What Are the Correct Steps to Determine the Thevenin Equivalent Circuit?

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In summary, the conversation is about finding the Thevenin equivalent circuit with respect to terminals a and b. The first attempt at a solution involves setting a short circuit across 72V and calculating Rth to be 6Ω. The person then asks a question about not being able to use this method in all problems. The responder suggests that this method should work unless there is a weird controlled source. The conversation then shifts to using the node voltage method to calculate Vth. The person shares part of their equation and asks for help with the next step. The responder points out that the 12Ω branch needs to be included in the equation for the Vth node. The person then mentions what their book says about Rth,
  • #1
orangeincup
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Homework Statement


Find the Thevenin equivalent circuit with respect to terminals a, b


Homework Equations


i1+i2+i3..=0
v1+v2+v3..=0
i=v/R


The Attempt at a Solution


I set a short circuit across 72V, and calculated that Rth is 6Ω. My first question is, how come I can't always do this? I get problems where I have to calculate Rth a completely different way. How can I tell when this is acceptable to do?

Now I'm using the node voltage method to calculate Vth

So I did part of my equation, which is:
(v1-72)/5+v1/20+(v1-Vth)/8=0
(Vth-v1)/8-...
But I'm not sure what I use next
 

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  • #2
orangeincup said:
My first question is, how come I can't always do this? I get problems where I have to calculate Rth a completely different way. How can I tell when this is acceptable to do?

Hm ... offhand, I don't remember this ever being an issue unless there is some kind of weird controlled source. Can you give an example of such a problem?
 
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  • #3
phinds said:
Hm ... offhand, I don't remember this ever being an issue unless there is some kind of weird controlled source. Can you give an example of such a problem?

I meant ones with controlled sources, I don't know how to solve those. Also ones like this.

It works if I pretend the 10kΩ isn't part of the circuit, but I'm not sure why that is...
 

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  • #4
If you pretend that the 10K is not part of the circuit, you cannot possibly get the right Rth, so I suggest your book has a wrong answer or something else is misleading you.
 
  • #5
orangeincup said:
Now I'm using the node voltage method to calculate Vth

So I did part of my equation, which is:
(v1-72)/5+v1/20+(v1-Vth)/8=0
(Vth-v1)/8-...
But I'm not sure what I use next

You need to include the 12Ω branch for the Vth node equation.
 
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  • #6
Here's what my book says for Rth, they calculated 12||20 +2.5, and ended up with 5kΩ
 

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  • #7
Exactly. Why did this make you think they ignored the 10K resistor? They obviously did not
 
  • #8
Why is it not in the equation though? It is because it's parallel to a zero resistor?
 
  • #9
orangeincup said:
Why is it not in the equation though? It is because it's parallel to a zero resistor?

Reread post #4. Forget what the book says. Do it right and you'll get the right answer (which is NOT the answer they got, 'cause they didn't do it right).

Why would you think it is parallel with a zero ohm resistor? WHAT zero ohm resistor?
 

Related to What Are the Correct Steps to Determine the Thevenin Equivalent Circuit?

What is Thevenin's Equivalent?

Thevenin's Equivalent is a method used to simplify complex electrical networks into a single equivalent circuit. It allows for easier analysis and calculations of the network's behavior.

How is Thevenin's Equivalent determined?

Thevenin's Equivalent is determined by finding the open circuit voltage and the internal resistance of the network. The open circuit voltage is the voltage across the output terminals when no load is connected, and the internal resistance is the resistance seen looking into the output terminals.

What is the purpose of finding Thevenin's Equivalent?

The purpose of finding Thevenin's Equivalent is to simplify complicated networks into a single circuit that behaves the same as the original network. This allows for easier analysis and calculations, and also helps in designing and troubleshooting electrical systems.

What is the difference between Thevenin's Equivalent and Norton's Equivalent?

Thevenin's Equivalent is a method of simplifying a network into a single voltage source and a single resistor, while Norton's Equivalent is a method of simplifying a network into a single current source and a single resistor. Both methods are based on the same principles, but the output parameters are different.

When is Thevenin's Equivalent most useful?

Thevenin's Equivalent is most useful when analyzing linear networks with multiple components and sources. It allows for easier calculations and predictions of the network's behavior, and can also be used to determine the maximum power that can be delivered to a load from the network.

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