Basic electronics college algebra course manual

Before measuring the resistance of an unknown resistor or electrical circuit, the test leads of the ohmmeter are first shorted together, as shown in figure 1-31.. Section 2.6 Rating of R

Electronics

Section 1.1 Electronics Safety

Safety is everyone’s responsibility Everyone must cooperate to

create the safest possible working conditions Where your personal

life and good health are concerned, safety becomes your

responsibility whether you step in front of a speeding truck, or

expose yourself to a lethal shock, are matters over which you, as an

individual have more control than anyone else

Safety is simply a matter of applying common sense precautions

The rules of safety are concerned with the prevention of accidental

injuries sustained when an accident occurs

The general rules for shop safety apply equally to the

electrical-electronics laboratory The following important shop rules should be

observed at all times

+ + + +

1-1 Electronics Safety

1-2 Applications of Electronics

1-3 Digital Number Systems

1-4 Representing Binary Quantities

+ + + +

1 Don’t clown around or engage in horseplay Many painful injuries are caused by the

carelessness and thoughtless antics of the clown

2 Get your teacher’s approval before starting your work This will save your time and

help prevent accidents Remember your teacher is there to help you

3 Report all injuries at once, even the slightest A small cut can develop serious

complications if not properly treated

4 Wear safety glasses- when grinding or working in areas where sparks or chips of

metals are flying Remember that your eyes is a priceless possession

5 Keep the floors around your work area clean and free of litter which might cause

someone to slip or stumble

6 Use tools correctly and do not use them if they are not in proper working condition

7 Observe the proper methods of handling and lifting objects Get help to lift heavy

objects

8 Do not talk nor disturb a fellow student when he is operating a machine

9 Never leave the machine while it is running down Stay with it until it stops

completely

10 Obtain permission before you use power tools

Students and teachers who work with electricity face hazard of electrical shock and should make

every effort to understand the danger

Electricity can cause fatal burns or cause vital organs to malfunction In general, a current of 5 mA or

less will cause a sensation of shock, but rarely any damage Larger currents can cause hand muscles

to contract Currents on the order of 100 mA are often fatal if they pass through the body for even a

few seconds

The Electronics Workshop is primarily concerned with low-voltage electronics The chance of injury

due to electric shock is very, very, low Experiments for younger students have been designed to be

easily completed without the use of soldering

Nonetheless, as in all laboratory situations, there are safety rules that must be followed

The two most important safety rules are:

1 Always have a knowledgeable adult to supervise work

Ask a teacher or parent to help you

2 Always use common sense and pay attention to the job you are working on

Doing so can prevent most laboratory accidents

Electricity-electronics is a tremendous field and most of us do well to understand small segments of it

Ask questions when in doubt Be humble!

Every possible precaution has been taken to ensure the safety of experiments and the correctness of

information

The study of electronics is interesting and exciting Enjoy yourself and be safe

between radio frequency equipment to produce the carrier wave radiated from the antenna and the

audio and video equipment in the studio that supplies the modulating signal with the desired

information

High-fidelity audio equipment can be considered with radio receivers The receiver itself has audio

amplifiers to drive the loudspeaker that reproduce the sound

Satellite communications is also a transmit-receive system using electro-magnetic radio waves The

satellite just happens to be orbiting around the earth at a height of about 22,300 miles order to

maintain a stationary position relative to the earth Actually, the satellite is a relay station for

transmitter and receiver earth stations

Electric Power These applications are in the generation and distribution if 60-Hz AC power, as the

source of energy for electrical equipment Included are lighting, heating, motors, and generators

Electronics plays an important role in the control and monitoring of electrical equipments

Digital Electronics We see the digits 0 to 9 on an electronic calculator or digital watch, but digital

electronics has a much broader meaning The circuits for digital applications operate with pulses of

voltage or current, as shown in the diagram below A pulse waveform is either completely ON or OFF

because of the sudden changes in amplitude In-between values have no function Note that ON and

OFF stage can also be labeled as HIGH and LOW, or 1 and 0 in binary notation Effectively the digital

pulses correspond to the action of switching circuits that are either on or off

Voltage or current variations with a continuous set of values form an analog waveform, as shown

below The 60-Hz power line and audio and video signals are common examples Note that the

values between 0 and 10 V are marked to indicate that all the in-between values are an essential part

of a waveform

Actually, all the possible applications in the types of electronic circuits can be divided into two just two

types- digital circuits that recognize pulses when they are HIGH or LOW, and analog circuits that use

all values in the waveform The applications of digital electronics, including calculators, computers,

data processing and data communications, possibly form the largest branch of electronics In addition

many other applications, including radio and television, use both analog and digital circuits

In addition to all the general applications in communications, digital equipment, and electric services,

several fields that could be of specific interest include automotive electronics, industrial electronics,

and medical electronics Both digital and analog techniques are used

In automotive electronics, more and more electronic equipment is used in cars for charging the

battery, power assist functions, measuring gages, and monitoring and control of engine performance

Perhaps the most important application is the electronic ignition This method provides better timing

of the ignition spark, especially at high speeds On-board computer monitor and control a wide auto

functions

Industrial electronics includes control of welding and heating processes, the use of elevator control,

operation of copying machines Metal detectors and smoke detectors, moisture control, and

computer-controlled machinery In addition there are many types of remote control-functions, such as

automatic garage door openers and burglar alarms Closed-circuit television is often used for

surveillance

Medical electronics combines electronics with biology Medical research diagnosis, and treatment

all use electronic equipment Examples are the electron microscope and electrocardiograph machine

