Thevenin equivalent of a circuit

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Discussion Overview

The discussion revolves around finding the Thevenin equivalent of a specific circuit as presented in a homework problem. Participants explore the application of Thevenin's theorem, particularly focusing on the calculation of Thevenin equivalent voltage and the implications of circuit components like current sources and voltage sources.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant attempts to calculate the Thevenin equivalent voltage and arrives at a result of V_{oc} = 0, expressing confusion about the validity of this outcome.
  • Another participant challenges the use of Kirchhoff's Voltage Law (KVL) in the left part of the circuit due to the presence of a current source, suggesting that the voltage drop across it is unknown.
  • A participant questions whether a specific circle in the circuit diagram represents an independent voltage source, indicating that it would fix the voltage across the network of passive components to its right.
  • There is a suggestion that the Thevenin equivalent voltage could be V_{oc} = 12 \angle 0 V, based on the isolation provided by the voltage source.
  • One participant proposes that the fixed voltage source allows for the neglect of the current source and its series resistor when determining the voltage across the voltage source, advocating for a stepwise application of Thevenin's theorem.

Areas of Agreement / Disagreement

Participants express differing views on the application of KVL and the impact of circuit components on the Thevenin equivalent voltage. No consensus is reached regarding the correct approach to solving the problem.

Contextual Notes

Participants highlight limitations in applying KVL due to the presence of the current source and discuss the implications of isolating components in the circuit. The discussion reflects uncertainty about the correct method to apply in this context.

aid
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Homework Statement



I am attempting to solve problem number 4 from the following picture:

http://img641.imageshack.us/img641/7950/imageisx.jpg

In the picture you can see the suggested line of "cut" for the first application of Thevenin's theorem.


The Attempt at a Solution



I have tried to find the Thevenin equivalent voltage (for the part on the left from the sketched line). I get:
V_{oc} = 12 \angle 0 - 2 * 6 \angle 0 = 0
which would render the whole exercise surprisingly trivial.

What the heck is wrong with the above equation? I would be grateful for any help on this one.
 
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KVL in left part is not possible, if that is what you are doing ! KVL is not possible because of the presence of current source across which you don't know the voltage drop !
 
Hi Aid. Is that circle with + and | marked on it supposed to represent an independent voltage source? If so, it fixes the voltage across the network of passive components to its right.
 
lazyaditya said:
KVL in left part is not possible, if that is what you are doing ! KVL is not possible because of the presence of current source across which you don't know the voltage drop !

What method is possible to apply in this case, then? Is it really so that the Thevenin equivalent voltage of the marked part is equal to:

V_{oc} = 12 \angle0 V?
 
aid said:
What method is possible to apply in this case, then? Is it really so that the Thevenin equivalent voltage of the marked part is equal to:

V_{oc} = 12 \angle0 V?

A fixed voltage source effectively isolates branches parallel to it. In this case the six amp current supply and its 2 Ω series resistor cannot affect the voltage across that voltage source in any way. So, if you are looking to find the voltage Vo which is to the right of the voltage source, you can discard anything to the left of that isolating supply from consideration. That is, ignore entirely the 6 Amp supply and its 2 Ω series resistance; the 12V voltage source will be the starting point for your march across the circuit towards Vo.

The question requires you to employ Thevenin's theorem to find the result, so I'd suggest applying it several times, accumulating chunks of the circuit into successive Thevenin models (source voltage and series impedance) as you go. Where you've drawn your first "cut" in pencil is a good staring point -- the Thevenin equivalent of the voltage source and capacitor alone is very straightforward.
 

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