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

Question about LC circuits

  1. Dec 14, 2009 #1

    This is such a great community! I've got some great input from here in the past!

    I took a bit of a break but I'm back to playing around and trying to learn about electronics and circuits and I wonder if I can pick your wonderful brains again :)

    Ive learned that a circuit with a properly tuned capacitor connected to a coil will create an oscillator that will oscillate until the capacitor loses its charge.

    Now, I'm experimenting with harmonic resonance

    Id like to make an LC circuit... Normally the current in the coil is AC as the circuit oscillates... Correct?

    Id like the circuit to drop its ac component but maintain the same frequency of dc pulses. I can use some diodes for this?

    I am thinking of diverting the return oscillation to a second, smaller coil to generate a harmonic of the fundamental frequency and mix it back into the first coil in time to repolarize the cap so it can deliver its next DC Pulse (along with the harmonic)

    If I wind the smaller coil on top of the larger one (fewer winds) I should be able to introduce the harmonic into the larger coil by induction correct?

    My question is, how can I feed the end product back into the cap so it can deliver its next dc pulse at the self resonant frequency of the coil?

    Thanks guys :)
  2. jcsd
  3. Dec 14, 2009 #2


    User Avatar
    Science Advisor
    Gold Member

    It doesn't work. A sequence of "DC pulses" has a pretty complicated frequency spectrum (with in theory an infinite number of harmonics), but at the same the you have an LC circuit which is quite litteraly a bandpassfilter (meaning everything except the resonance frequency is filtered).
    So no, you can't make a passive oscillator that works with pulses.

    You can create all sorts of strange effects if you add non-linear elements like diodes to resonanators, but not the effect you seem to be after.
  4. Dec 14, 2009 #3
    couldn't he turn an ac wave into a crude dc square wave using a rectifier and a couple of zener diodes? it would have to be on the output of the LC circuit though, not interfering with the circuit itself.
  5. Dec 14, 2009 #4


    User Avatar
    Science Advisor

    I read back through your previous posts.


    You seem to be stuck on the idea of LC circuits oscillating by themselves.
    In fact, LC circuits only oscillate briefly and under very special circuitry. "Ringing" on pulsed inputs is one.

    Usually, tuned circuits just modify incoming waveforms like this:

    effect of tuned circuit on pulse.PNG

    This shows the harmonics of an incoming waveform. If you remove the harmonics the waveform looks more and more like a sinewave. If you remove all of them, it will be a sinewave.

    A tuned circuit isn't perfect and it may remove all but the lower harmonics, producing a rounded version of the original but without the fast rise and fall times of the original.

    You can tune the tuned circuit to just a harmonic if you like. This would give you a sinewave at the harmonic frequency.
  6. Dec 14, 2009 #5
    LC won't oscillate by itself.

    To utilize the resonator as an oscillator the circuit must amplify the oscillations of LC and feed them back to itself in correct phase - that way it will sustain stable oscillations, otherwise LC will die as soon as it was energized.

    Also, because the amplifier is usually non-linear and components not ideal, the oscillator will output the signal at a fundamental frequency and a couple of its harmonics (integer multiples), if that poses a problem the output is low-pass filtered.

    Once you have a stable oscillator, you can apply it to a schottky diode bridge rectifier which will produce sharp pulses, or apply it to some other non-linear diode scheme that produces combs.
  7. Dec 14, 2009 #6
    Can you explain what you are trying to make. It sounds a bit like a complicated way of making an oscillator (which would require a power source such as a battery).

    On the other hand, are you trying to make a system to keep working forever without power?
    Last edited: Dec 14, 2009
  8. Dec 14, 2009 #7
    It seems from others comments that the "perpetual motion machine" is in fact your aim.
    Please try to understand that power losses in circuit components and wiring make this impossible.
    Any such system will always run down as energy is lost, usually as heat.

    People have been trying to do this in different ways (e.g. mechanical) for centuries - it never works.
  9. Dec 15, 2009 #8
    Hi again! :)

    Yes, this sounds good
  10. Dec 15, 2009 #9

    TYVM for your input :)

    This is sort of what I was thinking. Though from what youre suggesting, it sounds like I need some kind of timing circuit to generate dc pulses in a second circuit.
  11. Dec 15, 2009 #10
    It sounds like in order to get an LC circuit to emit pulsed DC, the first dc pulse through the coil would have to be routed around the coil on its return but somehow fed back to the cap with the right latency to be properly in phase.

    I'll have to play around with what kind of signals are the most interesting - This is great info though, thanks very much!! :D
  12. Dec 15, 2009 #11
    I thought about it, and youre right, there must be an easier way to generate dc pulses.. Not sure how you'd do that though :\ Can you point me in the right direction?

    Like I said, I've got myself interested in harmonic resonance. My previous posts arnt really related to what I'm looking at here. I'm just interested in the properties of harmonic resonance and the interaction effects of harmonic frequencies.


    That sounds a lot more interesting than what I'm doing but I dont really think you can actually do that... Can you explain what that has to do with harmonic resonance and LC circuits?
    Last edited: Dec 15, 2009
  13. Dec 15, 2009 #12
    Hello again

    I'm not sure what gave you the impression that I'm trying to defy the laws of physics.. I'm aware that energy in a system is finite unless new energy enters the system. In either case, I said I'm interested in the interaction of harmonic frequencies and harmonic resonance. What gave you the impression that this relates to perpetual motion? Is there something that I'm missing?

    Please elaborate - Your previous post gives me the impression that you perceive a link here..
  14. Dec 15, 2009 #13
    Hello Phantasm,

    Well, I think that I probably misunderstood your earlier posts - sorry. I had thought that perhaps you were trying to rectify the output from a resonator, so as to recover dc which would in turn be used to power the oscillator driving the resonator in the first place...

    I was just trying to make sure that you were not wasting your time going down that particular false trail. Since evidently you are not, that's all I have to say about it.
  15. Dec 15, 2009 #14

    Sorry I didnt mean to come off harsh... and I can see what you mean about how I said I wanted to recycle half of the AC phase as implying that I'm trying to build a perpetual motion machine =p You must get a lot of those kinds of people around here =p

    I was just trying to kill 2 birds with one stone by using half the AC phase to somehow generate a second frequency while maintaining the same fundamental frequency of dc pulses. Since I only have 1 signal generator and I'd need more than one to study harmonic interactions I thought I could just generate my signals with coils with their number of turns in the same ratio as the harmonic number I'm trying to generate.

    Am I on the right track here at least?


    On another note,

    Someone suggested I could speed up my test setups by using circuitry simulation software to design the desired test setup so I can build it with minimal tweaking and re-designing (since im new at all this)

    OrCad was suggested but I'm told there are easier to use solutions that are free and still do much of the same thing.

    I'd like to be able to just plug in the circuit components and then configure their properties and see the resulting current at various points in the circuit so I know how the circuit is behaving. Any tips in this regard?
    Last edited: Dec 15, 2009
  16. Dec 15, 2009 #15
    LC doesn't generate DC pulses, it generates AC sinewaves.
  17. Dec 15, 2009 #16


    User Avatar
    Science Advisor
    Gold Member

    Download LTSpice from www.linear.com
    It is basically a free SPICE implementation with a reasonably easy-to-use user interface.
  18. Feb 5, 2010 #17
    hi friends!!!
    i have a doubt too in LC parallel circuits..how is the impedance related to quality factor and inductive reactance..Z=(Q,XL)...i would like to have an expression in this form...thank u in advance
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook