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equal charge between capacitors connected in series (Read 1537 times)
jjnkxb
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equal charge between capacitors connected in series
Sep 13th, 2017, 8:55am
 
I have a question regarding equal charge between two capacitors in series. The charge voltage relationship is Q1=f(V1) and Q2=f(V2). f is the function which describes the complex behavior of the capacitor. I want to force the Q1=Q2, not only that the current is equal, since the initial condition is different for the two capacitors.

One solution I am thinking is use an auxiliary terminal whose voltage is equal to the charge. In circuit simulation, I connect the two auxiliary terminals so that the charge is equal. But one problem with this is the loop formed between the auxiliary terminal and ground. The spectre simulator stops due to this loop.

Do you have any good suggestion on this?

Thanks.
Best,
Kai
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Geoffrey_Coram
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Re: equal charge between capacitors connected in series
Reply #1 - Sep 20th, 2017, 5:55am
 
Is the f() the same for both capacitors (as you wrote it), or is it Q1=f1(V1) and Q2=f2(V2)?
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jjnkxb
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Re: equal charge between capacitors connected in series
Reply #2 - Sep 20th, 2017, 8:00am
 
Thank you for your reply Geoffrey. The function is different for each capacitor. I am thinking to solve the equation by iteration. I just want to know if there is any other solution.

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Kai
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Geoffrey_Coram
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Re: equal charge between capacitors connected in series
Reply #3 - Sep 20th, 2017, 8:53am
 
I'm not really sure what you're trying to do. Do you want to force Q1=Q2 for simulation? Do you want to build a circuit that will drive Q2 to the same value as Q1? What sets the initial condition? Usually in Spice, Q1(0)=Q2(0) = 0, although I guess some simulators put a GMIN to ground on the node between the caps, to provide a dc path to ground. Physically, I don't think you can measure how much charge is on the capacitor, other than by integrating the current from a known initial condition.
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