# Circuit analysis

Discussion in 'General Electronics Chat' started by sharanbr123, Oct 25, 2014.

1. ### sharanbr123 Thread Starter Member

Sep 29, 2014
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Hello All,

This is my first attempt at analysis a circuit in terms of working out voltages and currents across different paths of the circuit. I have used superimposition theorem for analysis. Figure is attached that describes the details.

I would appreciate if forum members can comment if my approach is correct.

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2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,324
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hi,
Re-check your V1 and also the current polarities.

E

3. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
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With respect, Eric, voltages have polarities, currents have directions.

4. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,324
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hi s,
Of course, I was trying to keep the wording as simple as possible, perhaps in hindsight too simple.
It was intended to bring his attention to the mathematical 'signs' of the current values he was using in his calculations.

Eric

Jan 29, 2010
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6. ### sharanbr123 Thread Starter Member

Sep 29, 2014
49
0
Dear Eric,

Yes. That is the book is I am reading. I decided to work out on my own after seeing the circuit and posted by workout in this forum.
Yes, I do see a difference in current direction. I have still not figured it though why my analysis shows direction the way it does.
In fact, in my example, the current flowing out of 28 V source was 5 A which I intuitively assumed correct since being a 28 V source (compared to 7 V), I assumed that this source has higher potential to source current. Anyway, I will go back and analyze more ...

7. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,173
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In figure 1 you assume the "positive" current directions. Ir3 is positive if flow from +7V
through R3 into V1 node. Now notice that in figure 2 Ir3 flow in opposite direction , so ir3 from fig2 is negative but in fi3 ir3 again is positive.
So finally IR3 = -4A + 3A = -1A.
We have the same situation for Ir1 current.
In fig2 the Ir1 is positive and in fig3 is negative so
IR1 = 6A + -1A = 5A. And for Ir2 all current are positive.

8. ### sharanbr123 Thread Starter Member

Sep 29, 2014
49
0
I got it ... direction of Ir3 is incorrect in my work out ...

9. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
519
Do remember that it is not your working that is incorrect when you get a negative answer: the signs are important because they tell you which way currents are flowing.

At least in electrical engineering there is really only one convention. Other branches of engineering have to put up with authors etc using different conventions.

I'm glad Eric wants to stress that these are sign conventions.

We constantly see posters confused by the these conventions and indeed have a couple of threads running at any one time on the subject.