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

[orangeincup]

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

 

[phinds]

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?

 

[orangeincup]

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...

 

[phinds]

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.

 

[gneill]

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 V_th node equation.

 

[orangeincup]

Here's what my book says for Rth, they calculated 12||20 +2.5, and ended up with 5kΩ

 

[phinds]

Exactly. Why did this make you think they ignored the 10K resistor? They obviously did not

 

[orangeincup]

Why is it not in the equation though? It is because it's parallel to a zero resistor?

 

[phinds]

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?