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- 1. ELECTRICAL CIRCUITSElectrical CircuitsA complete path, or circuit, is needed before current path if too much current flows. Too muchvoltage can cause a current flow through current is called an overload, which couldresistances to perform work. damage conductors and working devices. A list of five things to look for in any circuit:There are several types of circuits, but all requirethe same basic components. A power source 1. Source of Voltage(battery or alternator) produces voltage, or 2. Protection Deviceelectrical potential. Conductors (wires, printedcircuit boards) provide a path for current flow. 3. LoadWorking devices, or loads (lamps, motors), 4. Controlchange the electrical energy into another form ofenergy to perform work. Control devices 5. Ground(switches, relays) turn the current flow on andoff. And, protection devices (fuses, circuit We will be identifying these items when we look atbreakers) interrupt the Automotive Circuits a little later in this book. Page 1 © Toyota Motor Sales, U.S.A., Inc. All Rights Reserved.
- 2. ELECTRICAL CIRCUITSTypes Of CircuitsThere are three basic types of circuits: series,parallel, and series-parallel. The type of circuitis determined by how the power source,conductors, loads, and control or protectivedevices are connected.SERIES CIRCUITA series circuit is the simplest circuit. Theconductors, control and protection devices, loads,and power source are connected with only onepath for current. The resistance of each devicecan be different. The same amount of current willflow through each. The voltage across each willbe different. If the path is broken, no currentflows.PARALLEL CIRCUITA parallel circuit has more than one path forcurrent flow. The same voltage is applied acrosseach branch. If the load resistance in each branchis the same, the current in each branch will be thesame. If the load resistance in each branch isdifferent, the current in each branch will bedifferent. If one branch is broken, current willcontinue flowing to the other branches.SERIES-PARALLEL CIRCUITA series-parallel circuit has some components inseries and others in parallel. The power sourceand control or protection devices are usually inseries; the loads are usually in parallel. The samecurrent flows in the series portion, differentcurrents in the parallel portion. The same voltage isapplied to parallel devices, different voltages toseries devices. If the series portion is broken,current stops flowing in the entire circuit. If aparallel branch is broken, current continuesflowing in the series portion and the remainingbranches. Page 2 © Toyota Motor Sales, U.S.A., Inc. All Rights Reserved.
- 3. ELECTRICAL CIRCUITSSERIES CIRCUITSIn a series circuit, current has only one path. All voltage on the other side of the load. The drop orthe circuit components are connected so that the loss in voltage is proportional to the amount ofsame amount of current flows through each. The resistance. The higher the resistance, the highercircuit must have continuity. If a wire is the voltage drop.disconnected or broken, current stops flowing. Ifone load is open, none of the loads will work. When troubleshooting, then, you can see that more resistance will reduce current and less resistanceUse of Ohms Law will increase current. Low voltage would also reduce current and high voltage would increaseApplying Ohms Law to series circuits is easy. current. Reduced current will affect componentSimply add up the load resistances and divide the operation (dim lamps, slow motors). But, increasedtotal resistance into the available voltage to find the current will also affect component operation (earlycurrent. The voltage drops across the load failure, blown fuses). And, of course, no current atresistances are then found by multiplying the all would mean that the entire circuit would notcurrent by each load resistance. For calculation operate. There are electrical faults that can causeexamples, see page 6 in the Ohms law section. such problems and knowing the relationshipVoltage drop is the difference in voltage between voltage, current, and resistance will help(pressure) on one side of a load compared to the to identify the cause of the problem. Page 3 © Toyota Motor Sales, U.S.A., Inc. All Rights Reserved.
- 4. ELECTRICAL CIRCUITSPARALLEL CIRCUITSIn a parallel circuit, current can flow through more the smallest load resistance. This makes sensethan one path from and to the power source. The because current can flow through more than onecircuit loads are connected in parallel legs, or path. Also, remember that the voltage drop acrossbranches, across a power source. The points each branch will be the same because the sourcewhere the current paths split and rejoin are called voltage is applied to each branch. For examples ofjunctions. The separate current paths are called how to calculate parallel resistance, see page 6.branch circuits or shunt circuits. Each branchoperates independent of the others. If one load When troubleshooting a parallel circuit, the loss ofopens, the others continue operating. one or more legs will reduce current because the number of paths is reduced. The addition of one orUse of Ohms Law more legs will increase current because the number of paths is increased. Current can also beApplying Ohms Law to parallel circuits is a bit reduced by low source voltage or by resistance inmore difficult than with series circuits. The reason the path before the branches. And, current can beis that the branch resistances must be combined to increased by high source voltage or by one orfind an equivalent resistance. Just remember that more legs being bypassed. High resistance in onethe total resistance in a parallel circuit is less than leg would affect component operation only in that leg. Page 4 © Toyota Motor Sales, U.S.A., Inc. All Rights Reserved.
- 5. ELECTRICAL CIRCUITSSERIES-PARALLEL CIRCUITSIn a series-parallel circuit, current flows through The total resistance is then divided into the sourcethe series portion of the circuit and then splits to voltage to find current. Voltage drop across seriesflow through the parallel branches of the circuit. loads is current times resistance. Current inSome components are wired in series, others in branches is voltage divided by resistance. Forparallel. Most automotive circuits are series- calculation examples, see page 6.parallel, and the same relationship betweenvoltage, current, and resistance exists. When troubleshooting a series-parallel circuit, problems in the series portion can shut down theUse of Ohms Law entire circuit while a problem in one leg of the parallel portion may or may not affect the entireApplying Ohms Law to series-parallel circuits is a circuit, depending on the problem. Very highmatter of simply combining the rules seen for resistance in one leg would reduce total circuitseries circuits and parallel circuits. First, calculate current, but increase current in other legs. Verythe equivalent resistance of the parallel loads and low resistance in one leg would increase totaladd it to the resistances of the loads in series. circuit current and possibly have the effect of bypassing other legs. Page 5 © Toyota Motor Sales, U.S.A., Inc. All Rights Reserved.
