RL Circuit Calculator
Analyse a series RL circuit for impedance, time constant and phase angle.
About RL Circuit Calculator
The RL time constant is L divided by R, unlike RC where the two multiply. Current rises exponentially toward its steady-state value, which is limited by resistance alone once the transient has settled. Interrupting current in an inductor produces a large voltage spike, which is why freewheel diodes are fitted across relay and motor coils.
Formula
RL Circuit
Z = √(R² + X_L²); τ = L ÷ R
- τ
- Time constant of the RL circuit, inductance divided by resistance
Worked example
With the default values below, RL Circuit Calculator returns Circuit Impedance of 101.2262. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Resistanceminimum 0.01 ohm
- 100 ohm
- Inductanceminimum 0 H
- 0.05 H
- Signal frequencyminimum 0.01 Hz
- 50 Hz
- Supply voltageminimum 0 V
- 24 V
Results
- Circuit Impedance
- 101.2262 ohm
- Time Constant
- 0.000500 s
- Inductive Reactance
- 15.7080 ohm
- Phase Shift
- 8.927 degrees
- Steady-State DC Current
- 0.24000 A
Step by step
- 1Values used
- 2RL Circuit
- 3Circuit Impedance= 101.2262 ohm
- 4Time Constant= 0.000500 s
- 5Inductive Reactance= 15.7080 ohm
- 6Phase Shift= 8.927 degrees
- 7Steady-State DC Current= 0.24000 A
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or fees or charges into account.
- Figures are rounded for display to 6 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is RL Circuit calculated?
Z = √(R² + X_L²); τ = L ÷ R The RL time constant is L divided by R, unlike RC where the two multiply. Current rises exponentially toward its steady-state value, which is limited by resistance alone once the transient has settled.
Why does RL Circuit matter?
Interrupting current in an inductor produces a large voltage spike, which is why freewheel diodes are fitted across relay and motor coils.
What values do I need to enter?
This calculator takes 4 inputs: Resistance, Inductance, Signal frequency, Supply voltage. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.