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Transformers - Find n1 and n2

  1. Nov 3, 2011 #1
    1. The problem statement, all variables and given/known data

    [PLAIN]http://img192.imageshack.us/img192/8428/circuits.png [Broken]

    2. Relevant equations

    [itex]\frac{V_L}{V_s} = \frac{nR_L}{n^2 R_s + R_L}[/itex]

    [itex]R_s = \frac{R_L}{n^2}[/itex]

    [itex]P = \frac{V_L^2}{R_L}[/itex]

    3. The attempt at a solution

    How should I go about solving this problem? Do I have to convert it to an equivalent circuit? If so how would the end result look exactly? I haven't seen any good examples in the book of creating equivalent circuits of transformers so I'm a little unsure of what to do.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Nov 3, 2011 #2


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    Staff: Mentor

    It looks like n2 is their way of expressing the turns ration of that transformer, 1:n2. So in place of the transformer with 8k in its secondary, you can show a new resistor RS in series with your resistor R, that new resistor bring RL transformed by the square of the turns ratio, as they indicate.
  4. Nov 3, 2011 #3
    In part a) I found n2 to be 10, but how do I find the max power now? I used the equation I posted in the OP to find the load voltage then used [itex]P=\frac{V_L^2}{R_L}[/itex] but I get 115.2 W as power when the book has 10,000 W roughly. Perhaps it's because I use the wrong value for [itex]R_s[/itex]. I used 20 Ohms. Should it be different? Or it could be because of the n value I use. I put in 10.
    Last edited: Nov 3, 2011
  5. Nov 4, 2011 #4


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    Staff: Mentor

    as do I
    Are you talking about part (a) or part (b)?
    It can't be thousands of watts. Isn't that a 20 ohm source resistance? So you want the transformed impedances to be of that order, too, so the current from the 120v source is going to be less than 3 amps. (Even were you to short circuit the transformer windings, that 20 ohm will limit the current to 6 amps.)
    I see nothing to indicate a different Rs.
    In part (b) you are told n1 is 5, and required to find the new n2. So there is no place for using an n of 10.

    FWIW, I calculate 1.3125 A is drawn from the 120v supply, in part (b).
    Last edited: Nov 4, 2011
  6. Nov 5, 2011 #5
    Yeah I was talking about part a) Apparently some others were able to get the 10,000 W power for part a that was int he back of the book though I couldn't really follow what they did.
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