Capacitor and inductor calculations?

Thread Starter


Joined Aug 24, 2007
Hello, i was wondering if any one knew a calculation to determine the rise time of a current from a capacitor? also for inductors aswell?

like how long after the capacitor is switched on will attain peak voltage and amperage. again also for the inductor aswell, lol

I found this equation for inductor's

(uh \ voltage) * Peak amperage

is this correct?

also I found these capacitors by just doing a google search and they all have insanely small inductance and small farads to, but I do not know if the equation I used for inductor's would work for the capacitor's.

By the way when doing the Time constant equations here

how do you find the time like is us or ns or 1 second?
it probably is something simple that I just don't know.


Joined Apr 7, 2010
Have you looked at volumes 1 and 2 of the e-books on this site? More than you'll ever want to know.


Joined Feb 24, 2006
You take the solution to the differential equation that gives voltage or current as a function of time. Then by algebra you solve for time as a function of voltage.

For example if the input voltage is a step function and you want the time to 90% of the step input you can easily solve for the time.


Joined May 30, 2010
Uh huh, expect to study harder if you want to get your degree, Wes. Not to be condescending... it's just experience talking ;).

Have you dealt with differential equations before? Or better yet - have you learnt about laplace transforms yet?

Thread Starter


Joined Aug 24, 2007
lol, No , I have no idea what Laplace transform is.
I know what differential equation is though (I think, lol). The hardest thing for me when I see some complex math problems is that I have no idea what the different letter's mean and little symbol's to, If I had a book with like all the different formulas and what all the different letter's and symbols mean then it would be so much easier to figure this stuff out, lol.


Joined Apr 24, 2007
It depends on what circuit they are in, even if it's just a single resistor with one of them. e.g. A larger parallel R will make a given capacitor discharge more slowly.

Basic v/i equations for C and L: i = C(dv/dt) and v = L(di/dt)

For a simple circuit, define each current and voltage and then use one of Kirchoff's Laws to write the differential equation(s). (Kirchoff: 1. Voltages around each loop sum to zero. 2. Currents into each node sum to zero.)

The differential equation's solution is the current or voltage as a function of time.

The "Laplace Tranform" converts an ordinary differential equation into (gasp!) an ALGEBRAIC equation. Solve algebraically (easier!). Use "Inverse Laplace Transfrorm" on algebraic solution: Have solution of diff. eq. as function of time (or whatever it was about)!

You can DO it! It's not as much about being "smart" as it is about not giving up. i.e. You have to spend a lot of TIME, trying to learn and understand, and not giving up until you've really got it. (Learning everything in the proper sequence will help a whole lot, too.)