# Circuit analysis?

So far I've learned these circuit analysis techniques for:

• Tellegen's Theorem

• Using only KCL

• Using only KVL

• Solving a system of equations that consist of all KCL equations and all KVL equations

• Nodal Analysis

• Superposition

• Mesh Analysis

When do these techniques allow you to completely analyze a circuit? Please note, I'm not asking when I should to use each technique. I'm asking what conditions must the circuit fulfill for a technique to allow me to solve the circuit. For instance, I noticed that in some circuits only using KCL is enough to solve for everything but in other circuits using only KCL doesn't allow you to completely analyze the circuit.

I've only done resistive circuits so far.

Circuit that can't be solved using only KCL:

because using KCL at the nodes gives:

which are three linearly independent equations with 5 variables, so can't be solved.

• What do you mean to distinguish by listing "mesh analysis" and "using only KVL" as two separate methods? (Also "nodal analysis" and "using only KCL") Mar 19, 2014 at 16:15

"Superposition" is not a method for solving a circuit. It is a part of the other methods that is used when multiple sources are present in the circuit.

"Tellegen's Theorem", although I don't recall it from school, doesn't seem to produce a practical method of solving circuits since it only produces one equation for the circuit (according to a quick scan of Wikipedia). However I'd be glad to corrected on this point --- its not something you run into often in the real world.

Nodal analysis can not solve a circuit that contains a voltage source.

Mesh analysis can not solve a circuit that contains a current source.

If you want to solve a network that contains both voltage sources and current sources, you will need the modified nodal analysis. Using duality you could also imagine a "modified mesh analysis", however this method is not commonly used. The modified nodal analysis is the most commonly used method of solving circuits because it is easier to set up the equations for a computer.

Edit

You clarified,

"just KCL", I mean not using Ohm's Law to rewrite the equations in terms of voltages. Instead, keeping the equations in terms of currents and solving the equations for the currents.

Just the KCL and/or KVL equations are not sufficient to solve any circuit. These equations can be written simply from the topology of the circuit and take no account of what kind of elements are on each branch. For example they don't make any distinction between a branch being a resistor or a voltage source or a current source. You will always need to also include the characteristic equations for the elements on the branches to fully describe a circuit.

• Thank you. If I have circuit that doesn't contain any voltage sources, just KCL still doesn't always give me an answer (nodal analysis does however). When would KCL directly give me an answer?
– dfg
Mar 19, 2014 at 16:30
• Also when would using KCL and KVL together allow me to completely solve the circuit?
– dfg
Mar 19, 2014 at 16:30
• @dfg, To me, "just KCL" and "nodal analysis" are the same thing. Can you explain what you mean by those two different terms? Also an example of a circuit with no voltage sources that can't be solved by KCL? Mar 19, 2014 at 16:32
• By "just KCL", I mean not using Ohm's Law to rewrite the equations in terms of voltages. Instead, keeping the equations in terms of currents and solving the equations for the currents.
– dfg
Mar 19, 2014 at 16:43
• I also added in an example of a circuit that can't be solved using "just KCL".
– dfg
Mar 19, 2014 at 16:46