In hospitals, oscilloscopes are commonly used as the display to monitor the heartbeat of patients in

extensive care

Job titles

Job titles

Different specialties in electronics are indicated by the following titles for engineers: antenna, audio,

computer, digital, illumination, information theory, magnetic, microwave, motors and generators,

packaging, power distribution, radio, semiconductor, television, and test equipment Many of these

fields combine physics and chemistry, especially for semiconductors

The types of jobs in these fields include engineer for research, development, production, sales, or

management, teacher, technician, technical writer, computer programmer, drafter, service worker,

tester and inspector Technicians and service workers are needed for testing, maintenance and repair

of all the different types of electronic equipments

Resistors

The resistor's function is to reduce the flow of electric current This

symbol is used to indicate a resistor in a circuit diagram,

known as a schematic

Resistance value is designated in units called the "Ohm." A 1000

Ohm resistor is typically shown as 1Ohm ( kilo Ohm ), and 1000

K-Ohms is written as 1M-Ohm ( mega ohm )

There are two classes of resistors; fixed resistors and the variable

resistors They are also classified according to the material from

which they are made The typical resistor is made of either carbon

film or metal film There are other types as well, but these are the

most common

The resistance value of the resistor is not the only thing to consider

when selecting a resistor for use in a circuit The "tolerance" and the

electric power ratings of the resistor are also important

The tolerance of a resistor denotes how close it is to the actual rated

résistance value For example, a ±5% tolerance would indicate a

resistor that is within ±5% of the specified resistance value

The power rating indicates how much power the resistor can safely

tolerate Just like you wouldn't use a 6 volt flashlight lamp to replace

a burned out light in your house, you wouldn't use a 1/8 watt resistor

when you should be using a 1/2 watt resistor

The maximum rated power of the resistor is specified in Watts

Power is calculated using the square of the current ( I2 ) x the

resistance value ( R ) of the resistor If the maximum rating of the

resistor is exceeded, it will become extremely hot, and even burn

Resistors in electronic circuits are typically rated 1/8W, 1/4W, and

1/2W 1/8W is almost always used in signal circuit applications

When powering a light emitting diode, comparatively large current

flows through the resistor, so you need to consider the power rating

of the resistor you choose

This is the most general purpose, cheap resistor Usually the tolerance of the resistance value is

Carbon film resistors have a disadvantage; they tend to be electrically noisy Metal film resistors are recommended for use in analog circuits However, I have never experienced any problems with this noise

The physical size of the different resistors are as follows

From the top of the photograph

1/8W 1/4W 1/2W

Rough size Rating power

(W)

Thickness (mm)

Length (mm)

In the photograph on the left, 8 resistors are housed in the package Each of the leads on the package is one resistor The ninth lead on the left side is the common lead The face value of the resistance is printed ( It depends on the supplier )

Some resistor networks have a "4S" printed on the top of the resistor network The 4S indicates that the package contains 4 independent resistors that are not wired together inside The housing has eight leads instead of nine The internal wiring of these typical resistor networks has been illustrated below The size (black part) of the resistor network which I have is as follows: For the type with 9 leads, the thickness is 1.8 mm, the height 5mm, and the width 23 mm For the types with 8 component leads, the thickness is 1.8 mm, the height 5 mm, and the width 20 mm

Metal film resistors

Metal film resistors

Metal film resistors are used when a higher tolerance (more accurate value) is needed They are much more accurate in value than carbon film resistors They have about ±0.05% tolerance They have about ±0.05% tolerance I don't use any high tolerance resistors in my circuits Resistors that are about ±1% are more than sufficient Ni-Cr (Nichrome) seems to be used for the material of resistor The metal film resistor is used for bridge circuits, filter circuits, and low-noise analog signal circuits

From the top of the photograph 1/8W (tolerance ±1%) 1/4W (tolerance ±1%) 1W (tolerance ±5%) 2W (tolerance ±5%)

Rough size Rating power (W)

Thickness (mm)

Length (mm)

There are many types of these devices They vary according to light sensitivity, size, resistance value etc

Pictured at the left is a typical CDS photocell Its diameter is 8 mm, 4 mm high, with a cylinder form When bright light is hitting it, the value is about 200 ohms, and when in the dark, the resistance value is about 2M ohms This device is using for the head lamp illumination confirmation device of the car

resistor is the Ceramic resistor These are wirewound resistors in a ceramic case, strengthened with

a special cement They have very high power ratings, from 1 or 2 watts to dozens of watts These resistors can become extremely hot when used for high power applications, and this must be taken into account when designing the circuit These devices can easily get hot enough to burn you if you touch one

The photograph on the left is of wirewound resistors

The upper one is 10W and is the length of 45 mm, 13

The photograph on the left is a ceramic (or cement) resistor of 5W and

is the height of 9 mm, 9 mm depth, 22 mm width

Thermistor ( Thermally sensitive resistor )

Thermistor ( Thermally sensitive resistor )

The resistance value of the thermistor changes according to temperature

This part is used as a temperature sensor.There are mainly three types of thermistor

NTC(Negative Temperature Coefficient Thermistor)

: With this type, the resistance value decreases continuously as the temperature rises

PTC(Positive Temperature Coefficient Thermistor)

: With this type, the resistance value increases suddenly when the temperature rises above a specific point

CTR(Critical Temperature Resister Thermistor)

: With this type, the resistance value decreases suddenly when the temperature rises above a specific point

The NTC type is used for the temperature control

The relation between the temperature and the resistance value of the NTC type can be calculated

using the following formula

R : The resistance value at the temperature T

T : The temperature [K]

R0: The resistance value at the reference temperature T0

T0 : The reference temperature [K]

B : The coefficient

As the reference temperature, typically, 25°C is used

The unit with the temperature is the absolute temperature(Value of which 0 was -273°C) in K(Kelvin)

