understanding thevenin theorem

[PG1995]

Hi

I'm trying to understand Thevenin's thoerem through an example problem. Given below are two scans of the example problem.

1: The author says: Because of the presence of the dependent source, however, we excite the network with a voltage source Vo...

Okay, what would the author have done if there were no dependent source?

2: Why does the author choose subscripts "o" and "oc" in Vo and Voc when the author is concerned about the terminals "a" and "b" of the circuit? Why didn't the author choose more meaningful subscripts?

3: I1 = 1/6 A, I2 = -1/18 A, I3 = -1/6 A

The above currents, I1, I2, and I3, are for three loops. The circuit consists of three loops, right? The current supplied by the 1V source is assumed to be Io. This is the current being fed to the entire circuit. The authors then says Io = -I3. Why? The I3 is the current only in ONE loop which is only one part of the circuit, then how can we equate something such as Io which is being fed into the entire circuit to something which is only being fed in a part of that entire circuit?

4: The author says: 4(I1 - I2) = Vx.
Is this correct? Previously, the author has said: -4I2 = Vx = I1 - I2.

Please help me with the above queries. It would be really kind and nice of you. Thank you.

Scans
1: part one of the example:
https://img197.imageshack.us/img197/5708/theveninpart1.jpg
2: part two of the example
https://img709.imageshack.us/img709/3244/theveninpart2.jpg

 

[Ian Rogers]

All I know is Io has to equal I1+I2+I3 Thevenin said all currents flowing into / out off a node in a circiut add up to 0A. so the answer should be I2! -1/18A as the other two cancel out.

Ian

Sorry about the rest but it's been a while since I did these.

 

[Ratchit]

PG1995 ,

1: The author says: Because of the presence of the dependent source, however, we excite the network with a voltage source Vo...

Okay, what would the author have done if there were no dependent source?

The same thing really, only he would have been able to do it directly by inspection. For instance, if you have two parallel resistors, you can easily find their combined resistance by a cut and dried formula. Or you can mathematically apply 1 voltage across each of them and calculate the total current. Then divide the current into 1 volt to get to parallel resistance. Dependent sources are not so easy to figure out by inspection, so a loop calculation was done to obtain the correct value.

Why does the author choose subscripts "o" and "oc" in Vo and Voc when the author is concerned about the terminals "a" and "b" of the circuit? Why didn't the author choose more meaningful subscripts?

The subscripts mean Vo = voltage output, and Voc = voltage open circuit. In one case he is driving Vab with a 1 volt source and in the other case he is calculating what the Vab voltage will be from the 5 amp independent source and dependent voltage source.

I1 = 1/6 A, I2 = -1/18 A, I3 = -1/6 A
The above currents, I1, I2, and I3, are for three loops. The circuit consists of three loops, right? The current supplied by the 1V source is assumed to be Io. This is the current being fed to the entire circuit. The authors then says Io = -I3. Why? The I3 is the current only in ONE loop which is only one part of the circuit, then how can we equate something such as Io which is being fed into the entire circuit to something which is only being fed in a part of that entire circuit?

Yes, 3 loops.

All three loops are connected to each other, so any change in one loop will affect the other loops. The dependent voltage source of loop 1 is affected by the current in loop 2. The current in loop 2 is affected by the current in loop 3, which in turn is affected by Vo. Io is calculated and divided into Vo to get the Thevinen resistance.

Look at Fig. 4.32a . i3 is assumed to exist in a clockwise direction, and Io is assume to exist northwards. Those are opposite directions so there is a sign change when they are equated.

The author says: 4(I1 - I2) = Vx.
Is this correct? Previously, the author has said: -4I2 = Vx = I1 - I2.

Yes, he is correct. You are comparing Vx being driven by Vab = 1 volt in Fig. 4.32a to Vx during the open circuit Vab driven by the 5 amp independent source in Fig. 4.32b. That's a no-no. Look at the loop equations (4.9.5) to see how he derives Vx.

Please help me with the above queries. It would be really kind and nice of you. Thank you.

You are welcome. Now, be good enough to tell me the title and author of the book you scanned.

Ratch

 

[PG1995]

Now, be good enough to tell me the title and author of the book you scanned.

Hi Ratch

Thank you very much. It's Fundamental of Electric Circuits - Alexander Sadiku.

 

[PG1995]

Hi again,

I'm trying to understand how a circuit is thevenized practically. Please have a look on the attachment. When the switch is pressed one can measure the voltage on the terminals 'A' and 'B' by connecting a voltmeter. In theory it is said to short the voltage supply and open the current source. I believe in this case it would mean that disconnect the battery's terminals from the rest of the circuit and connect the points 'a' and 'b' with a wire. Then, connect an ohmmeter to the terminals 'A' and 'B' to note the equivalent resistance. Am I thinking along the same lines? Please let me know. Thanks a lot.

Regards
PG

 

[Ratchit]

PG,

There are terabytes of information on applying the Thevenin method. Nobody wants to explain it all over again.

Ratch

 

[PG1995]

PG,

There are terabytes of information on applying the Thevenin method. Nobody wants to explain it all over again.

Ratch

Hi Ratch

I know how to apply Thevenin theorem theoretically. I was trying to understand how it works in reality. By the way, the reason for there exists terabytes of information is that we all learn in different ways! Thousands books of a library cannot substitute for a teacher or knowledgeable persons like you and many others.

Thank you for helping me so many problems.

Best wishes
PG

 

[Ratchit]

PG,

All right, submit some nontrivial problems so we can show you. Try to include independent sources, dependent sources, and a mix of the two. Again, don't make them so easy that they can be done by inspection.

Ratch