40 GEARS January / February 2006

So far in Pushing Electronswe’ve looked at simple cir-cuits, series circuits and paral-

lel circuits. This time we’re going to look at series-parallel circuits; a hybrid that combines some of the characteris-tics of both series and parallel circuits.

While not a particularly common circuit in automotive applications, it can be useful to understand series-par-allel circuits for one important reason: That understanding can help you iden-tify problems in other types of circuits.

That’s because unwanted resis-tance behaves exactly the same as an expected resistance, so the unwanted resistance can turn a common series or parallel circuit into a series-parallel circuit. By being familiar with these unusual circuits, you can easily identify and diagnose a more common circuit that’s taken on additional resistance in its feed or ground sides.

A series-parallel circuit can take two basic forms:

1. The parallel component can be wired in series with the other resistanc-es in the circuit (figure 1). The overall circuit behaves the same as a series circuit, with the parallel component acting as a single resistance in the series circuit. So this type of series-parallel circuit should obey all the same rules as the series circuit.

2. The series component can be wired in as one leg of the parallel cir-cuit (figure 2); in fact, a better name for this might be a parallel-series circuit. The overall circuit behaves the same as a parallel circuit, with the series com-ponent acting as a single resistance with the parallel circuit. This type of circuit

will obey all the same laws as a parallel circuit.

In each case, one circuit acts as a subset of another circuit. We’ll call the main circuit the primary circuit, and the imbedded circuit the secondary circuit.

In every case, the circuit acts and obeys all the rules laid out for the primary circuit; the secondary acts as a single component in the circuit.

PUSHING ELECTRONS

Series-Parallel Circuits: Series-Parallel Circuits: One Circuit Wired Into AnotherOne Circuit Wired Into Another

by Steve Bodofsky

Figure 1: In this circuit, the parallel component (in the orange box) is wired in series with the other resistances in the circuit. So the parallel component acts as a

single resistance in the series circuit.

Figure 2: In this type of circuit, the series circuit (in the orange box) is one branch of the parallel circuit. This might be more properly called a parallel-series circuit.

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GEARS January / February 2006 41

Series-Parallel Circuits: One Circuit Wired Into Another

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Series-Parallel Circuit Type 1

Look at the first type of series-par-allel circuit (figure 1): In this case, the series circuit is the primary circuit. The parallel circuit is just one of two sepa-rate resistances in that series circuit.

When checking overall circuit resistance or current flow, this circuit will obey the rules of a series circuit. The difference here is that, to predict the overall circuit resistance or cur-rent flow, you must first determine the resistance of the secondary (parallel) component of the circuit, and treat it as a single resistance in the circuit.

Based on this, we know:• The total circuit resistance will

be equal to the sum of the individual resistances. The only difference is that we must first calculate the resistance of the parallel circuit to determine the resistance of that component of the cir-cuit. Or we can measure the resistance of the parallel circuit as a single com-ponent (figure 3).

• Current flow will be constant through all parts of the series compo-nents of the circuit (figure 4). The par-allel component will split that current flow, based on the individual resistance in each leg of the circuit.

• Voltage will be distributed between each resistance in the series circuit, based on its specific resistance (figures 5 and 6). But both legs of the parallel circuit will receive the same voltage (figure 7). The voltage drop across the parallel component will depend on the total resistance of the parallel circuit.

And, just as with any series circuit, all of the voltage will be used up push-ing the current through the resistances (figure 8).

Series-Parallel Circuit Type 2

In the second type of series-paral-lel circuit (figure 2), the parallel circuit is the primary circuit. The series circuit is the secondary circuit; it’s one com-ponent of the parallel circuit.

When checking overall circuit resistance or current flow, this circuit will obey the rules of a parallel cir-cuit. The difference here is that you must consider the secondary (series)

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42 GEARS January / February 2006

Series-Parallel Circuits: One Circuit Wired Into Another

Figure 3: The easiest way to anticipate this circuit’s behavior is to measure the parallel part of the circuit’s resistance as if it

were a single component.

