# Nodal analysis

Discussion in 'Homework Help' started by zeckk, Feb 16, 2015.

1. ### zeckk Thread Starter New Member

Feb 16, 2015
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0
Hello everybody!

I am struggling at the moment to define a reference point in this circuit, that will make things easier on me.

I'm supposed to solve for Vx and Iy, and I know by the use of nodal analysis at point C I will be able to solve the question. Problem is how these equations will be affected by the reference point, I can't seem to be able to wrap my head around the equations and which point to pick.

Kind regards, zeckk

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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You can choose any point you wish as the reference. Why not try something and post your solution....

3. ### zeckk Thread Starter New Member

Feb 16, 2015
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I know very well I can pick any point as I wish, but I also know that picking certain reference points can make problems significantly easier. In my case I only know that I can pick one, but I don't know how to distinguish between easier and harder ones. Where I am getting stuck here is that I am getting confused with all the elements at the bottom of the circuit. I just need a little push in the right direction. Equations or a reference point is all I need, not an answer to the question!.

4. ### WBahn Moderator

Mar 31, 2012
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4,788
Good decisions come from exercising good judgment. Good judgment comes from making bad decisions.

The reason you don't know how to distinguish between easier and harder ones is that you lack the necessary experience. As with most things, you gain that experience by making your best guess and then learning from how things turn out. Sometimes you will make the wrong guess, but that's fine because its THOSE decisions that you will learn the most from.

The best thing to do is to take each node and make a list of pros and cons, as you see them right now, as to why it should or shouldn't be chosen as the reference. Then rank order them, as you see them right now, from best to worst. Then proceed to work the problem with each choice of reference and make notes regarding whether your pros and cons turned out to be valid and what you think the proper rank ordering should have been. The time you invest will pay off handsomely.

Another point to keep in mind is that the "best" choice of reference is rather subjective and the pros and cons are somewhat personal in that they depend on how YOU view a problem and approach its solution. Thus this whole process is somewhat personal and can only be conveyed by someone else in an imperfect manner.

So how about you pick make the list of pros and cons for each node and list them here. We will then give you our thoughts about them and you can see whether that reinforces your thoughts or makes you rethink them. Also, work the problem with at least two different choices for the reference and present those and we will again give you our thoughts and observations.

5. ### zeckk Thread Starter New Member

Feb 16, 2015
4
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thank you for the great response! As of right now I believe node A, is the best option as I will immediately remove an unknown. Now, in order to solve the problem I will be needing to use nodal analysis around node C. Here is where I am stuck, because as each node, that node C is connected to is creating more equations and more unknows! and it seems like its an neverending cycle.

6. ### WBahn Moderator

Mar 31, 2012
17,715
4,788
Node A is the one that I would have picked and for the same reason. Nodal analysis does great with current sources but is more complicated with voltage sources. So by picking Node A you turn Node C into a known (it's 6V).

It's not a never ending cycle. You have four nodes and so, in general, you have three node equations. Choosing Node A as the reference doesn't change that, it just makes one of those three equations trivial by inspection and simplifies the other two.

How about if we had picked Node C as the reference? It also has the advantage of making Node A trivial (it's -6V). But it also makes the voltage at Node D pretty obvious since Vd/12Ω must yield -20A. That this voltage, relative to Node C, is this trivial is the case regardless of what node we pick. It's just that choosing Node C (or Node D, for that matter) makes it stand out as such.

As for Node B, the only thing it brings to the table is familiarity in that we are used to seeing circuits with the reference point at the bottom (and often bottom center) of the diagram. Don't write the value of that off too easily. The more familiar we can make the circuits appear to us, the more likely we are to NOT make stupid bookkeeping errors. That has real value, but it is hard to hang a price tag on it.

7. ### WBahn Moderator

Mar 31, 2012
17,715
4,788
Oh, and perhaps this is what is tripping you up. You WON'T be doing nodal analysis around Node C -- it's already solved! You be doing nodal analysis around (I prefer to say, "at") Nodes B and D.

8. ### zeckk Thread Starter New Member

Feb 16, 2015
4
0
ah! there we go! great thank you very much! I'll keep you guys posted if I still can't solve it!

9. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Picking node D as reference presents no more challenges than any other choice. Vc is known by inspection, Va falls out immediately and Vb is readily obtained from a single nodal equation. Vx is Vbc. What's the big deal about the choice of reference?