25°C are the 298 Kelvins

Section 2.2 Resistor color code

Because carbon resistors are small physically, they are color-coded to mark their value in ohms The

basis of this system is the use of colors for numerical values as listed in the table below In

memorizing the colors note that the darkest colors, black and brown, are for the lowest numbers, zero

and one, whereas white is for nine The color coding is standardized by the Electronic Industries

Association (EIA) These colors are also used for small capacitors

insulating body, which is usually tan Reading from left to right, the first band close to the edge gives

the first digit in numerical value of R The next band marks the second digit The third band is the

decimal multiplier, which gives the number of zeroes after the two digits

Resistors under 10ΩΩΩ For these values the third stripe is either gold or silver, indicating a fractional

decimal multiplier When the third digit is gold, multiply the first two digits by 0.1 Example, if the first

two digits are 25 then, 25 X 0.1 = 2.5 Ω Silver means a mult4iplier of 0.01 If the first two digits is still

25 then, 25 X 0.01 = 25 Ω

It is important to realize that the gold and silver colors are used as decimal multipliers only in the third

stripe However, gold and silver are used most often in the fourth stripe to indicate how accurate the

R value is

Resistor Tolerance The amount by which the actual R can be different from the color-coded value is

the tolerance, usually given in percent For instance, a 2000Ω resistor with 10 percent tolerance

can have resistance 10 percent above or below the coded value This R, therefore, is between 1800Ω

to 2200Ω The calculation are as follows:

Score:

Instructor’s signature: _

Date:

Remarks:

Exercise 2 Resistor Color Codes

I Fill up the table below for the expected value of the resistors in ohms and in kilo-ohms

given its color codes below (2 points per number)

Value in Ohms Value in K-ohms

1 Grey, Blue, Red, Silver

2 Yellow, Green, Gold, Gold

3 Violet, Brown, Black, Silver, Gold

4 Brown, Black, Red, Gold

5 Blue, Yellow, Orange, Silver

6 Brown, Black, Silver, Silver

7 Red, Red, Red, Gold

8 Green, Orange, Brown, Silver

9 Brown, Violet, Yellow, Gold

10 Blue, Black, Red, Orange, Gold

II Compute for the tolerance value of each resistor given its color codes.(2 points per number)

1 Red, Brown, Orange, Gold

The ohmmeter consists of a dc ammeter, with a few added features The added features are:

A dc source of potential (usually a 3-volt battery)

One or more resistors (one of which is variable) A simple ohmmeter circuit is shown in figure 2-1

The ohmmeter's pointer deflection is controlled by the amount of battery current passing through the

moving coil Before measuring the resistance of an unknown resistor or electrical circuit, the test

leads of the ohmmeter are first shorted together, as shown in figure 1-31

With the leads shorted, the meter is calibrated for proper operation on the selected range While the

leads are shorted, meter current is maximum and the pointer deflects a maximum amount,

somewhere near the zero position on the ohms scale Because of this current through the meter with

the leads shorted, it is necessary to remove the test leads when you are finished using the ohmmeter

If the leads were left connected, they could come in contact with each other and discharge the

ohmmeter battery When the variable resistor (rheostat) is adjusted properly, with the leads shorted,

the pointer of the meter will come to rest exactly on the zero position This indicates

ZZZZero Resistance ero Resistance ero Resistance

Between the test leads, which, in fact, are shorted together

The zero reading of a series-type ohmmeter is on the hand side of the scale, where as the zero reading for an ammeter or a voltmeter is generally to the left-hand side of the scale (There is another type of ohmmeter which is discussed a little later on in this chapter.) When the test leads of an ohmmeter are separated, the pointer of the meter will return to the left side of the scale

right-The interruption of current and the spring tension act on the movable coil assembly, moving the pointer to the left side (∞) of the scale

Figure 1-31 - A simple ohmmeter circuit

Using the Ohmmeter sing the Ohmmeter sing the Ohmmeter

After the ohmmeter is adjusted for zero reading, it is ready to be connected in a circuit to measure

resistance A typical circuit and ohmmeter arrangement is shown in figure 2-2

Figure 2-2 - Measuring circuit resistance with an ohmmeter

The power switch of the circuit to be measured should always

be in the OFF position This prevents the source voltage of the circuit from being applied across the meter, which could cause damage to the meter movement

The test leads of the ohmmeter are connected in series with the circuit to be measured (fig 1-32) This causes the current produced by the 3-volt battery of the meter to flow through the circuit being tested Assume that the meter test leads are connected at points a and b of figure 1-32 The amount of current that flows through the meter coil will depend on the total resistance of resistors R1 and R2, and the resistance of the meter Since the meter has been preadjusted (zeroed), the amount of coil movement now depends solely on the resistance of R1and R2 The inclusion of R1 and R2 raises the total series resistance, decreasing the current, and thus decreasing the pointer deflection The pointer will now come to rest at a scale figure indicating the

combined resistance of R1 and R2

If R1 or R2, or both, were replaced with a resistor(s) having a larger value, the current flow in the

moving coil of the meter would be decreased further The deflection would also be further decreased,

and the scale indication would read a still higher circuit resistance

Movement of the moving coil is proportional to the amount of current flow

O

Ohmmeter Ranges hmmeter Ranges hmmeter Ranges

The amount of circuit resistance to be measured may vary over a wide range In some cases it may be only a few ohms, and in others it may be as great as 1,000,000 ohms (1 megohm) To enable the meter to indicate any value being measured, with the least error, scale multiplication features are used in most ohmmeters For example, a typical meter will have four test lead jacks-COMMON, R X 1, R X 10, and R X 100 The jack marked COMMON is connected internally through the battery to one side of the moving coil of the ohmmeter

