## tdabboud 3 years ago Transient analysis problem http://imageshack.us/photo/my-images/827/99437379.jpg/ I need to find the value for C, where the voltage across the capacitor will not exceed a) 200V, b) 20V and c) 2V. I have done it on pspice and got answers but I want to understand how to do the problem by hand. Your help is much appreciated

1. imranmeah91

no worry, ee here

2. tdabboud

thank god, I have been working on this problem for 3 hours now

3. tdabboud

What I think I have come to is that i need to do an analysis for when t=0+ and t=100u- as well as when t=100u+ and t=infinity-

4. imranmeah91

Let me see , if I have that correctly There are two switches , one open at t=100 us other close at t=0 ?

5. tdabboud

yes that is correct

6. imranmeah91

It's a DC , right?

7. tdabboud

yes dc

8. tdabboud

i dont think that would matter though. I know when you have dc, the capacitor will open but i dont think it will in this case since the voltage across that resistor is the same as the capacitor

9. imranmeah91

Well we have to consider phase lag if it is AC

10. tdabboud

okay that makes sense

11. imranmeah91

This is DC Parellel RC circuit First time for me

12. imranmeah91

I got some Differential equations that might work, hopefully

13. tdabboud

Yeah same here..all of the examples are usually series RC and parallel RL. I think thats why it is so challenging. I made an attempt the first time to change the source and resistor into a non ideal current source. then find the current through the resistor. From that I feel that you may be able to get the right voltages but its very confusing. Another thing is I think it is two transient problems in one. one being 0<t<100us and the second being t>100us

14. imranmeah91

Did you get 9.91 uF using PSPICE?

15. imranmeah91

for 200 V

16. tdabboud

well for 200 V you could have any value capacitance because the voltage across the resistor will peak at 150 V.. so you will never have a value of capacitance that can make it go higher

17. imranmeah91

But isn't capacitor connected in parellel across the 300 V source?

18. tdabboud

|dw:1322624761701:dw|

19. tdabboud

the capacitor is parallel with the resistor....which is in series with the next resistor...which is in series with the voltage source

20. imranmeah91

oh, I was looking at it wrong

21. tdabboud

so at t=0, the 300V will drop to 150V after the first 2 ohm resistor, since the capacitor and resistor are in parallel they will both have a voltage drop of 150 V. So this is the max voltage that can be across the capacitor no matter what the value. once 100us comes the capacitor stops charging and then discharges as you can see from my sloppy grapg

22. imranmeah91

So , I am assuming at t=0 , \[I_c= infintiy\]

23. tdabboud

that cant be true

24. tdabboud

at t=0 the current through the resistor is not infinity so the current through the capacitor cannot be 0

25. tdabboud

sorry cannot be infinity

26. imranmeah91

oh, it should start at 75 amps?

27. tdabboud

i do not think so. I am not sure how you got that. are you converting to NICS?

28. imranmeah91

I don't know what to do. Try www.physicsforums.com , if you haven't already; They may be able to help you . Good luck

29. tdabboud

ok thanks I will try that! I too am an EE, junior, goodluck with your studies

30. imranmeah91

I am Sophomore,