# Question about thevenin and norton's theorems

Discussion in 'Electronics Homework Help' started by 9905mi, Jul 13, 2018.

1. ### 9905mi

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Jul 5, 2018
Hello everyone, so there is a circuit in which I need to find Thevenin's resistance and Norton's current between terminals A and B. I have found the resistance but when I try to calculate the current it appears to be wrong, I assume that I need to short the terminals and then to calculate the current considering that the current will flow through resistors R1 and R3 because it's the shortest and less resistive path. Can someone point out where is my mistake?

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2. ### Harald KappModeratorModerator

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Nov 17, 2011
Current doesn't flow simply through the path of least resistance, it divides between parallel paths inversely with resistance.

Your mistake is in line 22: You calculate Rtm as the parallel circuit of R1, R2 and R34, but only R2 and R34 are in parallel, then in series with R1.

Tip: It helps to draw a new diagram with appropriately labelled components for each step/equation. This way you won't loose track of what you're doing by substituting components.

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3. ### 9905mi

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Jul 5, 2018
So the current will divide between R1, R2 and R3 but R4 will get no current?

4. ### Harald KappModeratorModerator

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Nov 17, 2011
No, each resistor will carry its share of current. How could currrent flow through R3 but not through R4? Impossible!

5. ### 9905mi

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Jul 5, 2018
It's not shown in the diagram but terminals A and B are connected with conductive wire with 0 ohm

6. ### Laplace

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184
Apr 4, 2010
Points A&B are shorted only when calculating the short-circuit current, but when finding the equivalent resistance then only the sources are zeroed.

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7. ### 9905mi

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Jul 5, 2018
Ok. I understand
Thanx for help

8. ### Harald KappModeratorModerator

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Nov 17, 2011
It would have helped a lot to know this from the beginning. R4 thus is effectively 0 Ω and your calculation of R34 is useless.

9. ### Ratch

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Mar 10, 2013
The universal way to find the Thevenin impedance is to divide the open circuit voltage by the shorted current. That method will work even if there are dependent sources, which your circuit does not have. The open circuit voltage across A-B is easily seen to be 0.5 volts. The shorted current across A-B is easily calculated to be 7.5 ma. Dividing 0.5/0.0075 = 66.66... Since your circuit lacks dependent sources, you can double check the answer by shorting out the voltage source and calculating the resistance across A-B directly.

By the way, the phrase current flow is redundant. You should just say current, current is present, or current exists. "Current flow" means charge flow flow.

Ratch

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