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Homework Help: Transformers and induced current

  1. Oct 5, 2008 #1

    3) A step up transformer increases the mains voltage of 230V to 5000V.

    a) calculate the ratio Number of primary turns/Number of secondary Turns

    I know the formula input voltage/output voltage = Primary turn/ Secondary Turns

    So i did 230/5000 = 0.046. This as a fraction is 23/500. Is this correct?

    b)Calculate the current in the primary when the secondary current is 0.5A.

    I really am clueless here. I know of a formula Input voltage x input current = output voltage x output current. Is this the correct one to use

    4) The diagram shows a solder gun. It is a step down transformer with a single turn winding. When the voltage across the primary coil is 240V, the primary current is 0.6A and the induced voltage in the secondary coil is 2.0V

    a) Calculate the number of turns of wire in on the primary coil.

    I don't know which formula to use here.

    b) Calculate the current in the secondary winding.

    This would have something to do with part a so if i find out which formula to use i should be able to do this.

    c) Why is the secondary winding made from thick aluminium rod.

    Is this to do with resistance?

    d) Why is the nib of the solder gun thiner than the rest of the winding.

    Is this also to do with resistance

    Thanks for any help. I've tried my best to have a go. Thanks
  2. jcsd
  3. Oct 5, 2008 #2


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    Your first answer is correct. To answer the second, you need to know that
    I_sec = I_pri / R
    where R is the ratio of secondary to primary turns (reciprocal of the ratio you found).

    You could have found this from the two equations you listed!

    4a) Use the equation you know for the voltages.
    b) you have this equation, too.

    Try these and get back to us if you need more help.
  4. Oct 6, 2008 #3
    3b) input current x input voltage = output voltage x output current

    240V x input current = 5000 x 0.5

    input current = 10.42A

    is this correct?

    4a) i got 120 primary turns

    b) I used the formula from 3b) and i got 72 in the end.

    I would like some clarification here.

    thanks alot marcus
  5. Oct 6, 2008 #4


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    It would be if you used 230V as stated in the problem.

    Both correct.

    4c) Think about what kind of wire is needed to carry 72 amps!
    4d) Now think about where you'd like this wire to get hot, and where you want it cool.
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