ELEC 103
LABORATORY EXERCISE 1
USE OF THE MULTIMETER
PURPOSE OF EXPERIMENT
The purpose of this experiment is to determine the resistance of two circuit components, light bulb #1
and light bulb #2. In this experiment, you will determine the resistance of the light bulb's by measuring
the current flowing through each bulb and the voltage drop across each bulb. Then Ohm's Law will be
used to calculate the resistance of each bulb by dividing the voltage measured across the light bulb by
the current measured flowing through the light bulb.
EQUIPMENT LIST
1 − DC Power Supply
2 − 6V Light Bulb's
1 − Simpson Model 260 VOM
PROCEDURE − PART 1
1.
Carefully examine the Simpson Model 260
multimeter and note the location and configuration of
the following controls and scales:
RANGE/FUNCTION SWITCH is located in the center of
the control panel. It has 12 positions including five
positions for voltage. They are oriented to the left.
Four positions for current are located across the top
and upper right. And three positions for resistance
located to the right.
STATUS/POLARITY SWITCH is located on the left side
of the control panel. This switch usually has an off or
transport position, a +DC position, a −DC position,
and an AC position.
ZERO OHMS ADJUST KNOB is located on the right side
of the control panel. This knob is used to adjust the
meter pointer to zero ohms on the resistance ranges
when the meter leads are connected together.
DC SCALE is the second scale down from the top under the
glassed portion of the multimeter. It is sometimes
located directly under the parallax mirror. This scale is used for all DC voltage and current
ranges. The scale is divided into 50 divisions and is similar to the scale used in the figures
during your Unit One learning experiences. When using the 2.5 volt range switch position and
when using the 500 volt range/function switch position, you must mentally move the decimal
point to the appropriate position. On all other range switch positions the numbers on the scale
are appropriate.
COMMON (−) AND PLUS (+) RECEPTACLES are located in the lower left corner of the control
panel. Always make certain the black lead is connected to the minus or negative receptacle and
the red lead is connected to the plus or positive receptacle.
Page 1
ELEC 103
LABORATORY EXERCISE 1
USE OF THE MULTIMETER
PROCEDURE − PART 2
NOTE: It is not always possible to determine the
approximate value of the current flowing in a circuit before
you measure it with the milliammeter. Therefore, when the
milliammeter is connected in series with the element you
should start with the range switch adjusted to the highest
current measuring range. After power has been applied to
the circuit, adjust the range switch to lower current ranges
until you obtain an appropriate current measurement. If the
meter pointer moves to the left, reverse the meter leads and
continue with the experiment.
1. Connect the circuit as shown in Figure 1−1. Be
certain to connect the milliammeter to the circuit so it
is connected in series with the circuit. Additionally,
connect the meter to the circuit observing correct
polarity. The negative lead of the milliammeter is
connected toward the negative or ground side of the
power source, and the positive lead is connected
toward the positive side of the power source. Use the
highest current range possible.
2. Close the switch. (Turn the switch on.) You should see no current flow indicated by the meter
pointer since you are on a very high
current range. Adjust the range/function
switch to the next lower current
measuring range position. Continue
doing this until the meter pointer moves
up scale when you have adjusted the
range/function switch to a lower range.
Your reading on the meter will tell you
if it is safe to go to the next lower scale.
For example, if the meter pointer shows
a reading of 15 milliamperes when it is
Figure 1−1
on the 100 milliampere range, then it
would NOT be safe to go to the 10
milliampere range.
CONCLUSION
The resistance of a component can be determined by calculation when the voltage across the
component and the current through the component has been measured.
NOTE: The resistance of the bulb is different when the bulb is HOT than when it is cold. The
resistance of the bulb cannot be measured directly when it is hot and has current flowing
through it. However, the resistance of the bulb's filament can be determined indirectly using
the voltage and current measurements, and Ohm's Law as was done in this experiment.
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ELEC 103
LABORATORY EXERCISE 1
USE OF THE MULTIMETER
DATA TABLES
4.
5.
6.
