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is there an equation for primary transformer current when secondary current = 0

Discussion in 'General Electronics' started by Mark, Dec 10, 2007.

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  1. Mark

    Mark Guest

    Suppose I have a transformer such that the primary current is Ip when the
    secondary current is Is, if I flip a switch on the secondary , the Is'
    then is 0. I know that if you do this, the primary current does go
    down but it doesn't go to 0 amps. In fact this is obviously so because
    a transformer will remain warm even if the secondary is disconnected.

    Is there a general equation that will estimate that or is it just something
    you have to measure on the bench for each transformer? I'm
    just wondering a bit about how much my various wallwarts waste when
    the thing they go to are turned off.

  2. Eric Sears

    Eric Sears Guest

    Just a couple of comments -
    1. I suspect that you need to measure them. Most small transformers
    seem to waste more (ie get hotter) as a percentage of their rated
    power, than large ones in my opinion.

    2. Not all "wall warts" are alike. Many today ( such as lightweight
    cellphone chargers) are actually switched-mode supplies - not
    transformers. They OUGHT to be less lossy - but feeling how warm they
    get - I wouldn't bet on it!

    Just my tuppence worth.

    Eric Sears
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