Figure 1: In an RL low-pass filter, the inductor is in series and the resistor shunts to ground. At low frequencies the inductor has low impedance. At high frequencies it blocks the signal.
Table of Contents
What Is an RL Filter?
An RL filter uses a resistor and inductor to create a frequency-dependent response. The cutoff frequency is fc = R/(2πL). As a low-pass filter (L in series, R to ground), the inductor’s increasing impedance with frequency blocks high-frequency signals. As a high-pass filter (R in series, L to ground), low frequencies are shunted through the inductor.
RL filters are less common than RC for signal processing but essential in power electronics, EMI suppression, and applications where inductors are already present. The RC Filter Calculator covers the more common RC alternative.
The Formula
At fc: XL = R, gain = −3 dB
Worked Example — EMI Suppression
fc = 50/(2π × 0.01) = 795.8 Hz
The choke attenuates noise above ~800 Hz while passing the power or signal below. The Low Pass Filter Calculator shows the full frequency response.
Worked Example — Speaker Crossover
fc = 8/(2π × 0.0015) = 848.8 Hz
This creates a low-pass to the woofer, rolling off above ~850 Hz. The Crossover Calculator designs complete multi-way crossover networks.
Worked Example — Finding Required L
L = R/(2πfc) = 75/(2π × 100) = 119.4 mH
Inductor DCR Matters
Real inductors have winding resistance (DCR) that adds to the external R, shifting the cutoff frequency. For a 50 mH inductor with 10 Ω DCR in a circuit with 100 Ω external R, the effective R is 110 Ω, raising fc by 10%. Always check the inductor datasheet for DCR. The Filter Cutoff Frequency Calculator uses your entered R value directly — include DCR for accuracy.
RL vs RC — When to Choose
RC is preferred for most signal-level filtering because capacitors are cheap, small, and precise. RL is preferred when the signal carries significant DC current (inductors pass DC with low loss), in power supply input filters, and in high-current EMI suppression where capacitors alone cannot handle the current. The Band Stop Filter Calculator shows how RL and RC elements combine for notch filtering.
Inductive Reactance Calculator: The Inductive Reactance Calculator shows how inductive reactance increases with frequency, which is why inductors block high frequencies.
Frequently Asked Questions
What does first-order vs second-order mean?
Can I use this for audio applications?
What is the phase shift at cutoff?
How do I make a bandpass filter?
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