Record the current you
measured in step 3 in the box to
the right.
It = ____________ mA
Measure the voltage across
light bulb #1 and record it as
V1. The correct polarity must be
observed when connecting the
voltmeter to the circuit.
V1 = ___________ V
Measure the voltage across
light bulb #2 and record it as V2
The correct polarity must be
observed when connecting the
voltmeter to the circuit.
V2 = ___________ V
7. Calculations using Ohms Law:
R1 =
Volts
Amps
R2 =
Volts
Amps =
8. Calculations using Ohms Law:
Page 3
=
R1 = ____________ Ω
R2 = ____________ Ω
ELEC 103 LABORATORY EXERCISE 2 VOLTAGE MEASUREMENTS
PURPOSE OF EXPERIMENT
To familiarize the user with the DC voltmeter scales and ranges of a general purpose multimeter such
as the Simpson, Model 260 multimeter.
EQUIPMENT LIST
2 − 6 Volt Dry cell batteries
2 − 1.5 Volt Dry cell batteries
1 − Simpson, Model 260 multimeter
1 − 1000Ω, 1 watt resistor
PROCEDURE
1. Move the meter STATUS/POLARITY switch to the +DC
position. Move the RANGE SWITCH to the 10 Volt
range position. Check the meter leads to be certain they
are plugged into the correct receptacles. The black lead
should be plugged into the minus/common receptacle and
the red lead should be plugged into the plus receptacle.
2. Connect the negative lead (black lead) of the voltmeter to
the negative (minus) post on one of the 6 Volt batteries.
Connect the positive lead (red lead) of the voltmeter to the
positive (plus) post on the same 6 Volt battery. The meter
pointer should have moved up the scale (to the right). If the
meter pointer moved to the left, you must immediately
disconnect the meter and return to step 1 and start the
experiment again.
3. Read the meter and record the voltage in the appropriate
place on the datasheet supplied with this experiment.
4. Without disconnecting the meter from the circuit, move the
RANGE SWITCH to the 50 Volt range. Read the meter and record the voltage in the
appropriate place on the datasheet.
5. Without disconnecting the meter from the circuit, move the RANGE SWITCH to the 250 Volt
range. Read the meter and record the voltage in the appropriate place on the datasheet.
6. Repeat step 2 through step 5 for the other 6 Volt battery and record the voltage measurements in
the appropriate place on the datasheet.
7. Using one of the test leads to connect the batteries
together, connect the plus (+) terminal of one of the 6
Volt batteries to the negative (−) terminal of the other 6
Volt battery. This is a SERIES AIDING
CONNECTION. Draw the fully labeled schematic, with
appropriate electrical symbols, including the voltmeter,
Figure 2−1
on the datasheet. Refer to the schematic in Figure 2−1
for some hints in drawing the schematic.
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ELEC 103 LABORATORY EXERCISE 2 VOLTAGE MEASUREMENTS
8.
9.
10.
11.
12.
13.
14.
Move the RANGE/FUNCTION SWITCH to an appropriate range to measure the batteries when
they are connected as described in step 7. Observe correct polarity when connecting the
Voltmeter to the unused terminals of the 6 Volt batteries. Record the measured voltage on the
datasheet.
Measure the voltage of a 1.5 Volt battery using the correct RANGE SWITCH position. Record
the voltage on the datasheet.
Connect the negative terminal of the 1.5 Volt battery to the positive terminal of one of the 6 Volt
batteries. Again, as in step seven, this is called a SERIES AIDING CONNECTION. The battery
voltages add in a series aiding connection. Select the appropriate meter range and measure the
voltage across the two batteries. Draw a schematic of the circuit, including the voltmeter, in the
space provided on the datasheet. Also, record the measured voltage.
Connect the positive terminal of one 6 Volt battery to the positive terminal of a 1.5 Volt battery.
Draw the fully labeled schematic, including the voltmeter, in the space provided on the data
sheet. This is called a SERIES OPPOSING CONNECTION.
Measure the voltage across the two batteries using the correct RANGE SWITCH position.
