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RLC parallel circuit

AC circuits and filters · Advanced · about 20 minutes · part of Zen’s Electronics Design Mastery

With L and C in parallel, their branch currents are 180° apart. At resonance they cancel in the line, so the tank looks like a large resistor while a big current circulates inside it.

RLC parallel circuit: the circuit on the simulator's bench

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How it works

Ztank = ZL ∥ ZC = 6.37 kΩ at 159.2 Hz (resonance at 159.2 Hz)

Iline = Vs / |Rs + Ztank| = 1.357 mA, Vtank = 8.643 V

The coil branch includes its 1.5 Ω winding plus about 0.07 Ω of ammeter and leads; at resonance |Ztank| ≈ L / (rC), so every milliohm counts.

Inside the tank: IL = 86.42 mA, IC = 86.42 mA. At resonance they are far larger than the line current.

Try it on the bench

  1. At 159 Hz the line current dips to about 1 mA while about 87 mA circulates between L and C.
  2. Detune to 100 Hz or 250 Hz: the tank impedance collapses and the line current rises.
  3. Open Tools… → Bode plotter with the output across R: the peak is the resonant frequency and its width gives Q.

Parts on the bench

In the course

Zen’s Electronics Design Mastery, module “AC & Reactance”.

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