The jacks marked R X 1, R X 10, and R X 100 are connected to three different size resistors located within the ohmmeter This is shown in figure 2-3

Figure 1-33 - An ohmmeter with multiplication jacks

Some ohmmeters are equipped with a selector switch for selecting the multiplication scale desired, so only two test lead jacks are necessary

amount to cause a useful pointer deflection If the R X 100 range were used to measure the same

3,750-ohm resistor, the pointer would deflect still further, to the 37.5-ohm position This increased

deflection would occur because resistor R X 100 has about 1/10 the resistance of resistor R X 10

The foregoing circuit arrangement allows the same amount of current to flow through the meter's

moving coil whether the meter measures 10,000 ohms on the R X 10 scale, or 100,000 ohms on the

R X 100 scale

It always takes the same amount of current to deflect the pointer to a certain position on the scale

(midscale position for example), regardless of the multiplication factor being used Since the multiplier

resistors are of different values, it is necessary to ALWAYS "zero" adjust the meter for each

multiplication fact or selected

You should select the multiplication factor (range) that will result in the pointer coming to rest as near

as possible to the midpoint of the scale This enables you to read the resistance more accurately,

because the scale readings are more easily interpreted at or near midpoint

O

Ohmmeter Safety Precautions hmmeter Safety Precautions hmmeter Safety Precautions

The following safety precautions and operating procedures for ohmmeters are the MINIMUM

necessary to prevent injury and damage

Be certain the circuit is deenergized and discharged before connecting an ohmmeter

Do not apply power to a circuit while measuring resistance

When you are finished using an ohmmeter, switch it to the OFF position if one is provided

and remove the leads from the meter

Always adjust the ohmmeter for 0 (or ∞ in shunt ohmmeter) after you change ranges

before making the resistance measurement

Section 2.4 The Multimeter

A MULTIMETER is the most common measuring device used in the Navy The name multimeter

comes from MULTIple METER, and that is exactly what a multimeter is It is a dc ammeter, a dc

voltmeter, an ac voltmeter, and an ohmmeter, all in one package Figure 1-37 is a picture of a typical

multimeter

Figure 1-37 - A typical multimeter

The multimeter shown in figure 1-37 may look complicated, but it is very easy to use

You have already learned about ammeters, voltmeters, and ohmmeters; the multimeter is simply a combination of these meters

Most multimeters use a d'Arsonval meter movement and have a built-in rectifier for

ac measurement The lower portion of the meter shown in figure 1-37 contains the function switches and jacks (for the meter leads)

The use of the jacks will be discussed first

The COMMON or -jack is used in all functions is plugged into the COMMON jack The +jack is used for the second meter lead for any of the functions printed

in large letters beside the FUNCTION SWITCH (the large switch in the center) The other jacks have specific functions printed above or

below them and are self-explanatory (the output jack is used with the dB scale, which will not be

explained in this chapter) To use one of the special function jacks, except +10 amps, one lead is

plugged into the COMMON jack, and the FUNCTION SWITCH is positioned to point to the special

function (small letters) For example, to measure a very small current (20 microamperes), one meter

lead would be plugged into the COMMON jack, the other meter lead would be plugged into the 50A

AMPS jack, and the FUNCTION SWITCH would be placed in the 50V/IA AMPS position To measure

currents above 500 milliamperes, the +10A and -10A jacks would be used on the meter with one

exception

One meter lead and the FUNCTION SWITCH would be placed in the 10MA/AMPS position

M

Multimeter Controls ultimeter Controls ultimeter Controls

As described above, the FUNCTION SWITCH is used to select the function desired; the -DC, +DC,

AC switch selects dc or ac (the rectifier), and changes the polarity of the dc functions To measure

resistance, this switch should be in the +DC position

The ZERO OHMS control is a potentiometer for adjusting the 0 reading on ohmmeter functions

Notice that this is a series ohmmeter The RESET is a circuit breaker used to protect the meter

movement (circuit breakers will be discussed in chapter 2 of this module) Not all multimeters have

this protection but most have some sort of protection, such as a fuse When the multimeter is not in

use, it should have the leads disconnected and be switched to the highest voltage scale and AC

These switch positions are the ones most likely to prevent damage if the next person using the meter

plugs in the meter leads and connects the meter leads to a circuit without checking the function

switch and the dc/ac selector

M

Multimeter Scales ultimeter Scales ultimeter Scales

The numbers above the uppermost scale in figure 1-38 are used for resistance measurement If the

multimeter was set to the R x 1 function, the meter reading would be approximately 12.7 ohms

scale are used for the 2.5-volt ac function only

The lowest scale (labeled DB) will not be discussed The manufacturer's technical manual will explain the use of this scale

The table in figure 1-38 shows how the given needle position should be interpreted with various

functions selected

As you can see, a multimeter is a very versatile measuring device and is much easier to use than

several separate meters

PPPParallax Error arallax Error arallax Error

Most multimeters (and some other meters) have a mirror built into the scale Figure 1-39 shows the

arrangement of the scale and mirror

Figure 1-39 - A multimeter scale with mirror

The purpose of the mirror on the scale of a meter is to aid in reducing PARALLAX ERROR Figure

1-40 will help you understand the idea of parallax

Figure 1-40(A) shows a section of barbed wire fence as you would see it from one side of the fence

Figure 1-40(B) shows the fence as it would appear if you were to look down the fine of fence posts

and were directly in line with the posts You see only one post because the other posts, being in line,

are hidden behind the post you can see Figure 1-40(C) shows the way the fence would appear if you

moved to the right of the line of posts Now the fence posts appear to the right of the post closest to

you Figure 1-40(D) shows the line of fence posts as you would see them if you moved to the left of

the front post This apparent change in position of the fence posts is called PARALLAX

