# Solve Thevenian Theorem: Calculate Voltage Across A and B

• roshan2004
In summary, the conversation discusses the Thevenin's theorem and how to calculate the Thevenin voltage across terminals A and B. The 3 ohms resistor can be ignored, and the total resistance of the series circuit can be found to calculate the current. The voltage divider rule can also be used to find the voltage across the 2 ohms resistor. The 12 V battery is in series with the Thevenin voltage and can be treated as such.
roshan2004
This question is based on the thevenian's theorem,but my problem is on the basic rule of circuit.I have come at the stage where I have to calculate the Thevenian's voltage that is "V" across A and B.I know that V=I*6 since V across AB equals to the V across CD as there is no drop of current on the 3 ohms resistor but now I have to calculste I which can be found by V/R(total).V is 36 volt.Now I have to find Total R which is the stage where I am totally confused since I just know how to rearrange the given circuit.I don't know which one is parallel or in series with another.Please have a look on the picture (which is messy).
Your post is valuable or me.

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• Thevenian.bmp
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The Thevenin voltage is the open circuit voltage between the terminals A & B. Since you already realize that the 3 Ω resistor is irrelevant, just ignore it. You have a simple series circuit, for which you can find the total resistance and then the current. Use that to find the Thevenin voltage.

Thanks,I used the voltage divider rule and got the answer.

This is another type of question where I am troubled to calculate the thevenian voltage.The original question is to calculate the current through 10 ohms resistor.

#### Attachments

• thevenian.bmp
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Once again, the Thevenin voltage is the open circuit voltage between A and B. That means there's no current through the 8 Ω resistor (but don't neglect the 12 V battery). Use the voltage divider rule to find the voltage across the 2 Ω resistor.

Yeap I really don't know how to treat 12 V battery.

roshan2004 said:
Yeap I really don't know how to treat 12 V battery.
It's in series with the Thevenin voltage. (But oppositely oriented.)

Doc Al said:
It's in series with the Thevenin voltage. (But oppositely oriented.)

Can you give some more hint please...

Do this: Find the Thevenin voltage just before the 12 V battery. Then you can add the 12 V battery in series with it.

## What is Thevenin's theorem?

Thevenin's theorem is a fundamental principle in electrical circuit analysis that allows a complex circuit to be simplified into an equivalent circuit with a single voltage source and a single series resistor.

## How do you calculate voltage across A and B using Thevenin's theorem?

To calculate the voltage across A and B using Thevenin's theorem, you must first find the Thevenin voltage and resistance of the circuit. The Thevenin voltage is the open-circuit voltage at the terminals A and B, and the Thevenin resistance is the equivalent resistance seen from the terminals A and B when all independent sources are turned off. Once you have these values, you can use the formula Vab = Vth * (Rb / (Ra + Rb)), where Vab is the voltage across A and B, Vth is the Thevenin voltage, Ra is the resistance between A and the Thevenin voltage source, and Rb is the resistance between B and the Thevenin voltage source.

## When is Thevenin's theorem used?

Thevenin's theorem is used when analyzing complex circuits to simplify the calculations and make them more manageable. It is especially useful in situations where there are multiple sources and resistors in a circuit.

## What are the limitations of Thevenin's theorem?

Thevenin's theorem is only applicable to linear circuits, which means the current through a component is directly proportional to the voltage across it. It also assumes that the circuit is in a steady-state, meaning all transients have settled. Additionally, Thevenin's theorem is only accurate for circuits with independent sources and does not work for circuits with dependent sources or active elements like transistors.

## How is Thevenin's theorem related to Norton's theorem?

Thevenin's theorem and Norton's theorem are two equivalent ways of simplifying a complex circuit into a single voltage source and series resistor. Thevenin's theorem uses a voltage source and series resistance, while Norton's theorem uses a current source and parallel resistance. Both theorems can be used interchangeably to solve circuit problems.

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