Page 80 - LAPLACE TRANSFORM

Example 2 (RL series circuit)

By using Laplace transform, determine the current i(t) when S1 switch is closed.

Calculate the value of VR (t) and VL (t) when t = 0.1s. (Assume that all initial condition

is zero).

SOLUTION

Step 1: By using Kirchhoff’s voltage law, determine differential equation in the time-

domain.

VR + VL = Vs

di
Ri(t) + L = V
s
dt

Step 2: Apply the Laplace transform of the differential equation to put the equation

in the s-domain.

V s
RI(s) + L{ ( ) − } =
s

V s
RI(s) + L{ ( )} =
s

60
15I(s) + 3[sI(s)] =
s

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