Parallax can be a problem when you are reading a meter Since the pointer is slightly above the scale

(to allow the pointer to move freely), you must look straight at the pointer to have a correct meter

reading In other words, you must be in line with the pointer and the scale Figure 1-41 shows the

effect of parallax error

Figure 1-41 - A parallax error in a meter reading

(A) shows a meter viewed correctly

The meter reading is 5 units Figure 1-41(B) shows the same meter as it would appear if you were to

look at it from the right The correct reading (5) appears to the right of the pointer because of parallax

The mirror on the scale of a meter, shown in figure 1-39, helps get rid of parallax error If there is any parallax, you will be able to see the image of the pointer in the mirror If you are looking at the meter correctly (no parallax error) you will not be able to see the image of the pointer in the mirror because the image will be directly behind the pointer Figure 1-42 shows how a mirror added

to the meter in figure 1-41 shows parallax error Figure 1-42(A) is

a meter with an indication of 5 units There is no parallax error in this reading and no image of the pointer is seen in the mirror

Figure 1-42(B) shows the same meter as viewed from the right

The parallax error is shown and the image of the pointer is shown

in the mirror

Figure 1-42 - A parallax error on a meter with a mirrored scale

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Multimeter Safety Precautions ultimeter Safety Precautions ultimeter Safety Precautions

As with other meters, the incorrect use of a multimeter could cause injury or damage The following

safety precautions are the MINIMUM for using a multimeter

Deenergize and discharge the circuit completely before connecting or disconnecting a

multimeter

Never apply power to the circuit while measuring resistance with a multimeter

Connect the multimeter in series with the circuit for current measurements, and in parallel for

voltage measurements

Be certain the multimeter is switched to ac before attempting to measure ac circuits

Observe proper dc polarity when measuring dc

When you are finished with a multimeter, switch it to the OFF position, if available If there is

no OFF position, switch the multimeter to the highest ac voltage position

Always start with the highest voltage or current range

Select a final range that allows a reading near the middle of the scale

Adjust the "0 ohms" reading after changing resistance ranges and before making a resistance

measurement

Be certain to read ac measurements on the ac scale of a multimeter

Observe the general safety precautions for electrical and electronic devices

follows

4 Complete the table below

Resistor Number

Color- Code Value

Expected Value Measured

Value

% Error R1

R2 R3 R4 R5 R6 R7

Variable Resistors

Variable resistors can be wire-wound or the carbon type Inside the metal case, the control has a circular disk that is carbon composition resistance element It can be a thin coating pressed o a paper or a molded carbon disk Joined to the two ends are the external soldering-lug terminals 1 and 3 The middle terminal is connected to the variable arm that contacts the resistor element by a metal spring wiper As the shaft of the control is turned, the variable arm moves the wiper to make contact at different points in the resistor element The same idea applies to the slide control, except that the resistor element is straight instead of circular

When the contact moves closer to the end, the R decreases between this terminal and the variable arm Between the two ends, however, the R is not variable but always has the maximum resistance of the control

Carbon controls are available with a total R from 1000 Ω to 5 MΩ, approximately Their power rating is usually ½ to 2 W

Rheostats and Potentiometers

Rheostats and Potentiometers

These are variable resistances, either carbon or wire-wound, used to vary the amount of current or

voltage in a circuit The controls can be used in either DC or AC applications

A rheostat is a variable R with two terminals connected in series with a load The purpose is to vary

the amount of current

A potentiometer, generally called a pot for short, has three terminals The fixed maximum R across

the two ends is connected across a voltage source The variable arm is used to vary the voltage

division between the center terminal and the ends This function of a potentiometer is compared with

a rheostat in the table below

In series with load and V source

Ends are connected across V source

This symbol is used to indicate a variable resistor

The two resistors on the left are the trimmer potentiometers

There are three ways in which a variable resistor's value can change according to the rotation angle of its axis

When type "A" rotates clockwise, at first, the resistance value changes slowly and then in the second half of its axis, it changes very quickly

The "A" type variable resistor is typically used for the volume control of a radio, for example It is well suited to adjust a low sound subtly It suits the characteristics of the ear The ear hears low sound changes well, but isn't as sensitive to small changes

in loud sounds A larger change is needed as the volume is increased These "A" type variable resistors are sometimes called "audio taper"

potentiometers

As for type "B", the rotation of the axis and the change of the resistance value are directly related

The rate of change is the same, or linear, throughout the sweep of the axis This type suits a

resistance value adjustment in a circuit, a balance circuit and so on They are sometimes called

"linear taper" potentiometers Type "C" changes exactly the opposite way to type "A" In the early

stages of the rotation of the axis, the resistance value changes rapidly, and in the second half, the

change occurs more slowly This type isn't too much used It is a special use

As for the variable resistor, most are type "A" or type "B"

Section 2.6 Rating of Resistors

In addition to having the required ohms value, a resistor should have a wattage rating high enough to

dissipate the power produced by the current flowing through the resistance, without becoming too hot

Carbon resistors in normal operation are quite warm, up to a maximum temperature of 85°C, which is

close to 100°C boiling point of water Carbon resistors should not be so hot, however that they

“sweat” beads of liquid on the insulating case Wire-wound resistors operate at very high

temperatures, a typical value being 300°C for the maximum temperature If a resistor becomes too

hot because of excessive power dissipation, it can change appreciably in resistance value or burn

open

The power rating is a physical property that depends on the resistor construction Note the following:

