Why Is My Thevenin Resistance Calculation Different from the Textbook's?

[Questions999]

Homework Statement

We have the circuit.http://i.imgur.com/X4IYi4j.jpg
Find the equivalent thevenin resistance ..the result in my book is 8 ohm but I alway get 8/6 ..how so?

Homework Equations

Thevenin Teorem

The Attempt at a Solution

1/Re=1/4+1/2 and so Re=8/6 ..

 

[gneill]

You'll have to show your work so we can see where you are going wrong. Is R4 part of the circuit for which you want to find the Thevenin equivalent, or does it represent the load?

 

[Questions999]

I don't calculate R4 because I have to use the Tevenin's transformation between the points a and b..to find equivalent resistance, I took the R3 and R2 in parallel and found 1/Re=1/R3+ 1/R2

 

[rude man]

I get R thevenin = 80/21 ohms. I included R4.

Without R4 I get R thevenin = 5 ohms.

 

[gneill]

I don't calculate R4 because I have to use the Tevenin's transformation between the points a and b..to find equivalent resistance, I took the R3 and R2 in parallel and found 1/Re=1/R3+ 1/R2

So R4 is not part of the circuit under analysis. Fine.

What makes you think that R2 and R3 are in parallel? Even if you could suppress the two current sources (which you can't because one of them is an active source), wouldn't that leave the resistors in series?

You'll have to use another method to find the Thevenin equivalent since you have an active source in the circuit.

 

[Questions999]

Rude man,how about without R4?
I they are in series,than the result is 6 ,still different from my book..

 

[gneill]

Rude man,how about without R4?
I they are in series,than the result is 6 ,still different from my book..

The controlled source is going to muck with the Thevenin resistance. You need another analysis method.

@rude man: I'm seeing the book's answer of 8Ω.

 

[rude man]

Rude man,how about without R4?
I they are in series,than the result is 6 ,still different from my book..

As gneill pointed out, you have to include the effects of the voltage-dependent voltage source. You can't just parallel resistors etc.

The way I do it is to apply 1V to the input, then compute the input current i using KVL or whatever, then R thevenin = 1/i.

 

[rude man]

The controlled source is going to muck with the Thevenin resistance. You need another analysis method.

@rude man: I'm seeing the book's answer of 8Ω.

Looks like I struck out on this one!

 

[gneill]

Looks like I struck out on this one!

Hey, you do good work. A small slip every now and then can't be avoided. Besides, they tend to rejuvenate concentration

 

[rude man]

Hey, you do good work. A small slip every now and then can't be avoided. Besides, they tend to rejuvenate concentration

Still bugs me. Want to send me your analysis privately?

 

[gneill]

Still bugs me. Want to send me your analysis privately?

Sure, I'll see if I can slip it into a PM.

 

[NascentOxygen]

Homework Statement

i'm disappointed that you appear to have given up on this question. I seem to have worked it out needlessly, then.

Perhaps someone else would like clues on how to do it? It's a good exam-type question.

 

[gary32]

The circuit isn't very clear but if there's 2 power sources then Kirchhoff's voltage law and the use of simultaneous equations would be helpful