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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

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  1. imranmeah91
    • 3 years ago
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    no worry, ee here

  2. tdabboud
    • 3 years ago
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    thank god, I have been working on this problem for 3 hours now

  3. tdabboud
    • 3 years ago
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    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
    • 3 years ago
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    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
    • 3 years ago
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    yes that is correct

  6. imranmeah91
    • 3 years ago
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    It's a DC , right?

  7. tdabboud
    • 3 years ago
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    yes dc

  8. tdabboud
    • 3 years ago
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    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
    • 3 years ago
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    Well we have to consider phase lag if it is AC

  10. tdabboud
    • 3 years ago
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    okay that makes sense

  11. imranmeah91
    • 3 years ago
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    This is DC Parellel RC circuit First time for me

  12. imranmeah91
    • 3 years ago
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    I got some Differential equations that might work, hopefully

  13. tdabboud
    • 3 years ago
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    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
    • 3 years ago
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    Did you get 9.91 uF using PSPICE?

  15. imranmeah91
    • 3 years ago
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    for 200 V

  16. tdabboud
    • 3 years ago
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    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
    • 3 years ago
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    But isn't capacitor connected in parellel across the 300 V source?

  18. tdabboud
    • 3 years ago
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    |dw:1322624761701:dw|

  19. tdabboud
    • 3 years ago
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    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
    • 3 years ago
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    oh, I was looking at it wrong

  21. tdabboud
    • 3 years ago
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    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
    • 3 years ago
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    So , I am assuming at t=0 , \[I_c= infintiy\]

  23. tdabboud
    • 3 years ago
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    that cant be true

  24. tdabboud
    • 3 years ago
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    at t=0 the current through the resistor is not infinity so the current through the capacitor cannot be 0

  25. tdabboud
    • 3 years ago
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    sorry cannot be infinity

  26. imranmeah91
    • 3 years ago
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    oh, it should start at 75 amps?

  27. tdabboud
    • 3 years ago
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    i do not think so. I am not sure how you got that. are you converting to NICS?

  28. imranmeah91
    • 3 years ago
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    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
    • 3 years ago
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    ok thanks I will try that! I too am an EE, junior, goodluck with your studies

  30. imranmeah91
    • 3 years ago
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    I am Sophomore,

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