1 A larger physical size indicates a higher power rating

2 Higher-wattage resistors can operate at higher temperatures

3 Wire-wound resistors are physically larger with higher wattage ratings than carbon resistors

Section 2.7 Resistor Troubles

The most common trouble in resistors is an open circuit When the open resistor is a series

component, there is no current in the entire path

Noisy controls In applications such as volume and tone control, carbon controls are preferred

because the smoother change in resistance results in less noise when the variable arm is rotated

With use, however, the resistance element becomes worn by the wiper contact, making the control

noisy When a volume or tone control makes a scratchy noise as the shaft is rotated, it indicates a

worn out resistance element

Checking resistors with ohmmeter Resistance measurements are made with an ohmmeter The

ohmmeter has its own voltage source so that it is always used without any external power applied to

the resistance being measured Separate the resistance from the circuit by disconnecting one lead of

the resistor Then connect the ohmmeter lead across the resistance to be measured

An open resistor reads indefinitely high ohms For some reason, an infinite ohm is often confused

with zero ohms Remember, though, that an infinite ohm means an open circuit The current is zero,

but the resistance is infinitely high Furthermore it is practically impossible for a resistor to become

short-circuited in itself The resistor may be short-circuited by some other part of the circuit However,

he construction of resistors such that the trouble they develop is an open circuit with infinitely high

ohms

The ohmmeter must have an ohms scale capable of reading the resistance value, or the resistor

cannot be checked In checking a 10 MΩ resistor, for instance, if the highest R the ohmmeter can

read is 1 MΩ, it will indicate infinite resistance, even if the resistors’ normal value is 10 MΩ An ohms

scale of 100 MΩ or more should be used for checking such resistances

To check resistors of less than 10 Ω, a low ohms scale of about 100 Ω or less is necessary Center

scale should be 6 Ω or less Otherwise, the ohmmeter can read a normally low resistance value as

zero ohms

used in order to have equal distribution of I, V and P

In general, series resistors add for a higher RT With parallel resistors, REQ is reduced

Series Combinations of Resistors

Series Combinations of Resistors

Two elements are said to be in series whenever the same current physically flows through both of the

elements The critical point is that the same current flows through both resistors when two are in

series The particular configuration does not matter The only thing that matters is that exactly the

same current flows through both resistors Current flows into one element, through the element, out of

the element into the other element, through the second element and out of the second element No part

of the current that flows through one resistor "escapes" and none is added This figure shows several different ways that two resistors in series might appear as part of a larger circuit diagram

You might wonder just how often you actually find resistors in series The answer is that you find

resistors in series all the time

An example of series resistors is in house wiring The leads from the service entrance enter a distribution box, and then wires are strung throughout the house The current flows out of the distribution box, through one of the wires, then perhaps through a light bulb, back through the other wire We might model that situation with the circuit diagram shown below

In many electronic circuits series resistors are used to get a different voltage across one of the

resistors We'll look at those circuits, called voltage dividers, in a short while Here's the circuit

diagram for a voltage divider

Besides resistors in series, we can also have other elements in series - capacitors, inductors, diodes These elements can be in series with other elements For example, the simplest form of filter, for filtering low frequency noise out of a signal, can be built just by putting a resistor in series with a capacitor, and taking the output as the capacitor voltage

As we go along you'll have lots of opportunity to use and to expand what you learn about series combinations as you study resistors in series

Let's look at the model again We see that the wires are actually small resistors (small value of

resistance, not necessarily physically small) in series with the light bulb, which is also a resistor We

have three resistors in series although

two of the resistors are small We know

that the resistors are in series because

all of the current that flows out of the

distribution box through the first wire

also flows through the light bulb and

back through the second wire, thus

meeting our condition for a series

connection Trace that out in the circuit

diagram and the pictorial representation

above

Let us consider the simplest case of a series resistor connection, the case of just two resistors in

series We can perform a thought experiment on these two resistors Here is the circuit diagram for

the situation we're interested in

Imagine that they are embedded in an opaque piece of plastic, so that we only have access to the two nodes at the ends of the series connection, and the middle node is inaccessible If we measured the resistance of the combination, what would we find?

To answer that question we need to define voltage and current variables for the resistors If we take

advantage of the fact that the current through them is the same (Apply KCL at the interior node if you

are unconvinced!) then we have the situation below

Note that we have defined a voltage across each resistor (Va and Vb) and current that flows through both resistors (Is) and a voltage variable, Vs, for the voltage that appears across the series combination

What do we mean by series equivalent? Here are some points to observe

If current and voltage are proportional, then the device is a resistor

We have shown that Vs= Is X Rseries, so that voltage is proportional to current, and the constant

of proportionality is a resistance

We will call that the equivalent series resistance

There is also a mental picture to use when considering equivalent series resistance Imagine that you

have two globs of black plastic Each of the globs of black plasic has two wires coming out Inside

these two black plastic globs you have the following

In the first glob you have two resistors in series Only the leads of the series combination are

available for measurement externally You have no way to penetrate the box and measure things

at the interior node

In the second box you have a single resistor that is equal to the series equivalent Only the leads

of this resistor are available for measurement externally

Then, if you measured the resistance using the two available leads in the two different cases you

would not be able to tell which black plastic glob had the single resistor and which one had the series

combination

Here are two resistors At the top are two 2000W resistors At the bottom is single 4000W resistors

(Note, these are not exactly standard sizes so it took a lot of hunting to find a supply store that sold

them!) You can click the green button to grow blobs around them

After you have grown the blobs around the resistors there is no electrical measurement you can make

that will allow you to tell which one has two resistors and which one has one resistor They are

electrically indistinguishable! (Or, in other words, they are equivalent!)

