this:

    Scott connects a transformer to a 24V source and measures 8.0V at the secondary. If the primary and secondary were reversed, what would the new output voltage be?

    I'm not sure how to go about this. I know the basic ratios like

    Np/ns = Vp/Vs = Is/Ip but I can't think of how to do this question. I'm sure it's super easy but I'm super dumb :grumpy:
    Thanks for any help in advance! :biggrin:
     
  2. jcsd
  3. May 4, 2005 #2

    Doc Al

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    Staff: Mentor

    Hint: If the transformer steps down the voltage when used one way, then what must it do if the primary and secondary are reversed?
     
  4. May 4, 2005 #3
    If they are reversed the voltage must step up, but I still don't see how to answer the question :(
     
  5. May 4, 2005 #4

    Doc Al

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    The input voltage doesn't change. It's still 24V.
     
  6. May 4, 2005 #5
    Would the new voltage be

    24/8 = 8/x
    x = 2.7V?
     
  7. May 4, 2005 #6

    Doc Al

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    Not even close. :smile: Remember: When used one way, the transformer steps down the voltage by a certain ratio (what is that factor?). So... when used in reverse, it will step up the voltage by the same factor.
     
  8. May 4, 2005 #7
    Um ok, so originally you have 24/8, and now we are stepping it up by that factor (3). So now we end up with the original 24v x 3 which equals 72v?
     
  9. May 5, 2005 #8

    Doc Al

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    Exactly right.

    From Vp/Vs = Np/Ns, we know that Np/Ns = 3 for the original configuration. Now if we swap primary and secondary, Np/Ns = 1/3. Since the input voltage is still Vp = 24V, Vs = 72V.
     
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