resonant frequency

Discussion in 'Homework Help' started by mary_owen, May 4, 2008.

1. mary_owen Thread Starter New Member

Apr 30, 2008
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Calculate the resonnt frequency of a circuit consisting of a 400 ohm resistor in series with a 0.8h coil and a 3uf capacitor. could some one please give us a hand with this ?

Apr 26, 2005
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3. mik3 Senior Member

Feb 4, 2008
4,846
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The resonant frequency of a an RLC in series or parallel is given by the formula:

resonant frequency=1/sqrt(L*C)

4. Ron H AAC Fanatic!

Apr 14, 2005
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It would be interesting to see what the "official" answer to mary_owen's problem is.

5. mary_owen Thread Starter New Member

Apr 30, 2008
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thanks for the reply guys . while searching on the internet the formula 1/sqrt(L*C) poped up alot. i can supsotute the l and c for the capacitor and inductor value but where does the resistor value come into it?

6. mik3ca Active Member

Feb 11, 2007
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I thought it was 1 / (2 * pi * sqrt(L * C))

7. beenthere Retired Moderator

Apr 20, 2004
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Resistors aren't reactive - the resistance value is not affected by the applied frequency.

Guess I had better mention that a wire-wound resistor can cause problems in a high frequency application because that coil of wire in it is reactive - but it's probably safe to assume that's not the case here (there would have had to have been a mention of the diameter of the core and the number of turns for the resistor to have a calculable reactance).

Last edited: May 5, 2008
8. Ron H AAC Fanatic!

Apr 14, 2005
7,012
681
Here, we are talking about a series RLC circuit. In the previous thread, I was talking about a parallel resonant circuit with a resistor in series with the inductor, and driving that circuit with an AC current source. In that configuration, the damped resonant frequency (peak output voltage) is definitely a function of the resistor value. If you put the circuit in an oscillator, the oscillation frequency will be a function of the Q of the inductor.

With a series RLC circuit, the only thing that changes is the bandwidth of the notch.

9. Caveman Senior Member

Apr 15, 2008
471
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Guys,

You are correct about the formula for the undamped resonance frequency. However, an RLC circuit is a damped circuit, so it has a damped resonance frequency. Whether this is higher or lower than the undamped resonance frequency depends on the damping factor.

If you want more detail including the formula to solve for the resistance, read the RLC Circuit wikipedia article.

10. mik3ca Active Member

Feb 11, 2007
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I was wondering, in a parallel RLC circuit, shouldn't the resistance in AC mode be equal to slightly less than R at resonance, just because XL and XC are equal?

11. Digi Dave Member

Apr 25, 2008
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Digi_Dave, we don't give out answers in the Homework forum. We help questioners find their own answers.

-Thingmaker3

Last edited by a moderator: May 7, 2008