RL Circuit Help

Discussion in 'Homework Help' started by Digit0001, Mar 24, 2011.

Mar 28, 2010
89
0
Can someone help me solve this RL Circuit. My problem is how would i get the Resistance value?

I simplify the circuit using thevenins theorem and then i calculated two equations when i have two meshes, however this did not work.

P.S

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2. Georacer Moderator

Nov 25, 2009
5,175
1,284
Care to post your equations? We can help you better if we can see where you go wrong.

Also are you sure you want to short-circuit that inductor? It's ok if you want to study the DC state of the circuit, but if you want to find the transients, you aren't allowed to do it.

Mar 28, 2010
89
0
i didn't bother posting up the equations because even if i solved them it gave me zero.

Here it is anyways..

(1)
3I+I-I1-2(3I) = 0
-2I-I1 = 0

(2)
5I1-2(3I1)+I1-I = 0
-I = 0

Well i am trying to find the resistance so then i can find the time constant τ

where it is τ = L/R.

4. t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
The equations are incorrect. The problem requires a transient analysis method - either in the time domain by the use of differential equations or in the 's' domain by the use of Laplace Transform.

The Laplace transform approach is simpler, provided you are acquainted with the method - particularly with respect to the handling of initial conditions.

Mar 28, 2010
89
0
can someone tell me if the equations are correct. If they what should be the next step in solving?

V = 1/6 di/dt + 3i

(V-2x) + V/5 + i = 0

6. t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790

V-2Vx + V/5 + i =0

You need a third relationship, namely

Vx=-3i

With your three equations you can then eliminate V to give a first order differential equation in the variable 'i'.

Re-arrange the top equation in this post to give

V(1+1/5)=2Vx-i=2(-3i)-i=-7i

So

V= -35i/6

Equate this with the top equation in your last post, viz

V = 1/6 di/dt + 3i

Hence,

-35i/6=1/6 di/dt + 3i

Then re-arrange and solve your differential equation ... which I'll leave to you.

The necessary boundary condition will be

i(0)=5A