solving circuit problem

[jak9]

hello friends,
i have attached a image with this post in which i wanted to find the value of Ri. It is a dc circuit but the input voltage and current have some ripples so its not a pure dc. I have also frequency of ripple which can be used further to calculate Xc if in case needed. So with all these inputs how to calculate Ri?
i know by calculating the voltage across each element Ri,C1 and Cb(the voltage Vcb is given) the value of Ri can be calculated. But i am not able to calculate the voltage drop across Ri.
please help
thanks

 

[mago]

is this a theoretical problem or a pratical one? You can try to put this in Laplace domain and minimize the circuit in 2 blocks.

.
You have mentioned that you know the frequency, so with some algebra you can find the voltage drop in Ri. You can also vary the frequency and see, for instance, how the system behave if there is no ripple, like using a DC source.
Of if you are not comfortable with Laplace, you can analyze this system using phasors.
I hope that this can help you.

 

[jak9]

thanks friend i know with frequency i can find Xc but to find voltage again i dont know the current....i tried alll algebra but not working

 

[sina_extreme]

hey friend
since there is a cap in series with the input the circuit doesnt have any constant dc current
so the input dc impedance is INF !
the only current in this circuit is caused by the small input noise which is neglect able

 

[jak9]

@sina_extreme...thanks friend but when you have pure DC then the impedance is inf because of zero frequency

 

[mago]

Hi jk9, i have a suggestion for you. First of all, you said that you know the voltages Vcb and the input voltage, as well as the input current ok? So, you know the impedance of the whole branch, between the input and Vcb. You can also determine the impedance Ci, it is a given value and you know the frequency... Now you have to unknown values: Ri, which is what you want to know and Rt. Maybe you shoud try to separte and solve this problem by superposition. First treat all the components as if the source was pure DC (easy one) . Then solve again just using the AC part (lnot so easy). Then during the AC part you can try to fit the transfer function with the voltage drop you already know. The sum all the solutions.

 

[sina_extreme]

@sina_extreme...thanks friend but when you have pure DC then the impedance is inf because of zero frequency

yes sure
that's another point of view but any way the point is that you cant have a constant DC current in this circuit
i think the simplest way to calculate Ri is to consider the response of the circuit at the early stages of transmission where the caps do not have enough juice
this way you can neglect Rt and the ratio of V and I is your Ri
of course this would help if you have the early values