# Pulse Response Homework Help

Discussion in 'Homework Help' started by testing12, Mar 19, 2011.

1. ### testing12 Thread Starter Member

Jan 30, 2011
80
2
Hello everyone,
I have yet another question reguarding pulse response:

I need some help determining i(infinity). I have tried mesh analysis for il and tried using VL = L di/dt but i got an answer as a function of the derrivative of u(t).
I have 2 problems with the solution:

1) why can we assume u(t) is 0 at t=0, its an unknown function.

2) how do they get i(infinity)

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
u(t) is often referred to a s the Heavyside function which is defined as

u(t)=0 for t<0 and u(t)=1 for t>=0

For i(∞) simply consider the inductor as a short and solve the circuit as if it were a purely DC circuit.

The problem you posted doesn't appear to explicitly state the value of Vs. Was this given in the problem somewhere?

3. ### testing12 Thread Starter Member

Jan 30, 2011
80
2
ok i got it, but i have some problems with it:

How can they assume that as iL(0) =0 and Vs(infinity) = 9 volts.
when u(t) is there, its like it was completly ignored, then added in the final answer.

4. ### testing12 Thread Starter Member

Jan 30, 2011
80
2
quick reply t n k, i posted at the same time as you

thank you sir!

NOTE: yes Vs was given in the problem to be 9u(t) volts and i was asked to solve Vo (t).

Thanks for the information reguarding u(t), i had no idea what that was!! I will think of it as the on/off switch at t=0.
note for student reading this: http://en.wikipedia.org/wiki/Heaviside_step_function
Very interesting!

final note: i should also mention the solution manual is slightly older then the edition of my text and my text has the solution as
vo(t) = 3e^(-4t) u(t) volts, which is what i got when i solved.

Last edited: Mar 19, 2011