Figure 4: Just as with a regular series circuit, current will flow equally through all of the series parts of the circuit. The parallel component

will divide the current, based on the resistance of each leg.

Figure 5: As with a regular series circuit, voltage will be distributed between each resistance in the series circuit,

based on its resistance.

Figure 6: Notice that the voltage is considerably lower than half of the system voltage. That’s because the resistance of the parallel part of the circuit is much lower than the resistance of the single bulb, so more of the voltage gets used pushing the voltage through the bulb,

before it reaches the parallel circuit.

Figure 7: Both legs of the parallel component of the circuit will receive the same voltage, just as with a regular parallel circuit.

Figure 8: As with a regular series circuit, the resistances in the circuit will use up all of the voltage.

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GEARS January / February 2006 43

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component of the circuit as a single resistance in the parallel circuit.

Based on this, we know:• Each leg of the parallel circuit

should have full voltage and full ground applied to it (figures 9 and 10). The voltage applied to the series circuit will be divided between the two resistances in the circuit (figures 11 and 12).

• The current will be divided

between the legs of the parallel circuit, based on the resistance of each leg. But the resistance of the series leg will equal the sum of the resistances in that part of the circuit.

• The total circuit resistance is the reciprocal of the sum of the recipro-cals of the individual resistances of the parallel circuit. You should remember that from the last issue of GEARS.

But in this case, one of the resis-tances is a series circuit. To determine the resistance of that leg in the parallel circuit, you must first add the two resis-tances in the series circuit, or measure the together (figure 13). That’ll give you the total resistance in that leg of the parallel circuit, so you can calculate the total circuit resistance.

Figure 9: Each leg of the parallel component of the circuit will have full system voltage applied…

Figure 10: …and each leg of the parallel component will also have full ground applied.

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44 GEARS January / February 2006

Understanding Circuits Helps with Diagnosis

Over the last few issues of GEARS, we’ve examined the rules that apply to the different types of circuits. And we’ve tested those rules to see what they mean, and how they affect voltage, resistance and current flow.

But why? Why would we need to know that the resistances in a series circuit will each use its percentage of the applied voltage… or that a parallel circuit’s total resistance is equal to the reciprocal of the sum of the recipro-cals?

Because, believe it or not, it’ll be useful during electrical diagnosis.

By being able to identify the type of circuit you’re dealing with, and understanding how voltage, amperage and resistance behave within those cir-cuits, you can anticipate circuit opera-tion… before you begin to check the circuit. Which means that, simply by examining a wiring schematic, you should be able to determine exactly what to look for in terms of voltage, amperage or resistance within virtually any circuit.

Imagine how valuable it would be to be able to open a schematic and say, “Okay, this is a series circuit with system voltage applied. So I should have system voltage to this connec-tor. And since I have the components’ resistances, I know exactly how much amperage should be flowing through-out the circuit.”

And, when voltage or amperage doesn’t behave as anticipated, you can analyze your test results, know exactly what type of problem to look for, and have a pretty good idea of where to look.

All of a sudden, you’re no longer at the mercy of the manufacturers and their prepackaged diagnostic proce-dures. You can analyze circuit behavior based solely on your understanding of electrical principles. That’s a really comfortable way to be able to approach an electrical problem.

In the next issue of GEARS, we’ll start looking at specific circuits, and how to use what we’ve learned to diag-nose those circuits. Until then, keep on pushing those electrons!

Figure 11: The voltage applied to the series component of the circuit will be divided between the two resistances.

Figure 12: Since both resistances are roughly equal, the voltage drop between the two resistances should also be roughly equal.

Figure 13: To determine the resistance of the parallel circuit, you must consider the resistances in the series component as a single resistance. From there, the calcula-

tion is the same as for any other parallel circuit.

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