Sample problems
Topic: RC and RL circuits
Statement of problem
The switch in the circuit has been in position x for a long time.
18
mA
5
k
Ω
0.2
F
μ
x
y
0
t
=
20
k
Ω
10
k
Ω
v
α
Δ
v
Δ
+
−
At t=0, the switch moves instantaneously to position y.
(a) Find
so that the time constant for t>0 is 1ms.
(b) For the
found in (a), find
v
Δ
.
Solution
(a) After t=0, the resistance seen from the capacitor can be modeled as a Thevenin
internal resistance R.
20
k
Ω
10
k
Ω
v
Δ
v
Δ
+
−
v
+
−
i
iv
Δ
−
0.2
F
The resistance can be obtained from the time constant
1
5
0.2
ms
R
k
CF
τ
=
==
Ω
(1)
The resistance can be also obtained as follows
30 200000 (
)
R
k
=
−Ω
(2)
From equations (1) and (2),
is easily obtained
0.125
mS
=
Note that the unit for
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 Spring '05
 Unknown
 Electrical Engineering, RC circuit, RL circuit, 5k, 0 10k, siemens., RL circuits Statement

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