Please help with LCR Circuits...

Discussion in 'Homework Help' started by samantha3101, Mar 27, 2009.

  1. samantha3101

    Thread Starter New Member

    Mar 26, 2009
    2
    0
    I'm new to this so sorry if someone has already written something about this subject :)

    I'm stuck on what i'm actually meant to be looking for in LCR circuit. In my experiement we had to draw a table with three headings; frequency, voltage and current. voltage was a constant at 3.00v and we increased frequency (200 f/hz to 2000f/hz) to find the highest current. In one exp. C=0.047μF, L=0.5H and R=82 Ω (unsure if Ω is correct) and in the other exp. C=0.047μF, L=1H and R=239 Ω. Am i looking at different resistors or inductors and capicators?!

    So if this is very confusing. Any info, help, etc would be great!

    xxx
     
  2. davebee

    Well-Known Member

    Oct 22, 2008
    539
    46
    You're looking for resonance.

    When inductors and capacitors are used together in a circuit, they work together to make the circuit act differently at different frequencies. There is a special value of frequency called the resonant frequency where voltage and current reach maximum or minimum values, depending on how the parts are connected.

    Did the experiment tell you how to do the math to calculate the resonant frequency based on the values of C and L? (You can ignore the R for the resonance calculation)
     
  3. samantha3101

    Thread Starter New Member

    Mar 26, 2009
    2
    0
    Thanks! Im guessing that equation will be on this site? x
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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  5. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    I like to think of inductors and capacitors as variable resistors. Their resistance rises and falls as a function of frequency.

    For an inductor the impedence (variable resistance), XL = 2*pi*f*L
    For a capacitor the impedance (variable resistance), XC = 1/(2*pi*f*C)

    When a capacitor and an inductor are in parallel they resonate at the frequency that makes both their impedences the same, XL=XC. Equating these we get

    fr = 1/[(2*pi*SQRT(L*C)], the resonant frequency.

    At this frequency the signal voltage will be its highest and falls off when you vary the frequency to either side -- XL makes it fall as you decrease the frequency and XC makes the voltage fall as you increase frequency.

    You're measuring current to find the maximum voltage which is the resonant frequency. Most labs use oscilloscopes, but the principle is the same.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    "When a capacitor and an inductor are in parallel they resonate at the frequency that makes both their impedences the same, XL=XC."

    Likewise, when caps and inductors are in series, they also form a resonant circuit The difference is that the voltage across the network will DIP at the resonant frequency. Maximum LINE current (minimum impedance) at resonance in a series circuit and maximum CIRCULATING current (maximum impedance) at resonance in a parallel circuit. The end usage determines whether you want to notch out the voltage a specific frequency such reducing an interfering signal, or maximize the voltage at a specific frequency such as tuning in a particular radio station.
     
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