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Homework Help: Transformers - Don't understand VpIp = VsIs

  1. Apr 5, 2014 #1
    transformers -- Don't understand VpIp = VsIs

    1. The problem statement, all variables and given/known data

    If the equation below of any transformer is true, therefore it does not matter if i connect a lightbulb or anything else either to the primary or the secundary, it should be working the same, right? I have a 220v-3v step down transformer and i can connect an led without burning it. How is that possible? Do we lose power here? Shouldn't be the current much higher in the secondary?

    2. Relevant equations

    VpIp = VsIs

    3. The attempt at a solution
  2. jcsd
  3. Apr 5, 2014 #2


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    Gold Member

    Why should the current be any higher than what the load requires? I think you need to study transformer fundamentals and perhaps electronics fundamentals such as ohm's law.
  4. Apr 6, 2014 #3

    rude man

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    Homework Helper
    Gold Member

    The current IS much higher in the secondary. Your turns ratio is 220/3 = 73.3 = N1/N2.
    So if your load current is say 100 mA then your primary current is 100/73.3 = 1.36 mA. However, the primary current also includes the magnetizing current which depends on the primary winding inductance and your frequency of operation, and which is 90 deg. (approx.) out of phase with the secondary current.
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