Record the measured voltage on the data sheet.
Using test leads, connect a series combination of a 1.5 Volt battery, a 6 Volt battery and a 1000
Ω resistor with the voltmeter across the resistor, as shown in Figure 2−1. Draw a schematic of
the circuit. Record the measured voltage on the data sheet.
Reverse the test leads to the 6 Volt battery in the circuit shown in Figure 2−1. Connect the meter
to the circuit with the same polarity shown in Figure 2−1. However, before making the
measurement, move the STATUS/POLARITY SWITCH to minus DC. Draw a schematic of the
circuit. Record the measured voltage on the data sheet.
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ELEC 103 LABORATORY EXERCISE 2 VOLTAGE MEASUREMENTS
DATASHEET FOR EXPERIMENT 2
STEPS 3 − 6: Six Volt Battery Voltage Readings.
Battery #1
Battery #2
6 Volt battery using the 10 Volt Range
_________
_________
6 Volt battery using the 50 Volt Range
_________
_________
6 Volt battery using the 250 Volt Range
_________
_________
STEP 7:
Step 8:
Schematic − Two Batteries Connected in SERIES AIDING.
Series Aiding Voltage Measurement.
Voltage = _____________ Range = _____________
Step 9:
1.5 Volt Battery Measurement.
Voltage = _____________ Range = _____________
STEP 10:
Schematic − Two Batteries Connected in SERIES AIDING.
Voltage = __________
Range
Page 3
= __________
ELEC 103 LABORATORY EXERCISE 2 VOLTAGE MEASUREMENTS
STEP 11:
Schematic − Two Batteries Connected in SERIES OPPOSING.
Step 12:
Series Opposing Voltage Measurement.
Voltage = _____________ Range = _____________
STEP 13:
Schematic − Series Aiding Sources series circuit.
Voltage = _________
Range = _________
STEP 14:
Schematic − Series Opposing Sources series circuit.
Voltage = __________
Range = _________
Page 4
ELEC 103 LABORATORY EXERCISE 3 CURRENT MEASUREMENTS
PURPOSE OF EXPERIMENT
To familiarize the user with the DC milliammeter scales and ranges of a general purpose multimeter
such as the Simpson, Model 260 multimeter, and acquaint the user with current measuring techniques.
EQUIPMENT LIST
2 − 6 Volt Dry cell batteries
1 − 1.5 Volt Dry cell battery
1 − Simpson, Model 260 multimeter
1 − 1000Ω, 10,000Ω, 1 Watt Resistor
PROCEDURE I − CURRENT MEASUREMENT IN SERIES CIRCUITS
1. Using test leads, connect the series circuit as shown in
Figure 3−1. Do not connect the meter into the circuit.
On the meter, place the STATUS/POLARITY
SWITCH in the +DC position and the
RANGE/FUNCTION SWITCH in the 10 mA range
position.
2. NOTE Connecting the milliammeter into the circuit
Figure 3−1
requires you to place the meter in SERIES WITH
THE ELEMENT. To measure current one must first break the circuit at the place the
measurement is desired. Then, complete the circuit using the milliammeter leads. Connect them
with the negative lead toward the negative side of the source and the positive lead toward the
positive side of the source. In the circuit of Step #1, where two sources are used, you will use
the larger of the two sources to determine the polarity of the source.
Connecting the milliammeter as a voltmeter or reversing meter polarity, can cause
serious damage to the multimeter. Also, current measurements must be made with a
resistor in the circuit. A current meter must never be connected directly to the source.
3. Insert the milliammeter into the circuit. Draw a schematic of the circuit with polarities
indicated and the direction of current flow shown. Then, read and record the measured current
in the space provided on the datasheet.
4. Remove the meter from the circuit and place a test lead in the break. Now, break the circuit
between the negative terminal of the 1.5 Volt battery and the 1000Ω resistor. Insert the meter
into the break. Use the larger of the two batteries to determine the source polarity. Observe
correct polarity when inserting the milliammeter into the break.
5. Draw a schematic of the circuit as it is presently connected. Read the milliammeter and record
the measured current in the space provided on the datasheet.
