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DC applied to primary of transformer

Discussion in 'Electronic Basics' started by Thaqalain, Jun 10, 2005.

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  1. Thaqalain

    Thaqalain Guest

    I want to apply DC safely to the primary of a transformer,how can i
    *When limiting resistance is in series.
    *When limiting resistance is in parallel.
    *when neither of above
  2. The windings of a transformer act just as any other piece of wire
    where DC is involved, so think of how you could safely connect wire
    across a DC supply.

    If the supply is a voltage source (voltage essentially constant, no
    matter how much current is drawn from it), than adding other loads in
    parallel with the wire would only provide additional paths for more
    current, without limiting or reducing the current through the wire.

    Resistances connected in series (the wire is just a low value
    resistance) add. So a series resistor will reduce the current that
    passes through the transformer winding.

    If, however, the DC supply is a current source (which means that the
    current is nearly constant, regardless of how much resistance is
    connected across it because it varies its voltage to whatever is
    necessary to drive that current), then paralleling the wire with other
    resistances would detour some of that total current to other paths,
    lowering the total voltage the supply needed to produce to maintain
    the fixed, total current. This lowers the fraction of that total
    passing through the transformer.

    Since your mention of "applying DC" does not specify whether the
    source is more like a voltage source or a current source, or something
    in between, the question cannot be answered, except to discard the
    third choice.

    Tell your teacher that you learned this on the Internet, not in their
  3. Thaqalain

    Thaqalain Guest

    This was test question for ems comapny,they don't describe
    Volatge/Current Source?here is original question:
    DC is safely applied to the primary of a transformer:
    *When limiting resistance is in series with primary.
    *When -------------------------Parallel------------.
    *When neither of preceding applies
  4. Thaqalain

    Thaqalain Guest

    Thanks a lot Sir,I appreciate your detailed response due to that we get
    some knowledge by these forums.I am civil engineer working for ems cos
    in ON.
  5. Does my answer make sense to you?

    Since voltage sources are so much more common than current sources, I
    suspect they are expecting the first answer.
  6. Generalization...A.

  7. mike

    mike Guest

    True or false?
    Transformer cores are sized to be the smallest/lightest/cheapest
    they can be and still not saturate at rated drive levels.

    If true, then any significant amount of DC current is a bad thing.
    Matters not whether it was placed there by Mr. Thevenin or Mr. Norton.

    Return address is VALID but some sites block emails
    with links. Delete this sig when replying.
    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
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  8. Rich Grise

    Rich Grise Guest

    IOW, (D) None of the above. There is _no_ safe DC value that can
    be applied to the primary of a transformer. Unless, of course, it's
    a transformer that was designed for single-ended class A output. In
    that case, it'd be series resistance, or more strictly speaking,
    series transconductance.

    If this is a test question, then Thaqalain needs to reread the
    textbook before re-taking the test. Or ask the teacher what
    answer they're trying to extract from the students.

    One of the ways test questions are recognizable is by the date.
    It's FINALS WEEK! ;-D

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