time constant tau with capacitor in parallel

Discussion in 'Homework Help' started by aaron0000934, Dec 14, 2010.

1. aaron0000934 Thread Starter New Member

Dec 14, 2010
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I am stuck on the problem below. If the capacitor was not in parallel with the resistor I could solve for tau. I hope my make shift circuit looks ok.

For the circuit below, switch is closed at t=0. Determine:
1. the time constant, tau
2. the initial current in the capacitor
3. the final current in the capacitor
4. the capacitor voltage at t = 2 time constants
5. the capacitor voltage at t = 10 ms

see attached

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Last edited: Dec 14, 2010
2. Ron H AAC Fanatic!

Apr 14, 2005
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Hint: Do you know how to calculate Thevenin equivalent voltages and resistances?

3. Georacer Moderator

Nov 25, 2009
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1,266
Is the time constant troubling you?
In order to find the time constant, you need to reduce the whole circuit to a single resistance and capacitance and then multiply them. Nullify the voltage source and merge the two resistances (they are in parallel). Then the answer will be obvious.

4. aaron0000934 Thread Starter New Member

Dec 14, 2010
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I had a feeling that I needed to thevanize the circuit. So is this right Rth=300||600=200 ohms and tau=200ohm x 5uF= 1ms? Then my initial current is V/R = .09A and my final current is zero. Now I can plug my time constant in and solve for the two capacitor voltages.

5. Ron H AAC Fanatic!

Apr 14, 2005
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No, the current is wrong. You forgot to "thevanize" the voltage.

6. aaron0000934 Thread Starter New Member

Dec 14, 2010
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thanks Ron H, so Vth = 18V x (600 / 300 + 600) = 12V and then the current is Vth/Rth = .06A ?

7. Ron H AAC Fanatic!

Apr 14, 2005
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That's what I get.