6. In the previous circuits, the two batteries were connected in series opposing. In as much as
current in a series circuit increases with an increase in voltage, reconnect the circuit so the
batteries are connected in series aiding. This will increase the source voltage and, therefore, the
circuit current. Insert the milliammeter into the circuit in one of the two places it was located
previously. It makes no difference where you place the meter. Since this is a series circuit, the
current will be the same no matter where you insert the meter into the circuit.
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ELEC 103 LABORATORY EXERCISE 3 CURRENT MEASUREMENTS
7.
8.
9.
Draw a schematic of the circuit as it is presently connected. Read the milliammeter and record
the measured current in the space provided on the datasheet.
Connect the two 6 Volt batteries in a series aiding configuration and connect them with the
1000 Ω resistor in a series circuit. Before connecting the milliammeter into the circuit,
calculate the current flowing in the circuit. Adjust the RANGE SWITCH to the appropriate
current range. If you are unable to determine a suitable range, start at the highest range
available. Then, reduce the range, to give you a reading with maximum pointer deflection
without going beyond full scale.
Insert the milliammeter into the circuit. Draw a schematic of the circuit as it is presently
connected. Then, read and record the measured current in the space provided on the datasheet.
PROCEDURE II − CURRENT MEASUREMENT IN PARALLEL CIRCUITS
10. All the measurements in PROCEDURE I were performed using series circuits. That is, circuits
in which components were connected end−to−end so only a single path for current flow exists.
A series circuit schematic is shown in Figure 3−1. Measurement techniques in parallel circuits,
however, are not exactly the same as series circuits. The milliammeter is still placed in SERIES
WITH THE ELEMENT, or component, through which the desired current to be measured
flows. However, measured currents are different in different parts of the circuit.
Figure 3−2 shows a parallel circuit with a
milliammeter placed to measure the current flowing
through the 1000 Ω resistor and another to measure
the total current flowing in the circuit. The current
through the other resistor can be measured by
connecting the milliammeter in the same way it is
connected to measure the current through the 1000
Ω resistor. Since you were issued only one
milliammeter, you will be inserting the milliammeter
Figure 3−2
in one place and then moving it to the other points in
the circuit until you have measured the three individual currents.
You should be aware that the total circuit current in a parallel circuit is equal to the sum
of the individual branch currents. In this parallel circuit, the circuit shown in Figure 3−2, the
sum of the currents through the two resistors will equal the total circuit current, labeled IT in
the figure.
11. Using test leads. connect the 6 Volt battery, the 1000 Ω resistor and the 10,000 Ω resistor as
shown in Figure 2−Lab 3. This is a parallel circuit configuration. Since you have only one
milliammeter, the milliammeter should be connected to measure the total circuit current.
12. Using a safe current range on the milliammeter, and observing correct polarity, measure the
total current flowing in the circuit (the current flowing through the source). Draw a schematic
on the datasheet including the milliammeter placement used in the measurement. Also, record
the measured current in the same space.
13. Using a safe current range on the milliammeter, and observing correct polarity, measure the
current flowing through the 1000 Ω resistor. Draw a schematic of the circuit on the datasheet
including the milliammeter placement used in the measurement. Also, record the measured
current in the same space.
Page 2
ELEC 103 LABORATORY EXERCISE 3 CURRENT MEASUREMENTS
14. Using a safe current range on the milliammeter, and observing correct polarity, measure the
current flowing through the 10,000 Ω resistor. Draw a schematic of the circuit on the datasheet
including the milliammeter placement used in the measurement. Also, record the measured
current in the same space.
15. Add the current measured in step 13 to the current measured in step 14. Record the sum in the
appropriate space on the datasheet. This current should be the same as the current measured in
step 12.
