Working out Thevenin Equivalent

[eddysd]

Homework Statement

Work out the Thevenin Equivalent of this circuit:

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

The Attempt at a Solution

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

Any help would be much appreciated

 

[gneill]

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

 

[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.

 

[eddysd]

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

 

[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?

 

[eddysd]

I think the idea is that the load is on the right of the V_out 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!

 

[gneill]

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

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:

 

[phinds]

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:

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.

 

[eddysd]

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

 

[gneill]

I think the idea is that the load is on the right of the V_out 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?

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.

 

[eddysd]

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.

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

 

[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.

 

[NascentOxygen]

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

 

[eddysd]

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

 

[NascentOxygen]

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.