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Transformer impedance?

  1. Jul 29, 2014 #1

    Since the primary side of a transformer is a closed circuit (like the cubical adapter for the iphone charger), it would make sense that even if my iphone wasn't plugged into the secondary, the primary would still draw power.

    I know this is the case for at least some adapters, because i see arcing sometimes when i plug my laptop adapter into the wall without my laptop being plugged into the secondary.

    But some adapters only start getting hot when i plug in load at the secondary. Why?


    When the secondary load is connected to make the secondary a closed circuit, does its induced emf change the impedance for the primary side?
  2. jcsd
  3. Jul 30, 2014 #2


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    A transformer has a turns ratio, primary to secondary, of Np : Ns.
    The same power must flow through the primary and secondary circuits.

    Given the low impedance mains supply, the primary voltage is fixed at Vp.
    The secondary voltage will be Vs = Vp * Ns / Np
    The load, Rs, on the transformer will decide the secondary current, Is = Vs / Rs.
    The primary current will then be Ip = Is * Ns / Np.
    So the transformer transforms the voltage one way, and the current the other.

    Looking into the primary from the mains supply is seen; Rp = Vp / Ip.
    That is the apparent input impedance of the loaded transformer.
    Substituting everything back into everything else gives; Rp = Rs * (Np / Ns)2
    The impedance ratio is therefore the square of the turns ratio.

    The adapter you show is an electronic switching power supply. It is not a simple transformer, but it does a good imitation of a transformer. It operates at a much higher frequency than the mains supply, it can then use a much smaller transformer to isolate the DC output from the mains supply.
  4. Jul 31, 2014 #3
    why does higher frequency = able to use smaller transformer?

    is it perhaps.. because higher freq. = more inductive impedance = lower current = can use smaller wires. and therefore can make the transformer smaller in size?
  5. Jul 31, 2014 #4


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    Voltage is proportional to rate of change of flux.
    Higher frequency is faster change of flux, so it needs both less turns and less flux.
    Less flux gives a smaller core before saturation. Less turns gives a smaller winding.

    There are too many influences to predict size from frequency accurately. Core material will also change.
    A crude rule of thumb is something like; Mass ∝ Frequency-2/3
  6. Aug 1, 2014 #5
    In Switching Power Suply the mains voltage is always rectified and smoothed with capacitor of 10-100uF. The arcing is caused by charging that capacitor.
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