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Series resonance circuit

  1. Mar 9, 2010 #1
    hey guyz i was just reading series resonance circuits and a read a confusing statement ''at resonance frequency the voltage drop across capacitor and inductor may be much larger than the source voltage''
    how is that possible drop bieng greater than source voltage??????????
     
  2. jcsd
  3. Mar 9, 2010 #2
    If the capacitor and inductor are in series, the voltage drops across the capacitor and inductor are out of phase by about 180 degrees. So the sum of the two voltages is very small.

    Bob S
     
  4. Mar 9, 2010 #3

    sophiecentaur

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    It's just one of the many forms of resonance* - energy builds up in the form of current flowing from inductor and capacitor and back and, on each cycle, the input signal supplies the same amount of energy that is dissipated by the resistive parts of the circuit.
    *If you give a child's swing a very small push each time it returns to you the amplitude of the swing will build up to be much higher than it was from your first push. That's a mechanical analogue.
     
  5. Mar 10, 2010 #4
    so the voltage drops that are greater than the source just cancel that is why we can say that these drops can be greater than the source voltage right ?????????
    but it is a bit diffcult for me to understand how the concept of resonance is applied in these circuits
     
  6. Mar 10, 2010 #5

    sophiecentaur

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    Left to themselves, a C and L in series have a natural frequency. Charge up the C and leave them to it and you will get an oscillation with a gradually decreasing amplitude as the energy dissipates in the resistance present - just the same as a pendulum or a mass on a spring etc.
    The large voltage swing appears across the C two components and can be high - to the source, if applied in parallel with them, it looks like a high impedance and the voltage may not get much higher than the input volts (limited by the source resistance). If you feed them in series, the input looks like a low impedance (low voltage swing) but across the L and C are high voltages - in antiphase with each other and in quadrature with the exciter voltage.
     
    Last edited: Mar 10, 2010
  7. Mar 12, 2010 #6
    It's NOT possible! Since the source voltage must equal the voltage in the rest of the circuit (capacitor and inductor in series) the voltage of the series combination is never greater than the source voltage.

    What they are obviously referring to is that the voltage measured EITHER across the capacitor OR across the Inductor can be greater than the source voltage. This would be because at resonance the impedance of the capacitor and inductor pretty much cancel each other. This makes the total impedance of the series circuit quite low. That leads to a large current flowing. And that large current through EITHER the impedance of the capacitor or inductor gives a high voltage (by ohm's law) But together they cancel leading to the low impedance of the whole circuit. But such a cancellation only happens at one frequency known as "resonance".
     
  8. Mar 12, 2010 #7

    sophiecentaur

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    That last statement is a biT misleading. There is a band of frequencies about a peak value, which is the 'natural' frequency. The height and width of this 'response curve' depend on the resistive elements in the circuit. This is referred to as the Q (quality) factor of the resonator.
     
  9. Mar 22, 2010 #8
    i also read that in an r_l circuit the back EMF should be equal to the source voltage but would not that reduce the total voltage to minimum
     
  10. Mar 22, 2010 #9

    sophiecentaur

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    The 'back EMF' will settle down to zero, when the current, eventually, has reached its maximum (the current has an exponential value - approaching V/R). The rate of change of current is highest at switch on - when the actual value of the current is Zero.

    You are right in suggesting that the initial (instantaneous) impedance of the RL combination is at its highest at switch on. As the rate of change of current drops, so does the input impedance.
     
  11. Mar 24, 2010 #10
    Here is a site that explains this: http://www.allaboutcircuits.com/vol_2/chpt_6/3.html
    Tesla coils are a phenomenal example of series resonance. Other factors in Tesla coils play a part too like loose coupling, turns ratio, capacitor efficiency, etc., but it is mostly series resonance that produces the high voltages.
     
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