Inductor Reactance Calculator
XL (Ω)
37.7
How it works
Inductive reactance (X_L) is the opposition an inductor presents to alternating current: X_L = 2π × f × L, where f is frequency in Hz and L is inductance in henries. Unlike resistance, inductive reactance is frequency-dependent — inductors are essentially short circuits at DC and increasingly oppose higher-frequency signals.
**Phase relationship** In an inductive circuit, voltage leads current by 90°. This phase shift is fundamental to AC power analysis — reactive power (VAR) is the power stored and returned by inductors and capacitors. An ideal inductor dissipates no real power; all energy is stored in the magnetic field and returned to the circuit.
**Impedance vs. reactance** Reactance (X_L) is a component of impedance (Z). For a pure inductor, Z = jX_L. In a series RL circuit, Z = √(R² + X_L²), and the phase angle θ = arctan(X_L / R). At high frequencies, X_L >> R and the impedance is dominated by the inductance.
**Common applications** RF chokes: inductors used to pass DC while blocking RF signals — the high reactance at RF frequencies prevents interference from entering power rails. Switching regulators: inductors store energy during the switch-on phase and release it during switch-off, enabling power conversion. Filter networks: inductors and capacitors form low-pass, high-pass, and bandpass filters based on their reactance vs. frequency characteristics.
**Self-resonance frequency (SRF)** Real inductors have parasitic capacitance between turns. Above the SRF, the component behaves capacitively rather than inductively. Specify inductors with SRF well above the operating frequency for reliable performance.
Frequently Asked Questions
- For a buck converter: L = (V_in - V_out) × D / (f_sw × ΔI_L), where D is duty cycle (V_out/V_in), f_sw is switching frequency, and ΔI_L is desired ripple current (typically 20–40% of full load current). Example: 12V to 5V at 2A, 100 kHz, 30% ripple: L = (12-5) × (5/12) / (100,000 × 0.6) = 4.86 µH. Use the next standard value: 4.7 µH or 6.8 µH. Check that the inductor's saturation current exceeds peak current (I_avg + ΔI_L/2).
- An inductor saturates when its core can no longer store additional magnetic flux — the permeability drops sharply, inductance falls dramatically (sometimes to 10–20% of rated value). In a switching regulator, saturation allows current to rise uncontrollably, potentially destroying the switch (MOSFET or transistor). Signs of saturation: excessive heat in inductor, unstable output voltage, audible buzzing. Always choose inductors with saturation current rating above peak current. Gapped ferrite cores and powdered iron cores are more saturation-resistant than ungapped ferrite.
- Inductance (L, in henries) is a fixed property of the component — determined by number of turns, core material, and geometry. Inductive reactance (X_L = 2πfL, in ohms) is frequency-dependent — the same inductor presents different impedance at different frequencies. At DC (f=0), X_L = 0 (short circuit). At 1 MHz with L = 10 µH: X_L = 2π × 10⁶ × 10⁻⁵ = 62.8Ω. Impedance Z = √(R² + X_L²) includes both resistance (R, the DC winding resistance) and reactance. For most inductors in RF applications, R << X_L, so Z ≈ X_L.
- At frequencies above the self-resonant frequency (SRF), the parasitic capacitance between turns dominates and the inductor behaves as a capacitor. The SRF is where inductive and capacitive parasitics cancel — maximum impedance occurs here. Always choose inductors with SRF at least 2–3× above the operating frequency. A 10 µH inductor with SRF of 10 MHz should not be used above 3–5 MHz. For RF applications above 100 MHz, air-core inductors (no magnetic core) have higher SRF but lower inductance per unit size.