The RL circuit calculator computes the time constant (τ = L/R), -3dB cutoff frequency, and inductor current rise time for any resistor-inductor combination. Used for power supply filters, motor control, and RF choke design.
RL Circuit Calculator
How to Use an RL Circuit Calculator
RL circuits are fundamental in power electronics, motor control, and RF filtering. Power supply inductors filter switching noise; the inductor's impedance at the switching frequency (typically 100 kHz-1 MHz) attenuates high-frequency ripple. In motor control, motor winding inductance combined with drive resistance creates the RL time constant that limits current ramp rate.
Inductor Impedance
Inductive impedance XL = 2πfL increases with frequency, making inductors ideal for blocking high-frequency interference while passing DC and low-frequency signals. This property is used in RF chokes (ferrite bead inductors) on power supply lines to prevent switching noise from entering sensitive analog circuits.
FAQ
What is the RL time constant?
The RL time constant τ = L/R (inductance in henries / resistance in ohms). After one time constant, current rises to 63.2% of the final steady-state value. After 5τ, current is 99.3% of maximum — effectively the inductor's resistance has been overcome and steady-state is reached.
How is an RL circuit different from an RC circuit?
In an RC circuit, current flows freely at startup and stops when the capacitor is charged. In an RL circuit, current starts at zero and rises to maximum as the magnetic field builds. RC stores energy in an electric field (capacitor); RL stores energy in a magnetic field (inductor). Both have the same mathematical shape of response.
What is an RL low-pass filter?
An RL low-pass filter has the inductor in series and resistor to ground. At low frequencies, the inductor has low impedance and passes current. At high frequencies, the inductor's impedance (XL = 2πfL) blocks current. The cutoff frequency is fc = R / (2πL).
When would I use an RL circuit vs an RC circuit?
RL circuits handle higher power applications better (inductors can carry large currents). RC filters are preferred for small-signal audio and timing circuits (capacitors are cheaper and smaller for most frequencies). Inductors are chosen for power supply filtering, EMI suppression, and RF applications where large capacitors would be impractical.
Is this calculator free?
Yes, completely free with no signup required.