Score:

Instructor’s signature: _

Date:

Remarks:

Exercise 4 Resistors in Series

Here is a circuit you may have seen before Answer the questions below for this circuit

1.Are elements #3 and #4 in series? (Yes or No)

2.Are elements #1 and #2 in series? (Yes or No)

3.Is the battery in series with any element?

° Element 1

° Element 2

° Element 3

° Element 4

4.Is the series equivalent resistor larger than either resistor, or is it smaller? (Larger or Smaller)

5 What is the series equivalent of two 1000 W resistors in series?

6 What is the series equivalent of a 1000 W resistor and a 2700 W resistor in series?

7 What is the series equivalent of three 1000 W resistors in series? You may want to do this

problem in two steps

8 Imagine that you have a 100 W resistor You want to add a resistor in series with this 100 W

resistor in order to limit the current to 0.5 amps when 110 volts is placed across the two resistors in

series How much resistance should you use?

Note that we have defined the voltage across both resistor (Vp) and the current that flows through

each resistor (Ia and Ib) and a voltage variable, Vp, for the voltage that appears across the parallel

combination

Let's list what we know

The voltage across the two resistors is the same

The current through the parallel combination is given by:

Here, we take Rparallel to be the parallel equivalent of the two resistors in parallel, and the expression

for Rparallel is:

1/Rparallel = 1/Ra + 1/Rb

There may be times when it is better to rearrange the expression for Rparallel The expression can be

rearranged to get:

Rparallel = (Ra*Rb)/(Ra + Rb) Either of these expressions could be used to compute a parallel equivalent resistance The first has a

certain symmetry with the expression for a series equivalent resistance

Parallel Resistors

Parallel Resistors A Point to Remember A Point to Remember A Point to Remember

It is important to note that the equivalent resistance of two resistors in parallel is always smaller than

either of the two resistors

Score:

Instructor’s signature: _

Date:

Remarks:

Exercise 5 Resistors in Parallel

1 Is the parallel equivalent resistor larger than either resistor, or is it smaller?

2 What is the parallel equivalent of two 1000 W resistors in parallel?

3 What is the parallel equivalent of a 1000 W resistor and a 1500 W resistor in parallel?

4 What is the equivalent of three 1000 W resistors in parallel? You may want to do this problem in

two steps

5.What is the equivalent resistance of this resistance combination?

6 What is the equivalent resistance of this resistance combination?

7 What is the equivalent resistance of this resistance combination? Here all three resistors are 33

kW Remember to input your answer in ohms

is a resistor It has two leads at the left (marked here with red dots) and we'll assume that we want to

find the equivalent resistance you would have at those leads

We will use the following numerical values for the resistors in this example, and we will work through

using these values

We need to figure out where we can start We can start by trying to find any of the combinations

we've learned about So let's think about whether there are any series or parallel combinations and if

there are let's see if we can identify them Then we can apply what we know about series and parallel

combinations There's no guarantee that approach will work, but it is worth a try Let's look at two

resistors at a time

Now, we should be able to replace the two resistors in series with their series equivalent If we do

that, there's a node in the middle with a voltage, and we'll lose information about that voltage Right

now, we're not interested in that voltage, and we'll willing to lose that information Let's just replace

the two resistors with their series equivalent Click the red button to make that replacement

Depressing the button will remove the two resistors in series, and releasing the button will insert the

replacement

Now you should have the circuit with the two resistors in series replaced by their series equivalent

Now, we can see that there is another replacement we can make What's that replacement?

Ok, you see how it goes Let's take a numerical example using the values mentioned above

Here is the circuit

1 What is the equivalent resistance of the two resistors in series - 1000W and 2000W?

2 Next you should have two resistors in parallel What is the parallel equivalent?

3 Now you should have two resistors in series attached to the source What is the value of the

series equivalent?

4 With a 12v source - as shown in the figure - what is the current that is drawn from the source?

Give your answer in amperes here Give your answer in milliamperes here, if that's what you

want

1.) If a current of 3 A is divided by the following circuit, the current flowing through the 4 Ohm resistor

2.) The diagram at the right shows part of a circuit into which a current I is flowing Which ammeter

shows the highest reading?

a A1

b A2

c A3

d All three ammeters give the same reading

3.) The diagram to the right represents a part of a

circuit containing an ohmic resistor, a voltmeter and an ammeter If the reading on the ammeter A

increases the reading on voltmeter V …

a increases in the same ratio

b increases but not in the same ratio

c remains unchanged

d decreases in the same ratio

4.) A battery is connected to two identical light bulbs in parallel as well as another identical bulb in

series An ammeter and a voltmeter are also connected as shown in the circuit diagram below

Voltmeter reading Ammeter reading

a increases increases

b increases decreases

d decreases decreases

5.) A learner connects a circuit as shown in the diagram to the right He/she uses a source of

electricity with an electromotive force (emf) of 12 V Which one of the following best gives the

ammeter and voltmeter readings which the learner is most likely to get with this circuit?

Ammeter reading Voltmeter reading

a reads zero reads zero

b reads zero reads 12 V

c very large reading reads zero

d very large reading reads 12 V

6.) Three identical resistors of 4 Ω are connected to give a combined resistance of 6 Ω Which of the

following circuit diagrams illustrates how this was done?

a I

b II

c III

d IV

7.) In the circuit to the right B1, B2 and B3 are identical light bulbs The internal resistance of the

Which statement is true regarding the relative brightness of the bulbs?

a The three bulbs glow with the same brightness

b B2 and B3 glow with the same brightness but brighter than B1

X Y

a brighter brighter

b dimmer dimmer

c brighter not lit up

d not lit up brighter 9.) A student connects three identical resistors as shown in the sketch to the right The potential

difference across the battery is 12 Volt What are the readings on V1 and V2 respectively?

