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Lc circuit

  1. Aug 9, 2013 #1
    So I have a question about the circuit.
    Now we know that when the C is discharging there is a current through the inductor , now there can be these oscillations because the inductor time delays the current (because of back emf) that wants to run to the other side of the capacitor as it would normally want to do.

    The thing I don't understand is that even though the capacitor is being shorted by an inductor which delays the shorting due to the buildup of the magnetic field it still is a single wire over opposite polarity capacitor terminals , now when opposite charges meet they annihilate into a electric discharge typically.
    I don't understand why in the case of the inductor the opposite charges can sorta like "change lanes" on the same wire when if that wire would be just a typical shorted wire across the cap terminals it would result in as short circuit rather than a polarity change with a given characteristic frequency depending on the parameters of the C an L. ?
     
  2. jcsd
  3. Aug 9, 2013 #2

    mfb

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    Staff: Mentor

    An inductor is not a simple delay. While the capacitor is discharging, the inductor is charging (it builds up a magnetic field). When the capacitor is discharged, the inductor "tries to keep that current" (and the corresponding magnetic field). It discharges, and charges the capacitor at the same time.
     
  4. Aug 9, 2013 #3
    oh i guess i got it , the capacitor charges the inductor one way and then when the magnetic field collapses it reverses the current flow so those positive ad negative charges don't have to meet , their fate and flow is made at the very instant the magnetic field starts to collapse to form a current flow just the opposite way than before the field was made?

    Oh by the way , say I wanted to drive my lc circuit with pulsed or short square DC , when it would be best to drive the pulses in when the capacitor is charging up or when the inductor is "charging up" ?Or maybe it doesn't matter?
     
  5. Aug 9, 2013 #4

    berkeman

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    If your source is a voltage source, you would drive in phase with the capacitor voltage (through an isolating impedance so you don't short out the LC with the low output impedance of the AC voltage source).

    If your source is a current source, you would drive in phase with the inductor current.
     
  6. Aug 9, 2013 #5
    Say i have mains rectified and smoothed out to DC , I guess you could consider that a current source rather than a voltage one , even though it's kinda weird as every voltage has also some current, and what do you mean by isolating impedance? A capacitor comes to mind but I guess you can't drive a LC parallel circuit and insulate it from the source with a capacitor as that would form a series LC with the inductor and God knows what would happen at such circuit.. Or?

    I talked about the rectified mains because I have a crazy idea of making a smps which has a transformers primary in parallel with a capacitor and inductor or in other words a small smps , with a varying capacitor which would compensate for the frequency lag due to the load on the transformer.
    Well just an idea , theoretical right now.
    But something tells me this will not work the way i think it should.Well theoretically the transformers primary is also an inductor ...?
     
  7. Aug 9, 2013 #6

    berkeman

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    You smooth the output of a rectifier with a capacitor, which has a low impedance. So that would be a voltage source with a low output impedance.

    The isolating impedance I was referring to for driving the LC with an AC voltage source is a resistor.

    It doesn't sound like you should be messing around with AC Mains voltages and trying to build up a SMPS at this point. Give it a couple more years of learning electronics, and try to find a local mentor who can help you learn how to safely work with AC Mains circuits.

    You should also do a lot more learning about DC-DC converter circuits before you start trying to come up with new tricks to make them work better. Get a good book on DC-DC converters (like by Mitchell or by Chryssis), or use online resources (TI or Maxim, etc.).
     
  8. Aug 9, 2013 #7

    berkeman

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    BTW, depending on the value of the resistor and the frequency that you are driving, you may get some voltage lag through the RC circuit.
     
  9. Aug 10, 2013 #8
    No need to worry berkeman , I have worked on mains voltages for quite a time , repairing broken smps and building some amps also other stuff.
    I could agree on the "learning theoretical " part because I am the type of guy who always starts something even if he isn't perfectly sure of what he is doing but rather has a good idea.
    The local mentor thing is a bit tricky as I have had electricians that have helped me when I was still at middle school but somewhere it gets to a point where you grow to a level past the one which the man is and so beyond tat point he becomes less of a help.
    Well I was messing around some astable generator types etc looked at the waveforms i was getting in my oscilloscope.So now I thought that maybe just maybe one can make a simple smps out of an LC.by the way I didn't hear your commentary on this idea?

    By the way the LC with given parameters tend to oscillate at it's resonant frequency , so if it would be part of a transformers primary and the transformer would be under load that frequency would probably go down below the resonant frequency but if the power supply that feeds the LC is powerful enough would that tend to drive the frequency back to the resonant?
    Or in other words does the LC tend to lean towards resonance from frequencies above or below the resonant one due to certain condition being pushed upon the LC?
    I guess it should.

    Thanks for the answers so far , hoping to hear more :)
     
  10. Aug 12, 2013 #9
    Anyone would like to comment? ..

    @berkeman by the way i didn't understand about the resistor because you used the word impedance which I kinda forgotten since I'm not a native english speaker, so yes it makes sense now.
     
    Last edited: Aug 12, 2013
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