# time constant

#### gani

Joined May 15, 2005
46
hi
oooooops! i posted my query in the wrong forum..
sorry abt that.
can some one help me?when lc is coontd across dc or ac there is energy transfer btn L and C,right?
but this is only for the transient period.so how do we caluculate how long the transient period is?
hoping to hear from u guys.Thanx!

Joined Jan 19, 2004
220
when u caoonect a cap and inductor across dc the cap stores energy when given by .5*C*V^2
and the inductor stores energy given by .5*L*i^2

now while chargin V=Vo(1-e^(t/RC))
and i=Vo(1-e^tRL)/R

pluggin these values into the energy exp we get energy stored in the caps and inductors as a fn of time

energy is continually stored in the reactive devices however a majority of it is done with a few time constants

#### gani

Joined May 15, 2005
46
Originally posted by haditya@May 18 2005, 01:09 AM
when u caoonect a cap and inductor across dc the cap stores energy when given by .5*C*V^2
and the inductor stores energy given by .5*L*i^2

now while chargin V=Vo(1-e^(t/RC))
and i=Vo(1-e^tRL)/R
pluggin these values into the energy exp we get energy stored in the caps and inductors as a fn of time

energy is continually stored in the reactive devices however a majority of it is done with a few time constants
[post=7769]Quoted post[/post]​

yes i get that part abt the energy being stored in L and C.energy gets transferred btn l and C.but this is for only some period of time....the transient period.am i right?hmm... what i don get is how long is this transient period...?

#### hgmjr

Joined Jan 28, 2005
9,029
Gani,

Which case are you most interested in, under-damped, critically damped, or over-damped?

hgmjr

#### gani

Joined May 15, 2005
46
THANX FOR THE REPLY HGMJR, IF U DON MIND....CUD U TELL ME ABT ALL.
THANX!!!

#### hgmjr

Joined Jan 28, 2005
9,029
Originally posted by gani@May 18 2005, 09:01 PM
THANX FOR THE REPLY HGMJR, IF U DON MIND....CUD U TELL ME ABT ALL.
THANX!!!
[post=7795]Quoted post[/post]​
Gani,

Take a look at this write-up and let me know if you have questions.

You will see that I have described the general technique I used to get the decay time constant for the under-damped case (k<1).

I thought I would give you a chance to work through the process for yourself before I go into greater detail. You should be able to find the time-domain expressions for all three cases in a textbook or on the Internet. With these expressions you can use the approach I have decribed to confirm my results for the decay time-constants for all three cases.

Let me know if you need further assistance.

hgmjr

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