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Current and Voltage in Transformers

  1. Dec 9, 2012 #1
    Hi there,
    I've got a fairly simple question theoretical question about transformers and I suspect it stems from a misconception I have about them. I understand that step down transformers can be used in situations where a high current is desired but that they also reduce the voltage in the secondary turns relative to the primary turns.

    Here in a nutshell is what's causing me grief. I'm envisioning an AC power supply that is connected to just a single resistor and has a certain current through the resistor as determined by the resistance. If instead the AC power supply was connected to a step down transformer and the resistor was connected to the other side of the circuit, I'm faced with a bit of a paradox.

    One the one hand, I know that the current should be greater as a result of the stepping down of the current, but shouldn't a straightforward application of Ohm's law tell us that since the voltage applied to the resistor was stepped down that the current would also drop? What am I missing here?


  2. jcsd
  3. Dec 9, 2012 #2


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    Gold Member

    The current through a resistor is a direct function (Ohm's Law) of the voltage across it.

    The fact that the supply to the resistor is capable of providing more current than the resistor draws is irrelevant.
  4. Dec 10, 2012 #3
    Think of it in another way. Your 110V wall AC plug usually can supply up to 15A of current. But if you plug a 100W light bult, It only draw about 1A of current and no more. Just because the AC plug is capable to supply 15A does not mean it will drive 15A out no matter what device you plug into. It is only the capability of the wall plug.

    The current draw is govern by the impedance of the load, if you put a 110KΩ resistor across a 110V source, you are going to draw 1mA even though the source is capable to supply 15A.
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