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RL Filter Calculator

Resistor-Inductor filter calculator

Result

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Formulas

ττ = L / R

f_cf_c = R / (2πL)

X_LX_L = 2πfL

RL vs RC

RL filters: Better for high current, power applications

RC filters: Smaller, cheaper, better for signal processing

Understanding RL Filters

RL filters use a resistor and inductor to filter frequencies. Unlike RC filters, RL filters can handle higher currents and are commonly used in power applications, switching power supplies, and EMI filtering.

How RL Filters Work

Low-pass (output across R): At low frequencies, inductor has low impedance, allowing signal through. At high frequencies, inductor impedance increases, blocking signal.

High-pass (output across L): At low frequencies, inductor blocks signal. At high frequencies, inductor passes signal to output.

Time Constant

The RL time constant τ = L/R determines response speed. Unlike RC circuits where τ affects voltage, in RL circuits τ affects current buildup through the inductor.

Frequently Asked Questions

The cutoff frequency fc = R/(2πL) is the -3dB point where output is 70.7% of input voltage. At this frequency, the inductor's reactance (XL = 2πfL) equals the resistance R.

Time constant τ = L/R (in seconds when L is henries and R is ohms). It's the time for inductor current to reach 63.2% of final value. After 5τ, current reaches 99.3% of steady-state value.

Use RC for: signal filtering, audio, low-power applications (cheaper, smaller). Use RL for: power filtering, high current applications, EMI suppression. RC filters are more common in electronics; RL in power systems.

Inductors are larger, heavier, more expensive than capacitors at the same energy storage. They can also pick up electromagnetic interference and are harder to manufacture with precision. Capacitors are preferred for most filter applications.

Rearrange the formula: L = R/(2πfc). Choose a practical resistance value first, then calculate the required inductance. Verify the inductor can handle the expected current without saturating.

When current through an inductor exceeds its rating, the core saturates and inductance drops dramatically. This causes the filter to stop working properly. Always check the inductor's current rating for your application.