Finding Thevenin Equivalent Voltage for a Circuit with Multiple Voltage Sources

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Discussion Overview

The discussion revolves around finding the Thevenin equivalent voltage for a circuit with multiple voltage sources. Participants explore various methods and approaches to solve the problem, including nodal analysis and the maximum power transfer theorem. The conversation includes both conceptual understanding and technical calculations.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants express difficulty in finding the Thevenin equivalent voltage, particularly due to the presence of multiple voltage sources.
  • One participant suggests using the maximum power transfer theorem and questions whether the load resistance should be adjusted to match the Thevenin resistance.
  • Another participant proposes performing a "KVL walk" to sum potential changes between points A and B.
  • Some participants discuss the validity of obtaining a voltage of 0V at both points A and B, with one participant affirming that this result is acceptable.
  • One participant calculates the voltage at point B using nodal analysis and questions how to derive a nodal equation for point A.
  • There is a discussion about the concept of a reference node in nodal analysis, with explanations about its role in determining node potentials.
  • Some participants clarify that the reference node can be chosen based on convenience, with point A being a suitable choice in this context.

Areas of Agreement / Disagreement

Participants generally agree on the methods to approach the problem, such as using KVL and nodal analysis. However, there is no consensus on the specific calculations or interpretations of results, indicating that multiple competing views remain.

Contextual Notes

Participants mention limitations in understanding the question and the implications of having multiple voltage sources, which may affect their calculations and approaches.

Who May Find This Useful

This discussion may be useful for students studying circuit analysis, particularly those learning about Thevenin equivalents and nodal analysis in the context of circuits with multiple voltage sources.

Ash_M
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Homework Statement


See attached image
Screen Shot 2016-12-01 at 22.11.06.png


Homework Equations


Thevenin Equivalent

The Attempt at a Solution


a) Find the total resistance between A and B by short circuiting the voltage sources:

Rt = 100Ω in parallel with the two 150Ω

Two 150Ω in series: R = 300Ω
1/Rt = 1/100 + 1/300

Rt = 75Ω

I am having difficulty finding the thevenin equivalent voltage.

b) Don't entirely understand the question so I am unable to make an attempt

Any help would be greatly appreciated.
Thanks
 
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Ash_M said:
I am having difficulty finding the thevenin equivalent voltage.
What have you tried? What's the usual procedure?
b) Don't entirely understand the question so I am unable to make an attempt
Hint: Maximum power transfer theorem.
 
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gneill said:
What have you tried? What's the usual procedure?

Hint: Maximum power transfer theorem.

I have no idea so I have not tried anything. Usually for a system with a single voltage source I would try to find the open circuit voltage between A and B, however the two voltage sources are confusing me.

Oh so would I take the Rload to be 50Ω, and since for maximum power the thevenin resistance = load resistance, change one of the resistors to give me a thevenin resistance of 50Ω?
 
Ash_M said:
I have no idea so I have not tried anything. Usually for a system with a single voltage source I would try to find the open circuit voltage between A and B, however the two voltage sources are confusing me.
You can always do a "KVL walk" from A to B and sum up the potential changes along the way.
Oh so would I take the Rload to be 50Ω, and since for maximum power the thevenin resistance = load resistance, change one of the resistors to give me a thevenin resistance of 50Ω?
Yes.
 
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gneill said:
You can always do a "KVL walk" from A to B and sum up the potential changes along the way.

Yes.

I don't quite understand what you mean by a "KVL walk", but i tried going round the top of the circuit and noting how the potential would change I also wasn't sure if the current in the entire system could be calculated as I = 12/400 . Doing that would mean I get a voltage of 0V at A and then 0V at B which can't be correct.
 
Ash_M said:
I don't quite understand what you mean by a "KVL walk", but i tried going round the top of the circuit and noting how the potential would change I also wasn't sure if the current in the entire system could be calculated as I = 12/400 . Doing that would mean I get a voltage of 0V at A and then 0V at B which can't be correct.
:smile: You did well, and a "KVL walk" is just what you did.

0 V is a perfectly valid result.
 
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gneill said:
:smile: You did well, and a "KVL walk" is just what you did.

0 V is a perfectly valid result.
Haha thanks so much!
 
gneill said:
:smile: You did well, and a "KVL walk" is just what you did.

0 V is a perfectly valid result.

I took a different approach to this question, calculating Vb from 0 = (Vb-9)/300 + (Vb-(-3))/100, so Vb = 0. Is there a way to get a nodal equation for Va?
 
Conor_B said:
I took a different approach to this question, calculating Vb from 0 = (Vb-9)/300 + (Vb-(-3))/100, so Vb = 0. Is there a way to get a nodal equation for Va?
You've used A as the reference node for your first equation, so it's potential is defined to be zero. That is, Va = 0 is the node equation for A :smile:
 
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gneill said:
You've used A as the reference node for your first equation, so it's potential is defined to be zero. That is, Va = 0 is the node equation for A :smile:

What do you mean by reference node?
 
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Conor_B said:
What do you mean by reference node?
A node whose potential is taken as 0V for reference. All the voltages in the circuit are measured with respect to the reference node. It is also called as the "circuit ground".
 
  • #12
Conor_B said:
What do you mean by reference node?
When you do nodal analysis you are determining the potentials of various nodes of the circuit with respect to some reference node. The first step is always to choose the node that will be the reference node for all the potentials.

There's often an obvious or convenient node to choose for this role (what constitutes "obvious" or "convenient" and how to recognize a good choice for the reference node is something that comes with experience). In this case node A is suitable since it not only forms one of the "output" terminals of the Thevenin equivalent, but is located at the junction of the only two voltage sources in the circuit.
 
  • #13
cnh1995 said:
A node whose potential is taken as 0V for reference. All the voltages in the circuit are measured with respect to the reference node. It is also called as the "circuit ground".

gneill said:
When you do nodal analysis you are determining the potentials of various nodes of the circuit with respect to some reference node. The first step is always to choose the node that will be the reference node for all the potentials.

There's often an obvious or convenient node to choose for this role (what constitutes "obvious" or "convenient" and how to recognize a good choice for the reference node is something that comes with experience). In this case node A is suitable since it not only forms one of the "output" terminals of the Thevenin equivalent, but is located at the junction of the only two voltage sources in the circuit.

Ok. Cool. Thanks Guys. Any time we did a problem in lectures, the ground was always marked in, so I never realized you could 'pick' a ground.
 

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