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I'm an Electrical Engineering student, and circuit analysis can be really confusing to me. I sort of understand the concepts of solving circuits, but I don't really know when to use which method. Are there any simple rules on choosing which method I should use for solving circuits? Node-voltage analysis? Superposition? Mesh analysis? Just use reduced linear equation system (or whatever it's called in English)?

Should I just calculate how many equations each method will take and pick the one with least equations?

Also, how do I know if I should use Thevenin's / Norton's generator to substitute a network?

Please note that this is not really for practical purposes - I'm studying for an exam, but all the assignments in my textbook are categorized by methods, so I know which one to use. However, in the exam that info is not given to students.

Thanks.

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  • \$\begingroup\$ Some professors in my experience will ask you to use one method on one test problem, another for another so that they ensure you learned all of the methods. In the real world if you need to analyze something it does not matter what method you use as long as one condition is met, you get a correct useful answer. Which method do I use, whichever comes the fastest. I find when building something node voltage often makes sense as I normally have a reasonable number of nodes but the regularity with which I analyze a circuit that needs node voltage method is rare. \$\endgroup\$
    – Kortuk
    Jan 14 '12 at 4:16
  • \$\begingroup\$ @Kortuk Are you made to do these by hand and not use something like PSpice? WOW better start liking to do them :/ \$\endgroup\$
    – Dean
    Jan 14 '12 at 14:57
  • \$\begingroup\$ @Dean, I have done in my head circuit analysis many times at my job, I do not always need to know the exact waveforms on a circuit, I just need to work out what is happening so that I can move forward with what I am doing that is using them, or to fix them. In general I do not model anything out of this world at work, but how often do I really have something that doesn't have digital points that allow me to determine what the input is and then work out the next piece separately. \$\endgroup\$
    – Kortuk
    Jan 14 '12 at 17:59
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Basically there are few different things here that appear lumped together in your mind and which may be better seen separately.

First you have the classical methods for solving circuits, such as the mesh analyses, node-voltage analyses, full equation system, reduced equation system and maybe some more I forgot.

First, the full equation system has no excuse to be used, so don't use it. The reduced equation system should only be used for very simple circuits where the preparation to use one of the two other systems would take more work than solving the circuit using reduced system. After some practice, you should be able to determine how much work you'll need to solve the circuit using each system (so try using all 3 systems on some problems, just so you can compare how much work each requires).

Next, you should have good theoretical knowledge of how each system works. The circuit may be such that some system cannot be easily used with it. You should also note the special cases where the mesh analyses and nodal analyses are especially useful. For example, in mesh analyses, if you have lots of constant current sources, you get lots of trivial equations for mesh currents. Same thing with nodal analyses and voltage sources. So basically, you should calculate how many equations you'll get with each and pick one with least equations, but do take into account special cases of current source and voltage sources when calculating the number of equations!

After that, you have the theorems which you use. I'd say that you should first see if any theorems can be applied to a circuit and only after applying them try to solve it using the systems. For example, you should use Norton/Thevenin when you have a big circuit where one for few elements change. For example, you have a circuit with a rheostat and you need to calculate say current through it on various settings. In this case, just replace the rest of the circuit with Thevenin's generator, since it doesn't change. In case of superposition theorem, it's useful in cases where you have sources that turn on and off and you should calculate their effects on some part of the circuit. Same thing goes for other theorems, like bisection (where you have symmetrical circuit, so you only need to solve one half of it) or compensation (which is useful when you have sources whose values change).

So for theorems, the general idea is to find use cases where each of them will actually allow the circuit to be simplified. So when you have a problem, ask yourself not "How am I going to solve this using method X?" but "Why am I going to solve this using method X?". This should work even on problems in textbooks where they are divided by areas. So as I said before, try solving one problem using several different methods. See which ones can be applied to the problem, which ones can't be applied to the problem, ask yourself why for each and then take a look and see which method is the most optimal (in sense that the least number of equations needs to be solved or that you get a significant number of simple equations) and try to understand why the most optimal method is the most optimal method. This way, you'll see when it's counterproductive to apply some theorem, when you gain nothing by applying some theorem and when applying a theorem actually helps. Same story goes for reduced system, nodal and mesh analyses too.

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  • \$\begingroup\$ Whoa, we're from the same college! The internet is so small... ETF FTW :D \$\endgroup\$
    – Radiant
    Jan 14 '12 at 22:18
  • \$\begingroup\$ @Radiant Unfortunately, no. After being first year for 3 times (curse you OET1/2 and M1/2!), I dropped out of ETF and moved to the EE course at RAF. :( So be sure to study OET 1 and 2 hard and pass them as soon as possible and get to the good stuff later on! And yes, Internet is small :). \$\endgroup\$
    – AndrejaKo
    Jan 15 '12 at 2:28
  • \$\begingroup\$ Sorry to hear that. :( I saw in your profile link that you're an ETF Gaming Crew admin, so I assumed you're from ETF (which wasn't so untrue). Anyways, the same subjects are giving me problems, too... I guess just studying them very hard is the only solution, but I pretty much f'd up the 1st semester. \$\endgroup\$
    – Radiant
    Jan 15 '12 at 13:33
  • \$\begingroup\$ @Radiant Don't worry about that too much. It happens to everyone. The real problem is if that happens in the second semester too! \$\endgroup\$
    – AndrejaKo
    Jan 15 '12 at 13:59

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