The Next Step in Electrical Diagnosis: Series Circuits · The Next Step in Electrical Diagnosis: Series Circuits by Steve Bodofsky Figure 1: A series circuit connects multiple resistances

36 GEARS October 2005

In the last issue of GEARS we looked at simple circuits, and how Kirchoff’s Laws of Current and

Voltage applied to them. And we dis-covered how a voltage drop test can be an effective way to find unwanted resistance in a circuit.

In this issue, we’ll move on to a slightly more complex type of circuit: a series circuit (figure 1). As we examine series circuits, you’ll get a better idea of how Kirchoff’s Law of Voltage can help you identify both expected and unexpected resistances in a circuit.

The most familiar type of series circuit would have to be a simple string of Christmas tree lights. You know the ones: you plug in the string after being packed away all year, and… nothing happens. So then you start checking the bulbs, one at a time, until you find the single bulb that’s keeping the rest from lighting. Or you throw the whole string in the trash and buy a new one, which usually saves time in the long run.

The reason a single bulb can keep the rest from lighting is because the

string is wired in series: Each bulb pro-vides part of the path for electricity to reach the other bulbs. If one becomes loose or burns out, the circuit opens and keeps current from flowing.

It’s kind of a pain, but they’re cheap to produce, and by wiring the lights in series, they create the condi-

tions necessary for another familiar sight: The blinking string. You put a single blinking bulb in the string, and it causes the rest of the string to blink. Same reason: When the blinking bulb turns off, it opens the circuit, so the whole string goes out.

PUSHING ELECTRONS

The Next Step in Electrical Diagnosis:

Series Circuits

by Steve Bodofsky

Figure 1: A series circuit connects multiple resistances in series; that is, the nega-tive side of the first resistance connects to the positive side of the second, and so

on.

Figure 2: No matter where we move the current clamp along the circuit, current flow remains equal. So current is

equal throughout the series circuit. Figure 3: There’s 15.38 volts applied to the circuit.

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While series circuits aren’t particu-larly common in automotive applica-tions, they do provide us with a clear pic-ture of what to expect when performing a voltage drop test to identify unwanted resistance, such as a dirty or loose con-nection. So let’s examine the principles governing a series circuit, and see how they affect our diagnosis.

Principle 1: The current flow-ing through a series circuit is constant throughout the circuit.

Principle 2: The current must pass through each component in the circuit.

We’ll take the first two principles together, and this time, we’ll use a cur-rent clamp to measure the current at various places through the circuit (figure 2, page 36).

As you can see, no matter where we measure the current, it’s always the same. Which proves that current is indeed con-stant throughout the circuit, and that it does pass through each component.

This confirms a point about circuits that we discovered last time: It doesn’t matter where you check current flow in a circuit. Positive or negative side, or even in between the components, the current level will remain constant.

Principle 3: The amount of current that flows through the circuit depends on the total resistance of the circuit.

Actually, we’ve already proved this one. We know that current is always constant throughout the circuit; this is Kirchhoff’s Law of Current, which states:

The total current that goes into a junction is equal to the total current that comes out of a junction. There is no extra current, and none is lost.

We also know that current levels are a consequence of voltage and resistance: Ohm’s Law tells us that.

So, by applying these laws to our

38 GEARS October 2005

Figure 4: There’s no voltage left after the resistances, so the circuit resistances are using all of the applied voltage.

Figure 6: The resistance of the second bulb is 7.7 ohms.

Figure 5: The resistance of the first bulb is 6.6 ohms.

Series Circuits

Principle 1: The cur-

rent flowing through a

series circuit is constant

throughout the circuit.

GEARS October 2005 39

voltage, depending on its resistance. The sum of the voltage drops across the individual resistances will equal the applied voltage.

So each resistance in the circuit will only use part of the voltage — depending on its level of resistance — until all the voltage is used up. This is Kirchoff’s Law of Voltage, which says:

The sum of the voltage drops around the circuit will be equal to the voltage drop for the entire circuit.

Let’s check this out:We already measured the volt-

age before resistance 1 to prove principle 4; there was 15.38 volts applied (figure 3, page 36). A sim-ple voltage check shows 8.22 volts after the first resistance (figure 8).

Figure 8: The voltage after the first resistance is 8.22 volts; slightly more than half the applied voltage. So each resistance does use its share of

the applied voltage.

last proof, we know that principle 3 is correct: the total current depends on the total circuit resistance.

Principle 4: All of the voltage will be used up by pushing the current through the resistances in the circuit.