10.) A 9 V battery is composed of six 1,5 V cells, which are connected in series Each cell has an

internal resistance of 0,2 Ω What is the highest current that can be obtained from such a battery?

a 7.5A

b 1.5A

c 1.2A

d 0.3A

Exercises on resistor connections

Find total resistance RT given the following circuits

c

3 Simple Series-Parallel

a

b

OHM'S LAW

What is Ohm’s Law

What is Ohm’s Law????

A simple relationship exists between voltage, current, and

resistance in electrical circuits Understanding this relationship is

important for fast, accurate electrical problem diagnosis and repair

Ohm's Law says: The current in a circuit is directly proportional to the

applied voltage and inversely proportional to the amount of

resistance This means that if the voltage goes up, the current flow

will go up, and vice versa Also, as the resistance goes up, the

current goes down, and vice versa Ohm's Law can be put to good

use in electrical troubleshooting But calculating precise values for

voltage, current, and resistance is not always practical nor, really

needed A more practical, less time-consuming use of Ohm's Law

would be to simply apply the concepts involved:

SOURCE VOLTAGE is not affected by either current or resistance It

is either too low, normal, or too high If it is too low, current will be

low If it is normal, current will be high if resistance is low, or current

will be low if resistance is high If voltage is too high, current will be

high

CURRENT is affected by either voltage or resistance If the voltage

is high or the resistance is low, current will be high If the voltage is

low or the resistance is high, current will be low

RESISTANCE is not affected by either voltage or current It is either

too low, okay, or too high If resistance is too low, current will be high

at any voltage If resistance is too high, current will be low if voltage

is okay

NOTE: When the voltage stays the same, such as in an Automotive

Circuit current goes up as resistance goes down, and current goes

down as resistance goes up Bypassed devices reduce resistance,

causing high current Loose connections increase resistance,

causing low current

C H A P T E R

+ + + +

3-1 Ohm’s Law Formula

3-2 Applications of Ohm’s Law

Current, Voltage and Resistance Calculations in:

Section 3.1 Ohm’s Law Formula

When voltage is applied to an electrical circuit, current flows in the circuit The following special

relationship exists among the voltage, current and resistance within the circuit: the size of the current

that flows in a circuit varies in proportion to the voltage which is applied to the circuit, and in inverse

proportion to the resistance through which it must pass This relationship is called Ohm's law, and can

be expressed as follows:

E = I R Voltage = Current x Resistance

E Voltage applied to the circuit, in volts (V)

I Current flowing in the circuit, in amperes (A)

R Resistance in the circuit, in ohms

In practical terms "V = I x R" which means "Voltage = Current x Resistance"

1 volt will push one amp through 1 ohm of resistance

NOTE: E = IR, V=AR, or V=IR are all variations of the same formula How you learned Ohm's law will

determine which one you will use Personal preference is the only difference; anyone will get you the

correct answer

OHM'S LAW SYMBOL SHORTCUT

OHM'S LAW SYMBOL SHORTCUT

Mathematical formulas can be difficult for many who don't use them regularly Most people can

remember a picture easier than a mathematical formula By using the Ohms law symbol below,

anyone can remember the correct formula to use By knowing any two values you can figure out the

third Simply put your finger over the portion of the symbol you are trying to figure out and you have

your formula

Section 3.2 Application of Ohm’s Law

As an application of Ohm's law, any voltage V, current I or resistance R in an electrical circuit can be

determined without actually measuring it if the two others values are known

This law can be used to determine the amount of current I flowing in the circuit when voltage V is

applied to resistance R As stated previously, Ohm's law is:

Current = Voltage / Resistance

In the following circuit, assume that resistance R is 2 and voltage V that is applied to it is 12 V Then,

current I flowing in the circuit can be determined as follows:

This law can also be used to determine the voltage V that is needed to permit current I to pass

through resistance R: V = I x R (Voltage= Current x Resistance)

In the following circuit, assume that resistance R is 4 ohms The voltage V that is necessary to permit

a current I of 3 A to pass through the resistance can be determined as follows:

Still another application of the law can be used to determine the resistance R when the voltage V

which is applied to the circuit and current I flowing in the circuit are already known:

In the following circuit, assume that a voltage V of 12 V is applied to the circuit and current I of 4 A

flows in it Then, the resistance value R of the resistance or load can be determined as follows:

TYPES OF CIRCUITS

TYPES OF CIRCUITS

Individual electrical circuits normally combine one or more resistance or load devices The design of

the automotive electrical circuit will determine which type of circuit is used There are three basic

types of circuits:

Series Circuit

Parallel Circuit

Series-Parallel Circuit

Section 3.3 Series Circuits

A series circuit is the simplest circuit The conductors, control and protection devices, loads, and

power source are connected with only one path to ground for current flow The resistance of each

device can be different The same amount of current will flow through each The voltage across each

will be different If the path is broken, no current flows and no part of the circuit works Christmas tree

lights are a good example; when one light goes out the entire string stops working

A Series Circuit has only one path to ground, so electrons must go through each component to get

back to ground All loads are placed in series

Therefore:

1 An open in the circuit will disable the entire circuit

2 The voltage divides (shared) between the loads

3 The current flow is the same throughout the circuit

4 The resistance of each load can be different

SERIES CIRCUIT CALCULATIONS

SERIES CIRCUIT CALCULATIONS

If, for example, two or more lamps (resistances R1 and R2, etc.) are connected in a circuit as follows,

there is only one route that the current can take This type of connection is called a series connection

The value of current I is always the same at any point in a series circuit