16. Answer the questions on the Data Sheet in the space provided.
Page 3
ELEC 103 LABORATORY EXERCISE 3 CURRENT MEASUREMENTS
STEP 3:
− Schematic −
Current = __________
Range = __________
STEP 5:
− Schematic −
Current = __________
Range = __________
STEP 7:
− Schematic −
Current = __________
Range = __________
Page 4
ELEC 103 LABORATORY EXERCISE 3 CURRENT MEASUREMENTS
STEP 9:
− Schematic −
Current = __________
Range = __________
STEP 12:
− Schematic −
Current = __________
Range = __________
STEP 13:
− Schematic −
Current = __________
Range = __________
Page 5
ELEC 103 LABORATORY EXERCISE 3 CURRENT MEASUREMENTS
STEP 14:
− Schematic −
Current = __________
Range = __________
STEP 15:
Current measured in STEP 13. =
Current measured in STEP 14. =
Total current flow in Figure 2 =
QUESTIONS:
1.
State the conclusions you draw from the results of step 3 through step 9.
2.
What happens to the current in a series circuit if you increase the source voltage (Refer to steps
7 and 9)?
3.
What conclusions do you draw from STEP 13?
4.
What conclusions do you draw from STEP 15?
Page 6
ELEC 103 LABORATORY EXERCISE 4 RESISTANCE MEASUREMENTS
PURPOSE OF EXPERIMENT
To familiarize the user with the DC Ohmmeter scales and ranges of a general purpose multimeter such
as the Simpson, Model 260 multimeter. And, to acquaint the user with safety precautions, equipment
precautions, and measurement methods.
EQUIPMENT LIST
1 − Simpson, Model 260 multimeter
1 − 1000Ω, 10,000Ω, and 1,000,000Ω 1 Watt Resistor
1 − 10kΩ potentiometer.
PROCEDURE
1. In addition to being used as a voltmeter, and a
milliammeter, the Simpson, Model 260, multimeter
can be used as an Ohmmeter. Observe the top scale
line on you meter. Notice the following significant
differences between the Ohms scale and the DC
scale used previously:
a.) Zero is at the right end of the scale and the scale
markings increase in value going to the left.
b.) The divisions on the Ohms scale are not linear
(equal in size). In fact, note that the physical
length of the segment between 2 and 5 on the
Ohms scale is roughly the same as the segment
between 5 and 10. Yet 5 minus 2 = 3 units,
whereas 10 minus 5 = 5 units, different values
for roughly equal length scale segments. This
type of scale is generally referred to as a
nonlinear scale. Nonlinear scales are used for
Ohmmeters while linear scales are used for
voltage and current meters.
Now turn your attention to the RANGE/FUNCTION SWITCH. There are three resistance or
Ohmmeter ranges, R X 1, R X 100, and R X 10000. Depending upon the size of the resistance
to be measured, one of these ranges is selected and used with the Ohms scale.
2. On the R X 1 range, any scale reading is interpreted using the numbers printed on the scale.
That is, all meter readings are multiplied by 1. On the R X 100 range or the R X 10,000 range,
the meter readings are multiplied by 100 or by 10,000 respectively. Consequently, if the needle
is pointing at 5 on the Ohms scale, it means the reading is 5 Ω if you are using the R X 1 range.
It would mean 500 Ω if using the R X 100 range or 50,000 Ω if using the R X 10,000 range.
3. Before proceeding with the experiment, the following Ohmmeter precautions must be reviewed
since serious damage to the multimeter can result if they are not carefully followed.
OHMMETER PRECAUTIONS
A. Move the status switch to the +DC position.
B. Move the range switch to one of the three resistor ranges. Unlike the voltmeter and the
Page 1
ELEC 103 LABORATORY EXERCISE 4 RESISTANCE MEASUREMENTS
milliammeter, the Ohmmeter does not need to be adjusted to the highest range. The
Ohmmeter can be adjusted to any range if it is connected to the circuit properly.
C. Remove all power from the circuit.
D. Remove at least one end of the component being measured from the circuit.
E. Connect the ohmmeter leads together and use the zero ohms adjust knob to make the pointer
read zero ohms before each use.
