Calculating unknown current and resistance in circuit…need help!?!?
In the following circuit, I and Rx are not known. The switch opens at t = 0, and the voltage across the unknown resistor is shown for t > 0. What is your estimate of the values of the current source I and the unknown resistor?
This is the circuit and graph, http://www.flickr.com/photos/96668433@N04/
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Verified answer
V(t) = (V0)e^(-R/L)t
V(0.1 ms) = 5
V0 = 60
1/12 = e^(-R/40x10^-3 H)(1x10^-4 s)
ln(12) = R(1x10^-4 s)/(40x10^-3 H)
R = ln(12)(40x10^-3 H)/(1x10^-4 s)
R = 994 Ω
The voltage across the components was -60 V before the switch was opened, Without knowing the series resistance of the inductor we can only guess The known current source is 2(60/994) + 60/10 = 6.12 Amperes
I*Rx=60 volts. The time constant L/Rx is about 40 usec. Rx=40E-6/40E-3=1E-3. Therefore I is 60/.001=60,000.
I = 6A
Rx=~ 1 K