# Working out Thevenin Equivalent

1. Apr 11, 2012

### eddysd

1. The problem statement, all variables and given/known data

Work out the Thevenin Equivalent of this circuit:

https://www.physicsforums.com/attachment.php?attachmentid=46120&stc=1&d=1334164751

3. The attempt at a solution

I worked out the Rth to be 300Ω, not sure if this is correct, and then I have no idea how to work out Vth

Any help would be much appreciated

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2. Apr 11, 2012

### Staff: Mentor

Hi eddysd. Can you show your work so far? How did you arrive at Rth = 300Ω ?

3. Apr 11, 2012

### phinds

I agree w/ gneill. Since the 300 is wrong, you need to show how you got it so we can help you understand where you went wrong.

4. Apr 11, 2012

### eddysd

I used 1/R = 1/R2 + 1/R3 and then added that to R1.

5. Apr 11, 2012

### phinds

and why are you taking r2 as parallel to r3 ???

why are you NOT taking r1 as parallel to r3 ???

Seems to me like you need to go back and study what parallel and series MEAN.

EDIT: actually, I may be wrong about that. Perhaps what you actually need is to go back and study what a Thevenin equivalent IS. What becomes an open circuit? What becomes a closed circuit? What are the steps you are supposed to follow to GET the equivalent circuit?

Last edited: Apr 11, 2012
6. Apr 11, 2012

### eddysd

I think the idea is that the load is on the right of the Vout and not actually shown in that image judging by the lecturers previous notes, I apologise for not mentioning this in the original post. The lecturer told me that you follow the current round, and when there is a choice of direction, that part of the circuit is parallel, would this not make R3 parallel with R2?

EDIT: Posted this before noticing your edit!

7. Apr 11, 2012

### Staff: Mentor

Ah. Remember that you want to find the equivalent resistance when all sources are suppressed and you are "looking into" the network from its output end:

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8. Apr 11, 2012

### phinds

gneill, just as a general note, I ASKED him what becomes open circuit and what becomes closed circuit, so that he would THINK about it.

You TOLD him what the answer is so he doesn't HAVE to think about it.

In general it's more help to someone in the long run if you get them to think, not feed them answers.

9. Apr 11, 2012

### eddysd

OK, so would I add R1 and R2 then take the parallel with R3? Even if I have worked out Rth, since I don't know the current how would I work out Vth?

10. Apr 11, 2012

### Staff: Mentor

Another way to judge whether two components are in parallel or in series (or neither) is as follows:

Two components are in parallel when they share exactly two nodes.

Two components are in series when they are the only components that share a single node.

11. Apr 11, 2012

### eddysd

Thanks, that is a much better way of looking at it than my lecturer's!

12. Apr 16, 2012

### eddysd

So now I have worked out the Thevenin equivalent resistance to be 270Ω, but I still don't know how to work out the equivalent voltage. This isn't actually for coursework or homework, but is revision for an exam, so I would be grateful if anyone could give me a step by step method to work it out.

13. Apr 16, 2012

### Staff: Mentor

Hello eddysd. Can you determine the value of Vout in your original circuit?

14. Apr 16, 2012

### eddysd

No, this is what I was trying to do, but can't think of a way without knowing the current

15. Apr 16, 2012

### Staff: Mentor

Assume a current I flowing around each closed loop. You know all voltage sources and resistances in the circuit, so you can form some equations.