Thevenin resistance question

[jeffy]

Homework Statement

Hi,

I had a question about how my textbook finds the thevenin resistance (Rth) for this question

I have already solved the thevenin voltage(3V) and I have a question about finding the Rth

In the solution using a test current source of 1 A and I have been taught to short circuit
the 4V independent source and find the voltage across the 1A test source

Howver In the solution of the problem,

The author has kept the 4V independent source instead of shorting it

I was wondering why he/she decided not to short the 4V source


thanks

 

[vela]

The method of setting the sources to 0 to find the equivalent resistance works when the circuit contains only independent sources. Since the circuit contains a dependent current source, you have to use another method, like applying a test voltage or current, and with this method, you leave the sources alone.

 

[jeffy]

thanks for the reply
im sorry to bother you but in a previous question the authour short circuited an independent voltage source in a circuit with both dependent and independent sources when finding the thevenin resistance of the circuit, do you mind explaining why they shorted the independent voltage source for the following question

question:

solution:

I was wondering why in one case the author shortcircuited the independent voltage source and in the other case he kept the source

thanks

 

[vela]

Good question. I'm not sure now. I have dueling sources:

http://www.swarthmore.edu/NatSci/echeeve1/Ref/E72WhaKnow/WhaKnow.html#Thevenin Theorem
http://www.ces.clemson.edu/ge/staff/park/Class/ECE202/Handouts/CircuitAnalysis.html

Depending on which method I use, I get either +16.7 ohms or -16.7 ohms for the Thevenin resistance.

 

[DeeNos]

I would like to first clarify that the Thevenin resistance (Rth) is the equivalent resistance of a circuit when all voltage sources are replaced by short circuits and all current sources are replaced by open circuits. This means that the Thevenin resistance is the resistance that a load would see when connected to the circuit.

Now, regarding your question about finding the Rth, there are two methods commonly used - the voltage divider method and the test current method. The voltage divider method involves calculating the equivalent resistance of the circuit by dividing the voltage source by the total resistance. On the other hand, the test current method involves using a test current source and finding the voltage across it to determine the Thevenin resistance.

In the solution you mentioned, the author has chosen to use the test current method. When using this method, it is important to note that the 4V independent source cannot be shorted as it is a part of the circuit and will affect the current flow. Instead, the test current source is connected in parallel to the 4V source. This allows for the calculation of the Thevenin resistance by finding the voltage across the test current source.

I hope this explanation helps clarify why the 4V independent source cannot be shorted in this method. It is important to understand both methods and when to use them depending on the circuit at hand.