RL Filter Calculator

The RL filter is the dual of the RC: swap capacitors for inductors, swap 1/(jωC) for jωL, and the math rearranges into fc = R/(2πL) with a time constant τ = L/R. The earliest real-world RL filters date to the 1890s, when inductors ("chokes") were standard components for smoothing the pulsating DC of early rectified power supplies feeding telegraph and radio transmitters. In those high-current applications, filter capacitors of the required size didn't exist — wound chokes did.

LC composite filters — a blend of RL and RC thinking — were patented by George Campbell in 1917 and became the dominant filter topology for long-haul telephony through the 1940s and for radio IF strips through the 1960s. Pure RL signal filters mostly faded with the rise of cheap electrolytic and film capacitors, but they survive in two places: power electronics, where inductors are already present to limit di/dt, and EMI suppression, where common-mode chokes block high-frequency noise on AC mains cords.

Pick low-pass (L in series, R to ground) or high-pass (R in series, L to ground), enter R and L with engineering suffixes, or enter a target fc and let the tool solve for L. Returned values include cutoff frequency, time constant L/R, gain response, and phase shift.

Real inductors have DC resistance in series with the ideal L — at low frequency that DCR sits in a voltage divider with R and can raise the low-frequency floor above 0 dB on the response plot. Real cores also saturate at high current, which limits how large L can be for a given current handling. For signal-level filters, RC is usually a better pick; RL shines where an inductor is already present for other reasons. All math runs client-side.