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RL Time Constant Calculator

Work out rl time constant instantly with clear inputs, formula shown and shareable results.

About RL Time Constant Calculator

In an RL circuit the time constant is L/R, and current rises as I(t) = (V/R)(1 - e^(-t/tau)). Note that more resistance makes the circuit faster, the opposite of RC, because resistance limits the final current the inductor must build up to.

Formula

RL time constant

tau = L / R; I(t) = (V/R)(1 - e^(-t/tau))

Worked example

With the default values below, RL Time Constant Calculator returns Time constant of 5.32. Change any input above and every figure here is recalculated in your browser.

Inputs used

Inductance
250 mH
Circuit resistance
47 Ω
Supply voltage
24 V

Results

Time constant
5.32 ms
Final steady current
0.5106 A
Current at one time constant
0.3227 A
Time to 99% (5 tau)
26.60 ms

Step by step

  1. 1Values used
  2. 2RL time constant
  3. 3Time constant= 5.32 ms
  4. 4Final steady current= 0.5106 A
  5. 5Current at one time constant= 0.3227 A
  6. 6Time to 99% (5 tau)= 26.60 ms

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why does adding resistance speed it up?

Because the final current is lower, so less flux has to be established. The rate of change dI/dt = V/L is unchanged at the start.

Why do inductive circuits need a flyback diode?

Interrupting the current forces dI/dt to a huge value, and V = L dI/dt produces a destructive voltage spike unless the current has somewhere to go.

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RL Time Constant Calculator – Calculate Time constant