(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement:

What is the value of capacitance needed in Fig. 1 so that the voltage across the capacitor never exceeds (a) 200 V, (b) 20 V, and (c) 2 V ?

This is the Circuit

2. Relevant equations

Vc(0-) = 0V

Vc(0+) = 150V

ic = c v'

vc(t) = final + [initial - final] e^(-t/τau)

3. The attempt at a solution

since the capacitor is in parallel with the resistor the voltages are equal. which at t=0 will be 150V. up until 100us the capacitor is charging, then when t>100us the capacitor will discharge. so the max voltage across the capacitor will be 150V no matter what the value is. However you can get a smaller capacitance value to decrease this voltage but the voltage will never exceed 150V.

I broke the problem into two transient problems:

(1) t=0+ and t=100us-

(2) t=100us+ and t=infinity-

I tried to solve for Vc(t) for each of these problems:

for (1) Vc(t) = 20 - 20e^(-t/c)

for (2) Vc(t) = 20e^(-t/2c)

This did not work out right. So then I tried converting to non ideal current source, but got stuck.

4.PROBLEM

- I have never done a transient problem where the switch changes at some value other then 0.

-I simulated this circuit on pspice and I get the capacitance for each voltage, BUT I want to know how to do this by hand.

Your help is much appreciated!!

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# Transient Analysis RC

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