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Resistance in circuits with transformers

  1. May 7, 2013 #1
    I am starting to learn about transformers, and know the basics: they use electromagnetic induction to induce a current in a secondary coil, and the voltage and amperage of that second coil are dependant on the voltage and amperage of the first coil, and the ratio of turns in each coil.

    So, here is a little scenario, then I'll get onto the question.

    Say I have the primary coil running at 10V and 1A, with 100 turns in the coil. In the secondary coil, I have 20 turns in the coil.

    The secondary coil would then have 2V and 5A (assuming an ideal transformer).

    Now for the question. When using Ohm's Law (V = IR --> R = V/I), it would seem that the primary coil has 10V/1A = 10Ω of resistance, and the secondary coil has 2V/5A = 0.4Ω of resistance.

    Does this then mean that the effective resistance of the secondary coil is lowered in a step-down transformer? Does this also mean that a 10Ω resistor in the seondary coil would cause a larger power drop (P = VI) percentage-wise than it would in the primary coil? (10+10 = 20/10 = 200%, 0.4+10 = 10.4/0.4 = 2600%)

    I would like to learn more about reistance in transformer circuits, so any links would also be appreciated. Thanks for any replies.
  2. jcsd
  3. May 7, 2013 #2


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