# RC Circuit from Voltage equation

Discussion in 'Electronics Homework Help' started by Thelair, Mar 2, 2013.

1. ### Thelair

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0
Mar 2, 2013
Hey there, I've got a little problem with my electronics homework and I can't really find any hints over the internet nor in my notes.
We were given a table showing time and voltage values and we are to create a proper one-capacitor circuit from it.
So far I managed to create an equation showing the voltage in given t-time:
U(t) = 1.84 + 5.765 * e (-t/T)
Also, during the process, I got few variables which may be useful later:
T = 2,8s
U(5T) = 1.84V (5T = 14s)
Uinput = 10V

So far it was somehow clean math, but the question is how to transform voltage equation into a single-capacitor-single-resistor circuit.
I don't really want an answer, rather a hint, I really like that "process of creation", but I tried everything I know ( t-to-s domain transformation, time-dependant circuit builds, math ) but I get too many unknowns as I calculate and it goes to nowhere so far.

I hope someone can help me with this one a bit

Last edited: Mar 3, 2013
2. ### Laplace

1,252
184
Apr 4, 2010
Compare the given discharge equation to a common RC discharge equation. How does the variable 'T' compare to the RC time constant 'tau'? There are many different values of R and C that will give the required value for 'tau'.

3. ### Thelair

3
0
Mar 2, 2013
T is the time constant, same as tau, just easier to write on the forum
I'm gonna go and look for this common discharge equation, I think I have seen it somewhere, maybe it will be much more useful than I thought.

Last edited: Mar 3, 2013
4. ### Thelair

3
0
Mar 2, 2013
Ok, so far I noticed that/and why whole excersise is about discharging ( I've drawn the function ). And I've noticed that after being discharged the capacitor leaves 1,84V inside the circuit. I therefor belive that I should start with such circuit:

I did some calculations and the result shown that R1 = 4,434R2.
Now I think I'll try to fit a capacitor on the line between voltmeter node-clips.
Am I going the right path?