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Diode placement and AC waveform across load?

  1. Aug 29, 2011 #1
    Hi all.

    A transformer is driven with a square wave pulse.
    The transformer has three secondary coils all in series, a diode is placed between two of them which is forward biased during the on time. The coils are used to charge a capacitor and limit current.

    When the transformer is pulsed an Amplitude modulated AC waveform appears across each secondary coil... Can anyone explain why this occurs???

    The AM waveform also appears across the capacitor?
    -How can this occur when the circuit has a diode in it?

    Last edited: Aug 29, 2011
  2. jcsd
  3. Aug 29, 2011 #2


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    What is the application? Where is this used? Can you post the waveforms, including the drive waveform?

    Is this for school?
  4. Aug 29, 2011 #3
    No it's not for school.

    The circuit is used to charge an air gap in order to ionize the air moving through it.
    The input to the primary is a square wave.
    The pulsing is at the resonant frequency of the outer secondary coils in order to limit current. The capacitors across each coil are simply representing the coils capacitance.

    I have run this simulation on multisim and I get a AM waveform across the outer secondary coils....I believe this is due to the differences in the inductance and capacitance of each coil. Across the air gap (capacitor) I get a voltage which builds up to a high value.....But I have also seen instances where the AM waveform appears across the capacitor..

    Could I have a bad diode....It seems to me that the diode should rectify the AC and prevent it from going to the capacitor.
  5. Aug 30, 2011 #4
    It wouldn't be reasonable to drive this with a square wave, and your use of the word "pulses" makes me wonder if its actually short pulses--which would be reasonable for this ckt. Also, the pulse on-time should ideally be half the period of the resonant frequency. In that case, you'd see ringing after each pulse and that might be what you are describing as "AM waveform". As far as the diode goes, note that once that right cap is charged, the diode is reverse-biased, so its an open (except, of course, the load will discharge that cap--which is not shown). it might be that they are using the extra voltage from the ringing just as a car ignition coil does. This is mostly conjecture because little info is given.
  6. Aug 31, 2011 #5
    I still tend to wonder.... if the coils had different L and C values would they oscillate at different frequencies even thouigh they are all on the same core and driven by the pirmary coil? The reason i say this is because the modulation is large....I have noticed as well that it may not be AM modulation but beating.

    http://hep.physics.indiana.edu/~rickv/Beats.html" [Broken]
    Last edited by a moderator: May 5, 2017
  7. Aug 31, 2011 #6


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    Modulation IS beating. You will never get a tank circuit alone to generate new frequencies. It takes a component such as a diode or transistor along with a tank circuit to do this.
  8. Aug 31, 2011 #7
    So could the diode be causing it. This is where I'm getting confused as I know it takes at lest 2 frequencies to cause amplitude modulation.
  9. Sep 1, 2011 #8


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    It really doesn't take more than one input signal to make new frequencies. This can be something undesirable that can happen close to a transmitting antenna for instance. A corroded connection on a resonant length of guy wire on a tower can do this. The corroded connection forms a crude diode.
    At first glance I would not have said that the circuit you posted would have done this. However, I will not argue that it is impossible especially not knowing tank circuit values.
  10. Sep 1, 2011 #9


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    Hi hobbs. Are you certain it is modulation (beating) and not merely a decaying ringing at each edge. It's not really possible for us to know unless you post some pictures or sketches of the input and output waveforms (correctly time aligned).
  11. Sep 1, 2011 #10
    Yes it is modulation. I'll post some oscope waveforms and component values when I get the time.
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