What is the difference between series resonance and parallel resonance?

RLC parallel resonance circuit:

Image by V4711 Attribution-Share Alike 3.0 Unported

  • At low frequencies, the inductor will have the lower impedance and will dominate the higher impedance of the C and the R. This means that most of the current goes through the inductor. As frequency increases, the impedance of L increases and current decreases.
  • At high frequencies, the capacitor will have the lower impedance and will dominate the higher impedance of the L and the R. This means that most of the current goes through the capacitor. As frequency increases, the impedance of C decreases and current increases.
  • At resonance, the impedance of the L equals the impedance of C except they are 180 degrees out of phase with each other, cancel to create infiniteimpedance and you are left with R as your impedance. This means that all of the current goes through the resistor.

Formula from: Parallel RLC Circuit and RLC Parallel Circuit Analysis

Here is impedance plotted versus frequency for a parallel RLC resonance.

Graphic from: Parallel Resonance and Parallel RLC Resonant Circuit


RLC series resonance circuit:

Image by V4711 CC BY-SA 3.0

  • At low frequencies, the capacitor will have the higher impedance and will dominate the lower impedance of the L and the R. This means that the capacitor determines the amount of current flowing through the circuit.As frequency increases, the impedance of C decreases and current increases.
  • At high frequencies, the inductor will have the higher impedance and will dominate the lower impedance of the C and the R. This means that the inductor determines the amount of current flowing through the circuit. As frequency increases, the impedance of L increases and current decreases.
  • At resonance, the impedance of the L equals the impedance of C only they are 180 degrees out of phase with each other, cancel to create zeroimpedance and you are left with R as your impedance.

Formula from: Series RLC Circuit and RLC Series Circuit Analysis

Here is impedance plotted versus frequency for a series RLC resonance (ignore the weird kink at higher frequencies.).

Graphic from Series Resonance in a Series RLC Resonant Circuit

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