Because of multiple unknown values
Generally speaking, network analysis is any structured technique used to mathematically analyze a circuit (a “network” of interconnected components). Quite often the technician or engineer will encounter circuits containing multiple sources of power or component configurations that defy simplification by series/parallel analysis techniques. In those cases, he or she will be forced to use other means. This chapter presents a few techniques useful in analyzing such complex circuits.
To illustrate how even a simple circuit can defy analysis by breakdown into series and parallel portions, take start with this series-parallel circuit:
To analyze the above circuit, one would first find the equivalent of R_{2} and R_{3} in parallel, then add R_{1} in series to arrive at a total resistance. Then, taking the voltage of battery B_{1} with that total circuit resistance, the total current could be calculated through the use of Ohm’s Law (I=E/R), then that current figure used to calculate voltage drops in the circuit. All in all, a fairly simple procedure.
However, the addition of just one more battery could change all of that:
Resistors R_{2} and R_{3} are no longer in parallel with each other because B_{2} has been inserted into R_{3}‘s branch of the circuit. Upon closer inspection, it appears there are no two resistors in this circuit directly in series or parallel with each other. This is the main problem: in series-parallel analysis, we started off by identifying sets of resistors that were directly in series or parallel with each other, reducing them to single equivalent resistances. If there are no resistors in a simple series or parallel configuration with each other, then what can we do?
It should be clear that this seemingly simple circuit, with only three resistors, is impossible to reduce as a combination of simple series and simple parallel sections: it is something different altogether. However, this is not the only type of circuit defying series/parallel analysis:
Here we have a bridge circuit, and for the sake of example we will suppose that it is not balanced (ratio R_{1}/R_{4} not equal to ratio R_{2}/R_{5}). If it were balanced, there would be zero current through R_{3}, and it could be approached as a series/parallel combination circuit (R_{1}—R_{4} // R_{2}—R_{5}). However, any current through R_{3} makes a series/parallel analysis impossible. R_{1} is not in series with R_{4} because there’s another path for current to flow, i.e., through R_{3}. Neither is R_{2} in series with R_{5} for the same reason. Likewise, R_{1} is not in parallel with R_{2} because R_{3} is separating its bottom leads. Neither is R_{4} in parallel with R_{5}. Aaarrggghhhh!
Although it might not be apparent at this point, the main problem is the existence of multiple unknown quantities. At least in a series/parallel combination circuit, there was a way to find total resistance and total voltage, leaving total current as a single unknown value to calculate (and then that current was used to satisfy previously unknown variables in the reduction process until the entire circuit could be analyzed). With these problems, more than one parameter (variable) is unknown at the most basic level of circuit simplification.
With the two-battery circuit, there is no way to arrive at a value for “total resistance,” because there are two sources of power to provide voltage and current (we would need two “total” resistances in order to proceed with any Ohm’s Law calculations). With the unbalanced bridge circuit, there is such a thing as total resistance across the one battery (paving the way for a calculation of total current), but that total current immediately splits up into unknown proportions at each end of the bridge, so no further Ohm’s Law calculations for voltage (E=IR) can be carried out.
So what can we do when we’re faced with multiple unknowns in a circuit? The answer is initially found in a mathematical process known as simultaneous equations or systems of equations, whereby multiple unknown variables are solved by relating them to each other in multiple equations. In a scenario with only one unknown (such as every Ohm’s Law equation we’ve dealt with thus far), there only needs to be a single equation to solve for the single unknown:
However, when we’re solving for multiple unknown values, we need to have the same number of equations as we have unknowns in order to reach a solution. There are several methods of solving simultaneous equations, all rather intimidating and all too complex for the explanation in this chapter. However, many scientific and programmable calculators are able to solve for simultaneous unknowns, so it is recommended to use such a calculator when first learning how to analyze these circuits.
This is not as scary as it may seem at first. Trust me!
Later on, we’ll see that some clever people have found tricks to avoid having to use simultaneous equations on these types of circuits. We call these tricks network theorems, and we will explore a few later in this chapter.
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