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[Circuits] Finding the Thevenin and Norton Equivalents #3
In summary, the conversation is discussing a mistake made while determining the resistance value for a circuit. It is explained that the mistake is likely due to the direction of the current being injected into the circuit and the polarity of the resulting voltage measurement. The correct polarity for the voltage measurement is discussed and it is noted that this may have been contrary to the desired arrangement, resulting in a reversed sign for the voltage.
- #2
gneill
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ainster31 said:
No doubt it has to do with the direction of the current that you're "injecting" into the circuit versus the polarity of the resulting voltage across the terminals. The resistance of a component (or circuit for that matter) is defined as the current into the port divided by the resulting potential across that port. The polarity of the voltage "measurement" is such that the terminal where the current is injected is taken to be positive:
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- #3
ainster31
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So my mistake was that the 1A current source was pointing down when it should've been pointing up?
- #4
gneill
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ainster31 said:
Not a mistake per se, not if you take it into account when you assign the polarity of the voltage. You are looking for the potential at terminal a with respect to terminal b, so automatically that implies that the a terminal is the where you'd put the + lead of your meter (if you were measuring it). You are driving the current into the b terminal, so it becomes the "+" terminal for the resistance determination. That's contrary to the desired arrangement, and will result in a reversed sign for the voltage Vab.
- #5
rezeew
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It is possible that you made a mistake in your calculations or that there was a typo in the given circuit. Double check your work and make sure to consider the direction of current flow when determining the equivalent resistance. Also, keep in mind that the Thevenin and Norton equivalents are only valid for linear circuits, so if the circuit contains non-linear elements, the results may not be accurate. If you are still getting a negative value for Rth, it is best to consult with your instructor or a peer for assistance.
FAQ: [Circuits] Finding the Thevenin and Norton Equivalents #3
What is the Thevenin Equivalent Circuit?
The Thevenin Equivalent Circuit is a simplified representation of a complex circuit that consists of a single voltage source in series with a resistor. It is used to analyze the behavior of a circuit at a specific load without having to consider the rest of the circuit.
How is the Thevenin Equivalent Circuit calculated?
To calculate the Thevenin Equivalent Circuit, first remove the load resistance from the original circuit. Then, determine the equivalent resistance seen from the load terminals by shorting all voltage sources and opening all current sources. This is the Thevenin resistance. Finally, the Thevenin voltage is equal to the voltage across the load terminals when the load resistance is reconnected.
What is the Norton Equivalent Circuit?
The Norton Equivalent Circuit is another simplified representation of a complex circuit that consists of a single current source in parallel with a resistor. It is equivalent to the Thevenin Equivalent Circuit and can be used interchangeably in circuit analysis.
How is the Norton Equivalent Circuit calculated?
To calculate the Norton Equivalent Circuit, first remove the load resistance from the original circuit. Then, determine the equivalent resistance seen from the load terminals by shorting all voltage sources and opening all current sources. This is the Norton resistance. Finally, the Norton current is equal to the current through the load terminals when the load resistance is reconnected.
How do Thevenin and Norton Equivalents compare to each other?
The Thevenin and Norton Equivalents are mathematically equivalent to each other, meaning they can be used interchangeably in circuit analysis. However, the Thevenin Equivalent is more commonly used for circuits with voltage sources, while the Norton Equivalent is more commonly used for circuits with current sources.
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