RL Circuit Calculator
Calculate the RL time constant τ = L ÷ R and settling time for a series resistor–inductor circuit.
How it works
- 1Enter the inductance in millihenries (mH) and the resistance in ohms (Ω).
- 2The calculator applies τ = L ÷ R, converting mH to henries first.
- 3Results show τ in milliseconds and seconds plus the 5τ settling time.
Use cases
- Designing filter circuits where the RL time constant sets the corner frequency.
- Estimating motor or solenoid current rise time from winding inductance and resistance.
- Verifying relay or coil energise/de-energise timing in control systems.
Frequently asked questions
What is the RL time constant?
τ = L ÷ R (seconds). The current reaches ~63.2% of its steady value after one τ and ~99% after 5τ. A 100 mH coil with 10 Ω gives τ = 0.1 ÷ 10 = 0.01 s = 10 ms.
Why does it show — when resistance is zero?
With R = 0 Ω, τ = L ÷ 0 is undefined (infinite). A lossless inductor would never settle, so the calculator returns no result.
How does τ relate to the filter corner frequency?
fc = 1 ÷ (2π × τ) = R ÷ (2πL). For τ = 10 ms, fc ≈ 15.9 Hz; signals above it are attenuated.
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