- 6. ELECTRICAL CIRCUITSOhms Law sample circuits. Current found by dividing voltage by resistance. This can be very helpful whenFast, accurate electrical troubleshooting is easy diagnosing electrical problems:when you know how voltage, current, andresistance are related. Ohms Law explains the • When the resistance stays the same ... currentrelationship: goes up as voltage goes up, and current goes down as voltage goes down. A discharged battery• Current (amps) equals voltage (volts) divided by has low voltage which reduces current. Some resistance (ohms) ... I = E ÷ R. devices may fail to operate (slow motor speed). An unregulated alternator may produce too much• Voltage (volts) equals current (amps) times voltage which increases current. Some devices resistance (ohms) ... E = I X R. may fail early (burned-out lamps).• Resistance (ohms) equals voltage (volts) divided • When the voltage stays the same ... current goes by current (amps) ... R ÷ E = 1. up as resistance goes down, and current goes down as resistance goes up. Bypassed devicesUSING OHMS LAW reduce resistance, causing high current. Loose connections increase resistance, causing low current.The effects of different voltages and differentresistances on current flow can be seen in the Page 6 © Toyota Motor Sales, U.S.A., Inc. All Rights Reserved.
- 7. ELECTRICAL CIRCUITSSAMPLE CALCULATIONS Ohms law includes these two ideas:Here are some basic formulas you will find helpful 1. In a circuit, if resistance is constant, currentin solving more complex electrical problems. They varies directly with voltage.provide the knowledge required for confidenceand thorough understanding of basic electricity. Now what this means is that if you take a component with a fixed resistance, say a light bulb,The following abbreviations are used in the and double the voltage you double the currentformulas: flowing through it. Anyone who has hooked a six- volt bulb to a twelve-volt circuit has experienced E = VOLTS this. But it wasnt "too many volts" that burned out I = AMPS the bulb, it was too much current. More about that R = OHMS later. P = WATTS 2. In a circuit, if voltage is constant, current varies• Ohms Law inversely with resistance.Scientifically stated, it says: "The intensity Of the This second idea states that when resistance goescurrent in amperes in any electrical circuit is equal up, current goes down. Thats why corrodedto the difference in potential in volts across the connectors cause very dim lights - not enoughcircuit divided by the resistance in ohms of the current.circuit." Simply put it means that current is equal tovolts divided by ohms, or expressed as a formula, • Wattsthe law becomes: A watt is an electrical measurement of power or I=E/R work. It directly relates to horsepower. In fact, in the Sl metric standards that most of the worldor it can be written: uses, engine power is given in watts or kilowatts. E=IXR Electrical power is easily calculated by the formula:This is important because if you know any two of P=EXIthe quantities, the third may be found by applyingthe equation. For instance, a halogen high-beam headlight is rated or 5 amps of current. Figuring 12 volts in the system, we could write: P=EXI P = 12 X 5 P = 60 watts Page 7 © Toyota Motor Sales, U.S.A., Inc. All Rights Reserved.
- 8. ELECTRICAL CIRCUITSRESISTANCE That becomes:The effect of individual resistors on the totalresistance of a circuit depends on whether thecircuit is series or parallel. Which becomes:Series CircuitsIn a series circuit, the total resistance is equal tothe sum of the individual resistors: So there is a little more than one-half ohm resistance in the circuit. You can see that the moreSERIES: resistors in parallel, the less the resistance.total R = R1 + R2 + R3 + In fact, the total resistance is always less than the smallest resistor. This is why a fuse will blow ifThat is the basis of the concept of voltage drop. you add too many circuits to the fuse. There are soFor example, if you had a circuit with three loads in many paths for the current to follow that the totalseries (a bulb, resistor, and corroded ground) you resistance of the circuit is very low. That meanswould add the three together to get total the current is very high - so high that the fuse canresistance. And, of course, the voltage would no longer handle the load.drop across each load according to its value. B. For two resistors:Parallel CircuitsParallel circuits are a different story. In a parallelcircuit, there are three ways to find total For a 3 ohm and a 5 ohm resistor that would be:resistance. Method A works in all cases. Method Bworks only if there are two branches, equal ornot. Method C works only if the branches are ofequal resistance.A. The total resistance is equal to one over the C. For several identical resistors, divide the value sum of the reciprocals of the individual of one resistor by the number of resistors, or: resistors. That sounds confusing, but looking at the formula will make it clearer:PARALLEL: Where R1 is the value of one resistor and n is the number of resistors. So if you had three 4 ohm resistors in parallel it would be:n example will make it even clearer. Suppose thereis a circuit with three resistors in parallel: 4 ohms,2 ohms, and 1 ohm. The formula would look likethis: Page 8 © Toyota Motor Sales, U.S.A., Inc. All Rights Reserved.
- 9. ELECTRICAL CIRCUITSPage 9 © Toyota Motor Sales, U.S.A., Inc. All Rights Reserved.
- 10. ELECTRICAL CIRCUITSASSIGNMENT NAME:1. Draw and label the parts of a Series Circuit and a Parallel Circuit.2. Explain the characteristics of “Voltage” and how it differs between a Series Circuit and a Parallel Circuit.3. Explain the characteristics of “Current” and how it differs between a Series Circuit and a Parallel Circuit.4. Explain the characteristics of “Resistance” and how it differs between a Series Circuit and a Parallel Circuit.

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