Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

DC-Transformer circuit help

  1. Feb 15, 2014 #1
    I recently learned that if you send direct current through the primary coil of a transformer, no power would be transferred to the secondary circuit? Why is that?
  2. jcsd
  3. Feb 15, 2014 #2
    Can you tell us why it works for AC? If so, then you should know why it does not work for DC.
  4. Feb 15, 2014 #3
    Theoretically, it would work. Since the flux in the core of the transformer increases by the integral of the applied voltage (Faraday's law), in an ideal transformer, the flux would just continue to increase. However, if you tried this with an actual transformer, you would drive the core into saturation and create a lot of heat.

    Dr. Frank Ferrese
  5. Feb 15, 2014 #4


    User Avatar
    Gold Member

    A Straight Answer:
    A changing current produces a changing magnetic field, which in turn can induce a current in the secondary. If the current is not changing, the magnetic field is not changing, so no current is induced in the secondary.

    When you first apply a DC voltage the current ramps up, so the field is changing and an initial current is induced in the secondary. But when the current stops changing (limited by resistance, for example) there will be no current induced into the secondary.
  6. Feb 15, 2014 #5


    User Avatar
    Science Advisor
    Gold Member

  7. Jun 22, 2016 #6
    I dont know why but i have read on other sources that people use ac/dc transformers on high voltage dc applications of 60v or more. they used the simple wall-warts to convert the high dc voltage to lower dc voltage.
    however even then, they still said that some wall-warts worked and some didnt.
    Dont know why that works with higher DC voltages but know it does. sometimes!

    check out this link: people on that forum use them a lot.
  8. Jun 22, 2016 #7
  9. Jun 23, 2016 #8
    Referring to the picture, if most of the magnetic flux stays inside the iron core, how do moving field lines cut through the secondary coil?
  10. Jun 23, 2016 #9
    The inductor has 0ohm resistance at dc
  11. Jun 23, 2016 #10


    User Avatar
    Science Advisor
    Gold Member

    no, that is incorrect. There will be resistance and is easily measured with an Ohm meter
    for a transformer primary, it can easily be several 100 to several 1000 Ohms
    Last edited: Jun 23, 2016
  12. Jun 23, 2016 #11


    User Avatar

    Staff: Mentor

    The field lines do not have to be moving. The changing magnitude and direction (+/-) of the primary current generates a changing flux value, which in turn induces the changing secondary output voltage.
  13. Jun 25, 2016 #12
    If the flux value (concentration of field lines) changes, do the magnetic field lines move?
  14. Jun 25, 2016 #13

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    It's a quirk of the universe that the "lines" need not "cut" the conductors,
    only that they be encircled by the conductors.

    See https://en.wikipedia.org/wiki/Faraday's_law_of_induction and http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

    The field itself is more of a continuous fluid than discrete lines. It goes everywhere , just changes its intensity as you get farther from its source.
    But drawing lines helps us visualize it and gives us something we can count to gauge its intensity.

    A "line" is a called Maxwell, one of them per square centimeter is called a Gauss which is roughly the magnetic field strength of Earth that tugs your compass needle around.
    A field strength of ten thousand Gauss is called a Tesla, about the field strength of a good loudspeaker magnet.

    Any help ?

    old jim
  15. Jun 25, 2016 #14
    Many thanks, Jim. I posed a difficult question that has been puzzling me for a while. Good answer.
  16. Jun 25, 2016 #15

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    Thanks for the feedback !
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: DC-Transformer circuit help
  1. DC circuits (Replies: 4)

  2. DC transformer (Replies: 23)