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Homework Help: Calculating turns in a primary coil of a transformer

  1. Jan 17, 2013 #1
    1. The problem statement, all variables and given/known data

    A 100 W transformer (Ns = 1500) has an input voltage of 9.0V and an output current of 0.65 A

    2. Relevant equations
    P = IV
    Np/Ns = Is/Ip = Vp/Vs

    3. The attempt at a solution
    The problem is that I'm not sure what a 100 watt transformer means. Assuming it means that it outputs, not inputs, 100 watts of power, then:

    P = IV
    100 = 0.65 V
    V = 154 V

    Np/Ns = Vp/Vs

    Np/1500 = 9/154
    Np = 87.66 turns.

    I feel that mathematically I am doing everything correctly, but I thought that because voltage was being increased then the amount of coils in the primary coil should be more than in the secondary.

  2. jcsd
  3. Jan 17, 2013 #2


    User Avatar

    Staff: Mentor

    Looks like you've done just fine. A step-up transformer has more turns on the secondary than on the primary. It's the ratio of turns that sets the "magnification" factor for the voltage.
  4. Jan 17, 2013 #3
    Oh okay, I wasn't clear on that. Thank you very much!
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