Thevenin without removing load resistor

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Discussion Overview

The discussion revolves around finding a Thevenin model for an unknown circuit when the open-circuit voltage (V_oc) is not provided. Participants explore how to approach the problem using given current values through load resistors, while considering the implications of Thevenin and Norton equivalents. The scope includes theoretical and practical aspects of circuit analysis.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant expresses difficulty in applying Thevenin's theorem without knowing V_oc and mentions specific current values through load resistors.
  • Several participants emphasize the need for a schematic to provide effective assistance, noting the lack of visual representation of the circuit.
  • Another participant suggests that the question can be approached without a schematic by drawing a generic Thevenin circuit and hints at the potential utility of a Norton equivalent.
  • A participant points out that the load resistor is not the only resistor in the circuit and suggests using Kirchhoff's Voltage Law (KVL) to set up equations.
  • One participant shares their realization of the usefulness of Thevenin and Norton models for circuit analysis.
  • A later reply indicates that they successfully set up the Norton model as a current divider to solve the problem.

Areas of Agreement / Disagreement

Participants generally agree that a schematic would aid in understanding the problem, but there is no consensus on the necessity of one for solving the question. Multiple approaches to the problem are discussed, including Thevenin and Norton models, without a clear resolution on the best method.

Contextual Notes

The discussion highlights the challenge of determining V_oc without additional information and the reliance on assumptions about the circuit configuration. There are unresolved mathematical steps and dependencies on the definitions of circuit elements.

Who May Find This Useful

This discussion may be useful for students and practitioners interested in circuit analysis, particularly those learning about Thevenin and Norton equivalents and their applications in practical scenarios.

britishben
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I'm having trouble with Thevenin models, specifically this question where the V_oc is not given.

The problem:
Assume an unknown circuit provides the specified current through the load resistors as given below:
100k - 2.3mA
1.2M - 510uA
Find a Thevenin model for the unknown circuit.

All I've been able to find on Thevenin requires me to find V_oc, which I don't know how to do from here. I've tried just using Ohm's Law, but that nets me 230V for the first case, and 612V for the second. Suggestions on how to proceed would be appreciated.
 
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No one will be able to help you until you post a schematic of your circuit.
 
The Electrician said:
No one will be able to help you until you post a schematic of your circuit.

There is no schematic of the circuit given. The load resistor is connected across two terminals of a "black box", I assume.
 
Actually, this question does not require a schematic. Since this sounds like homework, I won't tell you the answer. However, you can very easily figure out what the question is actually asking for by drawing a generic Thevenin circuit. Actually, it might be easier to use a Norton equivalent in this case.

In either case, you'll have two equations and two unknowns.

EDIT: This illustrates one of the principles of the "black box": that you can figure out an equivalent model of the box without actually cracking it open, just by how it drives a load.
 
Last edited:
I've never actually thought of Thevenin or Norton models' usefulness, until I saw this thread. I've always thought they were to make analysis easier.
 
Hint: remember that the load resistor is not the only resistor in each of the 2 circuits. You have 2 unknowns, and 2 separate cases with part of the circuit resistance given along with the entire source currents. Think KVL.
 
Thanks for the help, guys. I just set the Norton model up as a current divider, and solved from there.
 

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