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Multisim schematic

  1. Nov 23, 2013 #1
    Hi once again I don't really understand multisim simulated results as i find them kinda not 1:1 with reality, I am not sure maybe someone here can help.
    In this schematic a switch the switches one one at a time.
    For the first time when each of the switches is switched on each of the caps charge.While they charge a current runs through them which also runs through the primary winding.
    When I continue switching the caps still maintain some if not most of their charge , so how come the oscilloscope show a clear square wave when switching as each next time because the capacitor is already atleast half charged the current rush towards it is far less than if it were empty so the flux in the transformer should change too and the reading on the oscilloscope too.
    Well check it out for yourselves.

    I zipped the ms. file because somebody has made it impossible to attach a multisim file directly.Kinda stupid.

    Attached Files:

  2. jcsd
  3. Nov 23, 2013 #2
    Please add diagram in PNG format, everybody don't have to use multisim.
  4. Nov 23, 2013 #3
    here you are:)

    Attached Files:

    • 111.jpg
      File size:
      56.1 KB
  5. Nov 24, 2013 #4
    What are you try to achieve ? With this strange circuit?
    Have you check what type of a model multisime is using to model the behavior of the transformer?
    Maybe this "model" also work with DC and don't behaves just like a real transformer. Have you checked this?
    Last edited: Nov 24, 2013
  6. Nov 25, 2013 #5
    No I haven't , the circuit is thought to produce a back and forth oscillations so that the transformer could work.
    Ofcourse it is only useful until /if the capacitors stay empty, if they charge up hen no charge flows any more so no change in the em field and the transformer stops doing it's job.
    so the question rather is does this can work , including the back emf etc etc , guess rather not, and secondly what is a dc transformer , as transformers don't work on dc ?
  7. Nov 26, 2013 #6
    Real transformers don't work on DC but in multimsime they work on DC also.
    Simply remove capacitors from your circuit and see if we get the same result.
  8. Nov 26, 2013 #7
    yes we get the same result, also what made me wonder is that when I leave one switch closed and the other opened , the oscilloscope reads either + or - 50 volts from the secondary , but normally that couldn't happen because as if there are no oscillations anymore in the primary there should be no induction in secondary.
    So I guess multisim uses somekind of a weird dc transformer , I can't understand the reason one would need that as if it doesn't work in real life.

    what do you think if both sides of the primary are equally loaded should there be any current on the switches , if a high frequency is used , and no capacitors ? Theoretically there should but doesn't the back EMF stop it if the switches are cut off before the flux in the core reaches it's peak 90 degrees after voltage is applied?
  9. Nov 26, 2013 #8
    We use this "weird transformer" to model basic principal of how ideal transformer work.
    Transformer can step up/down voltages and Pin = Pout nothing more. An ideal transformer is a perfect transformer of both AC and DC.

    In general flux in the core the integral of the applied voltage waveform. So only for a sin wave voltage, the flux in the core lags 90 degrees after voltage is applied. Because if you integral sin wave you will get a cosine wave.
    Last edited: Nov 26, 2013
  10. Nov 27, 2013 #9
    Ok so then by how many degrees the current lags a say , square wave , even though there is no true square wave , especially in circuit parts that have inductance like a inductor or transformer.
  11. Nov 28, 2013 #10
    How can we talk about phase shift if our input voltage is not a sinewave? For a square wave voltage across an inductor we get triangle wave for the current as well as for the flax.

    I think that you should back to basics and start studying DC/DC converters.
    And I highly recommend you to read this pdf starting from page 22 "Understanding the Inductor".
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