We proved this was true of a sim-ple circuit, and a quick check shows it’s also true of a series circuit. In this case we have 15.38 volts applied to the cir-cuit (figure 3, page 36), and zero volts after the resistances (figure 4, page 38). So all of the voltage is being used up in the circuit.

Remember, this is the basis behind a simple voltage drop test: Any change in voltage levels along a circuit indi-cates a resistance in the circuit. So if there’s voltage on the negative side of the circuit, it indicates unwanted resistance in the circuit. Or, if the volt-age on the positive side of a circuit is lower than system voltage, it indicates unwanted resistance in the circuit.

Principle 5: The total resistance of the circuit is the sum of the indi-vidual resistances.

The total resistance equals the sum of the resistances: makes sense, but is it true? Let’s check:

Resistance 1 (figure 5): 6.6 ohmsResistance 2 (figure 6): 7.7 ohmsTotal Resistance (figure 7): 14.3 ohms

So resistance 1 plus resistance 2 equals the total resistance; yep, it’s true.

Principle 6: Each resistance in the circuit will use its share of the applied

Figure 7: The total circuit resistance is 14.3 ohms; exactly the sum of the individual resistances.

And there was zero volts left after the second resistance (figure 4, page 38).

So the first resistance uses 7.16 volts (15.38 – 8.22), and the second resistance uses 8.22 volts (8.22 – 0.00); each resistance uses roughly half of

the voltage being applied — the first resistance using slightly less voltage than the second, because resistance 1 is slightly lower — just what principle 6 would indicate: The greater the resis-tance, the more voltage necessary to push the current through.

Principle 7: A series circuit is sometimes referred to as a voltage divider network.

Makes sense, based on the results of the checks for principle 6: A series circuit divides the voltage between the resistances, based on the amount of resistance in each one. In effect,

the resistances “divide” the voltage between them, creating a voltage divid-er network.

That’s all for now: Next time we’ll look at one of the more common types of automotive circuit: the parallel circuit. Until then, keep on pushing those electrons.

40 GEARS October 2005

Series Circuits

TEST1. A series circuit:

A. Has only one resistance.B. Has more than one resis-

tance, with all of the resis-tances wired directly to the voltage source and ground.

C. Has more than one resis-tance, with the negative for one resistance wired to pro-vide the positive for the next resistance.

D. Doesn’t really exist any-where in the world.

2. If one bulb burns out in a true series circuit…A. The rest won’t light.B. The rest will blink.C. The rest will get brighter.D. The rest will also burn out.

3. Tech A says current in a series circuit will be constant every-where in the circuit.Tech B says current must pass through all of the resistances in a series circuit.Who’s right?A. A only.B. B only.C. Both A and B.D. Neither A nor B.

4. The amount of current that flows through the circuit depends on the total resis-tance of the circuit. Which law

of electrical circuits backs this up?A. Ohm’s LawB. Kirchoff’s Law of CurrentC. Kirchoff’s Law of VoltageD. Both A and B.

5. Which of these conditions indicates unwanted resistance in a circuit?A. Applied voltage is lower than

the system voltage.B. Additional voltage on the

ground side of the circuit.C. Voltage is lost between two

of the resistances in a series circuit.

D. All of the above.

6. There are three resistances in a circuit:R1 = 5 ohmsR2 = 3 ohmsR3 = 6 ohmsWhat should the total resistance

of the circuit be?A. 2 ohmsB. 14 ohmsC. 9.8 ohmsD. 1.43 ohms

7. Tech A says the first resis-tance in the circuit will use up most of the applied voltage.Tech B says a series circuit is sometimes called a voltage divider network.

Who’s right?A. A only.B. B only.C. Both A and B.D. Neither A nor B.

8. Excess resistance in the nega-tive side of a series circuit will:A. Keep some voltage on the

negative lead.B. Reduce the applied voltage

to the circuit.C. Create a voltage drop

between the resistances.D. Prevent the last bulb in the

circuit from lighting.

9. Excess resistance between two bulbs in a series circuit will:A. Keep some voltage on the

negative lead.B. Reduce the applied voltage

to the circuit.C. Create a voltage drop

between the resistances.D. Prevent the last bulb in the

circuit from lighting.

10. The most familiar example of a series circuit is:A. Christmas tree lightsB. Traffic lightsC. 3-way bulbsD. Hanukkah menorah

Answer Key: 1.C, 2.A, 3.C, 4.D, 5.D, 6.B, 7.B, 8.A, 9.C, 10.A

Principle 7: A series circuit is

sometimes referred to as a

voltage divider network.

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