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Reactance of L and C

AC circuits and filters · Intermediate · about 15 minutes · part of Zen’s Electronics Design Mastery

On AC, inductors and capacitors limit current through reactance. X_L = 2πfL grows with frequency, so a coil passes low frequencies. X_C = 1/(2πfC) shrinks with frequency, so a capacitor passes high frequencies and blocks DC.

Reactance of L and C: the circuit on the simulator's bench

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How it works

At 100 Hz: XL = 2π·f·L = 62.8 Ω (plus 1.5 Ω of winding), XC = 1/(2π·f·C) = 159 Ω

IL = V / |ZL| = 79.54 mA, IC = V / XC = 31.41 mA

They are equal at f = 1/(2π√LC) = 159.2 Hz. Below that the coil carries more; above it the capacitor does.

Try it on the bench

  1. Set the source to 50 Hz, then 200 Hz. I_L halves each time the frequency doubles, and I_C doubles.
  2. Try 1 Hz: the capacitor passes almost nothing, while the coil is limited only by its winding resistance.

Parts on the bench

In the course

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

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