# Thevenin Theorem problem

Discussion in 'Homework Help' started by Cantafford, Jan 4, 2014.

1. ### Cantafford Thread Starter New Member

Jan 4, 2014
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The problem says: For the given circuit apply the Thevenin's theorem to find the equivalent circuit of the highlighted area(I assume they refer to the white colored area between a and b.

I don't understand how can I do this. I have watched a few tutorials and learnt how to use the Thevenin theorem to create an equivalent circuit for a given one. But here is asking me to do it only for the highlighted area. How to do this?

2. ### studiot AAC Fanatic!

Nov 9, 2007
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515
Are the pages in your book blue?

Or is the highlighted area the blue bit on a white page?

What do you understand Thevenin to tell you, do you need help with that?

3. ### Cantafford Thread Starter New Member

Jan 4, 2014
9
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The circuit is blue. On the circuit there is a white highlighted area as you can see.
The problem says use Thevenin to determine the equivalent circuit of the colored area(white) and I don't know how to do this.
I do know how to determine the equivalent circuit for the whole circuit but that's not what the problem is asking me. It got me very confused.

4. ### Ramussons Active Member

May 3, 2013
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92
Something's misunderstood. Can you give a colour snapshot of the whole page?

Ramesh

5. ### Cantafford Thread Starter New Member

Jan 4, 2014
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In the circuit that I have provided I have to calculate the voltage through the R4 resistance with Thevenin. Here's what I have done. Please tell me if it's good, thank you.

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6. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Your method is incorrect. Presumably the problem solution would involve finding the Thevenin equivalent looking into the network at the points (a & b nodes) when R4 is removed from the circuit. R4 is then applied to the Thevenin equivalent terminals to find the required voltage across R4.
In deriving the Thevenin resistance you correctly show R3 as shorted by the source. But the equivalent resistance seen at terminals a & b is not what you have derived.

Last edited: Jan 4, 2014
7. ### shteii01 AAC Fanatic!

Feb 19, 2010
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To begin with, R4 and R1 are in parallel. That means you can switch them places. Now R4 is the very last resistor when you go from right to left, this is where load resistor is typically placed when deriving Thevenin equivalent circuit.

8. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
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OK if the first part of my quote is the actual wording of your problem I would agree that it is not entirely clear.

However it does not say what you apply Thevenin's Theorem to.
In particular it does not tell you to apply the theorem to the resistor between the nodes a and b , it tells you to 'apply Thevenin's theorem to find.....'

Now I take it that you must apply the theorem to the remainder of the circuit connected to a and b to find the current in R4, which was the reason I asked a question you did not answer in the second part of my post#2.

The point is that Thevenins theorem replaces the entire rest of the circuit with a single voltage source in series with a single resistance placed across ab.

So once you have performed your Thevenin reduction, you can easily calculate the current in R4.

9. ### WBahn Moderator

Mar 31, 2012
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4,917
Are all of the circuit problems in the book shown on top of a solid blue background? Or are most of them just on a white background?

Look for other problems that have both blue and white regions and for which the problem talks about "the highlighted area"? Are they talking about the white region or the blue region?

If you can't find any, but most problems are on a white background, then imagine this problem starting out on a white background and someone using blue to highlight everything except the one resistor. I'm pretty sure this is what was intended.

BTW: The Thevenin equivalent circuit for portion of the circuit consisting of just R4 is trivial. What value of a voltage source in series with what value of a resistor will behave the same as R4?

10. ### Cantafford Thread Starter New Member

Jan 4, 2014
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0
Yes that is right! My english is not the best so sorry for not beiing able to explain it that well. I tried to solve this way(attachment) but seems it wasn't fine. I'm going to try again with the instruction the people aboved have given me.
PS: It is got me very confused when I have to calculate Vo(Output voltage) because I cannot use voltage divider as I do not have at least 2 resistances in parallel.

• ###### problema.jpg
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11. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Voltage divider is two resistors in series.

Current divider is two resistors in parallel.

12. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
I think a big part of your problem is that you are relying at throwing equations and formulas at the problem whether you understand them or not rather than taking the time to understand the concepts behind circuit analysis.

Each time you redraw a circuit, make it clear what the quantity is that you are solving for. In your first redrawn circuit, you are trying to find the resistance between points 'a' and 'b', yet you don't include 'a' and 'b' in the diagram. If you had, you would see that, from points 'a' and 'b', R1 and R2 are NOT in series. That's not the only mistake you made as a result, either.

13. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
Here is one way (there are many) that you might use Thevenin to solve your problem.

I have not done the numbers, that is your job.

I have broken it down into what I hope are easily digestible steps.

Remember we need two pieces of information
The Thevenin Voltage, Vt
The Thevenin Resistance Rt
These are calculated separately

Note you do not include R4 in the circuit when you calculate either Vt or Rt, as you tried to do in your posted working.

Step1

The original circuit.

Step2

Rearrange the circuit to its most convenient form.
Shteii01 was quite right here, about swopping R1 and R4
I have also swopped the voltage source and another resistor.

Step3

Break the circuit between a and b by removing R4 from the circuit.
This creates an open circuit condition between a and b through the R4path , (but not throgh the rest of the circuit).
Because the Thevenin voltage across a and b is the same whatever the resistance between a and b is (ie whatever the value of R4) if we let R4 be infinity or open circuit we can calculate the voltage between a and b and this is the thevenin voltage Vt

Step 4

Calculate Vt from V1 and the circuit.
I have again rearranged the remainder of the circuit without R4 to show one easy way to calculate Vt.
There are many other ways to calculate Vt the best depends upon the actual circuit.

Step 5

Replace all voltage sources by a short circuit and all current sources by an open circuit so there are only resistors left.
Calculate the resistance as seen between a and b by your preferred method.

You should notice something strange about Rt, this is perfectly OK. Can you say why this is?
This is what another poster meant earlier in the thread.

Step6

Form the Thevenin circuit and calculate the Thevenin current through R4.

It becones easier when you have done a few.

• ###### thev1.jpg
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14. ### Cantafford Thread Starter New Member

Jan 4, 2014
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Thank you! It was the R4 that got me confused.