# RC Circuit Problem

Discussion in 'Homework Help' started by ML90, Aug 6, 2010.

1. ### ML90 Thread Starter New Member

Jul 14, 2010
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In the attached image, I need to solve for the voltage equation of C2. I need to get an equation in terms of all other components. I don't know what method to use. R6 and R7 are in series and can be added, but I would prefer to keep them separate for when I plot the equation with different values for all the variables. Please start me off with a method. Thanks!

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2. ### hgmjr Moderator

Jan 28, 2005
9,030
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The way I would begin the solution to this problem would be to redraw the circuit into a more familiar form such the one I have attached.

hgmjr

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3. ### ML90 Thread Starter New Member

Jul 14, 2010
4
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Thanks for the help, but how would you calculate the impedance of the capacitors? In an AC system, the impedance of a capacitor is -j/(wC). This is a DC system though.

4. ### Ghar Active Member

Mar 8, 2010
655
72
Are you looking for a DC solution, a time-domain step response, a transfer function, what exactly?

If you just want a DC solution the capacitors don't do anything and they're open circuit, you just calculate the voltage across the resistors.

If you want a step response you can either write differential equations and solve them or use a transform (using impedances) and then go back to the time domain.

5. ### ML90 Thread Starter New Member

Jul 14, 2010
4
0
Thanks for the replies. I want the step response. But how would I use a transform with the impedances? There is no frequency. Please help start me off. Thanks guys!

6. ### Ghar Active Member

Mar 8, 2010
655
72
Since this is homework you must have had a lesson at some point about finding a step response.
Surely they taught you at least one of the methods?

7. ### Potato Pudding Well-Known Member

Jun 11, 2010
684
92
I would start with the conditions at turn on when Capacitors are shortcircuits. That will simplify things for the initial calculations. Then you charge the capacitors from that state and in the next steps you need to carry their acquired voltage from the previous steps.

As a hint - until C5 acquires some charge and voltage it is just the 2 sources and their resistances that will have electrical activity. Once you have voltage across the C5 nodes that adds a second equation of Voltage that conducts into C3. That adds another equation that discharges through R1.

I really should try and practice my calculus. I decided to go back to school in September and the next month should be used to recover some of wha little I knew and have mostly forgotten.

These will be some of the work on Δv, Δi and then you can integrate and get your stepwise.