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[SOLVED] Network analyse question How to find V

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olemariendal

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Hi, first sorry about my english, but I will try to do my best.

Im trying to find V from this circuit, but I can´t find an solution. I know without R5, the solution is 2mA*(9kΩ+4kΩ)*(3kΩ/(3kΩ+7kΩ+9kΩ+4kΩ))=3,39V, but how do i do it, if I have a resistor between R3 and R1.

Circuit 1 without R5:
circuitone.PNG


Circuit 2 with R5 on 6kΩ:
circuittwo.PNG

Thanks for your help. :-D
 

Notice that R5 becomes the bridge resistor in a Wheatstone bridge.

It does not change the voltage put out by the current source (no matter whether R5 is 0 ohms, or infinite).

The ground icon is just there to throw you off. Since it is there, and since it defines 0V, you only need to transpose node levels up or down, to find V.
 

BradtheRad said:
Notice that R5 becomes the bridge resistor in a Wheatstone bridge.

It does not change the voltage put out by the current source (no matter whether R5 is 0 ohms, or infinite).

The ground icon is just there to throw you off. Since it is there, and since it defines 0V, you only need to transpose node levels up or down, to find V.
Click to expand...

This is not quite right; the Wheatstone bridge is not balanced, so the addition of R5 changes things.

olemariendal, do you know any of the general methods for solving networks, such the mesh method or the nodal method?
 
I wrote:

It does not change the voltage put out by the current source (no matter whether R5 is 0 ohms, or infinite).
Click to expand...

The Electrician said:
This is not quite right; the Wheatstone bridge is not balanced, so the addition of R5 changes things.
Click to expand...

My insights were one too few. You are correct. The bridge values are unequal and disproportionate. Large changes in R5 require a small change in V.

But I do have a formula to find the voltage across R5. It involves calculating the weighted contribution from the other four branches, and the resulting proportion occupied by R5 between them.
 

The circuit can be solved easily by nodal analysis. Find the voltage at various nodes and once the node voltages are found, currents will follow from Ohms law. Since there are four nodes (including ground), three equations will be required.
See attachment
 

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  • Capture22.PNG
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The method we are leaning is Thevenin to analysis a circuit, it is possible to use that one? But thanks for the hints.
 

Do you intend to find Vth across R2? If yes then do the following:
1. Remove R2
2. Voltage drop across R1 is 2mAxR1
3. to find the voltage drop across R5, equate:
(2mA-I)xR3 = Ix(R4+R5) [I is the current through R4 and R5. (2mA-I) is the current through R3]
4. Add the voltage drops across R1 and R5 to get Vth across R2
 

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