# RLL Circuit

Discussion in 'Homework Help' started by ameriland, Sep 24, 2008.

1. ### ameriland Thread Starter New Member

Sep 24, 2008
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Alright this is the circuit we're given...

a)"Determine the differential equation that relates the input current x(t) to output current y(t)."

b) Find the characteristic equation for this circuit and express the roots of the characteristic equation in terms of L1, L2, and R.

c) Determine the zero-input response given initial inductor currents of 1 amp each. That is, find yo(t) given iL1(0) = iL2(0) = 1 A.

This is the only problem on my homework that I cannot figure out... I just need some help getting a starting point. Thanks!

2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Did you intend to attach a diagram? If not, it would be helpful if you would do that.

hgmjr

Sep 24, 2008
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4. ### hgmjr Moderator

Jan 28, 2005
9,030
214
You are correct ameriland. For some reason, I was initially unable to view it but now I can see it just fine.

hgmjr

5. ### SIcam Active Member

Aug 9, 2008
61
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Its been a while but this is what I would do.

1. From left to right ignore everything and reassign them basic a, b, c, d.
2. Do Kirchoff's law for current and voltage with the new letters.
3. Take each letter and convert it back into current or voltage.

Example of voltage across an inductor V = Lc di/dt.
(knowing this you can rearrange and integrate to make the equation for current through an inductor. (Instantanious charging/instantaneous discharging)
http://www.ecelab.com/inductance-equations.htm

You will need to do the same for the current.

Here is a series problem solved that helps to understand were I a going with this.

http://commons.bcit.ca/math/examples/elex/differential_eqns/index.html

Hope this helps.

Apr 19, 2008
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7. ### ameriland Thread Starter New Member

Sep 24, 2008
3
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well yeah... apparently my course is using the same book as you and I found your post, but no one ever really answered it... Did you ever figure it out?

8. ### silvrstring Active Member

Mar 27, 2008
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I remember seeing this. I believe Ratch had helped Vanush with it. Yeah, I just checked. Part of the problem was a lack of info.

There needs to be some clarification as to the source--does your problem use a sinusoidal current source or just x(t)?

9. ### Vanush Active Member

Apr 19, 2008
46
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The question is supposed to be solved in the time domain