(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Calculate the rise time (the time require for the output to go from 10% to 90% of its final, or steady state, value) for the circuit shown above when astep input of 5Vis applied for the following resistor values: 1k, 10k, 100k, 1000k with C equal to 0.01uF.

2. Relevant equations

I believe either

Vt = Vo * exp(-t/RC)

or

Vt = Vo * (1 - exp(-t/RC))

3. The attempt at a solution

For the sake of simplicity, I'll just show you my 10k Resistor calculations. I took that second equation (not even sure if I should be using that?) and for Vt I multiplied my source voltage (which should be the final voltage of the capacitor before discharging, right?) by 0.1 for 10% of the final which is 0.5 so:

0.5 = 5 * (1 - exp(-t/1E-4))

I then distributed the 5 and took the natural log of both sides so:

-0.693 = 1.61 - (-50000t)

-2.303 = 50000t

so t at 10% of the final = -46us

Now the fact that I got a negative number indicates that I did something incorrectly. I honestly have no clue what to do as the question seems vague as to how to answer it. If anyone can help I'd appreciate it a lot.

(also, as a note, for the first equation I got the number 14us at 10% of the final using .5 = 5(exp(-t/1E-4)). Don't think that's right either?)

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# Homework Help: RC Circuit rise time

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