Consider the series RL circuit given in Problem 9, in discharge mode, with voltage source removed. Parameters such as R = 10 Ω and L = 10 mH are the same. The switch has been closed for long period of time, such that the current has developed to the maximum or steady-state level 1.04 A. How much time would need to elapse for the current to drop to 0.5 A after the switch is opened.
Apply series RL current equation, Eq. 1.31.
i_L(t)=i_R(t)=i(0)e^{-\frac{R}{L}t}+\frac{V}{R}\left\lgroup 1-e^{-\frac{R}{L}t}\right\rgroupGiven:
t = ?
L = 10 × 10^{−3}H
R = 10 Ω
V = 0
i(0) = 1.04 A
i_L(t)= 0.5 A
0.5=(1.04)e^{-\frac{10}{0.01}t}
0.481=e^{-\frac{10}{0.01}t}
\ln(0.481)= \ln{\left\lgroup e^{-\frac{10}{0.01}t}\right\rgroup}−0.7324 = −1,000t
t = 0.00073 s or 0.73 ms