RL circuit time constant

Objective

Observe the exponential rise of inductor current and read the time constant τ = L/R.

Procedure

1. Run and open the Waveforms tab: I(L1) rises to 100 mA.
2. Double L: τ doubles.

Theory

i(t) = (V/R)(1 − e^(−t/τ)), τ = L/R = 100 mH / 100 Ω = 1 ms. After one τ the current is 63.2 % of its final value V/R = 100 mA.

Questions to answer

1. What is the inductor voltage just after the switch-on?

Components used

Function generator (square), Inductor 100uH, Oscilloscope (2-channel), Resistor 220 ohm 1/4W

About the model

Ideal wires; component models as listed in the catalogue (no parasitics, no temperature drift). Built without a breadboard so the schematic stays readable.

More Basic Electrical Lab experiments

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• Ohm's law
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• Lamp circuits in series and in parallel