4. Adjust the STATUS/POLARITY switch to the +DC position and make certain the multimeter
test leads are in the proper receptacles. Turn the RANGE switch to the R X 1 range and short
(connect together) the two meter leads. Observe that the meter pointer moves up the scale and
comes to rest somewhere around zero ohms. With your test leads shorted, you want your meter
to "think" it is reading ZERO OHMS of resistance since there is no resistance between the
meter leads.
To accomplish this strategy leave the meter test leads shorted. Turn the ZERO OHMS adjust
knob slowly to the extreme counterclockwise position and then to the extreme clockwise
position. Observe the pointer movement. You should see the pointer move from below full
scale to greater than full scale. If this does not occur, contact you lab instructor immediately.
5.
Adjust the pointer, using the ZERO OHMS ADJUST knob, until the pointer is over the zero on
the Ohms scale. Once this has been accomplished, you have calibrated your Ohmmeter for this
measurement and you can UNSHORT the meter test leads.
NOTE: It is mandatory that you ZERO OHMS ADJUST the Ohmmeter before each
initial measurement on a specific range. Therefore, each time you change ranges you must
zero ohms adjust the meter.
6.
Using appropriate ranges, measure the resistance of the 1000Ω resistor, the 10,000Ω resistor,
and the 1,000,000Ω resistor. Record the measured values on the datasheet. MAKE CERTAIN
YOU RE−ZERO THE METER WHENEVER CHANGING RESISTANCE RANGES.
7.
Re−measure the 1,000,000Ω resistor using the appropriate range. However, this time place one
hand on each end of the resistor so your hands are in contact with the resistor and the meter test
leads. Record the results of the measurement on the datasheet.
8.
Repeat Step 7, only wet your fingers before touching the resistor and the meter leads. Record
the results of the measurement on the datasheet.
9.
The potentiometer is a variable resistor that has three terminals. One pair
of terminals is fixed and you will measure the total potentiometer
resistance between these terminals. The other lead connected to a wiper.
When you connect the ohmmeter between the wiper and one of the fixed
terminals, you will measure a resistance that may be any value between 0
Ω and the maximum resistance value of the potentiometer. The resistance
value that you measure will depend on the position of the wiper.
Connect the ohmmeter across the fixed leads of a potentiometer. Adjust
the meter to make an appropriate resistance reading. Slowly adjust the
FIGURE 4−1
wiper and observe the reading on the meter but be careful not to touch the
leads with your fingers. Record your observation on the Data Sheet as either remains constant
or varies.
Page 2
ELEC 103 LABORATORY EXERCISE 4 RESISTANCE MEASUREMENTS
DATASHEET FOR EXPERIMENT 4
STEP 6:
Color Code Value Measured Value
Meter Range
Used
1,000Ω
10,000Ω
1,000,000Ω
STEP 7:
Measured resistance = _____________ Range = ___________
STEP 8:
Measured resistance = _____________ Range = ___________
STEP 9:
As you move the wiper, the resistance between the two fixed terminals ___________________.
As you move the wiper, the resistance between the wiper and the fixed terminal
____________.
QUESTIONS:
1.
When you read the Ohmmeter in STEP 6, you probably noticed a difference between the meter
reading and the resistance value predicted by the color code. Give a reason for the difference.
2.
What was the purpose of STEP 7 ?
3.
Using the measured values of the resistors in STEP 6, calculate the value you would measure if
you placed the three resistors in parallel. Show the formula and the calculations used to arrive
at the answer.
Page 4
ELEC 103 Unit 5 The Oscilloscope
PURPOSE
To determine whether a circuit is operating properly, or malfunctioning, it is often necessary to
observe the electronic signals in the circuit. A VOM or DMM will give the magnitude of an ac current
or voltage, but is unable to display the signal or provide information to allow the user to determine if
there is distortion on the signal, or if there is an improper phase shift in the circuit. The oscilloscope
allows the technician to view these signals and determine the circuit operating characteristics. The
purpose of this exercise, is to use the ac meter and the oscilloscope to make voltage and current
measurements in an ac circuit. The student will be able to describe the procedure to measure voltage,
period, frequency, and phase relationships for ac waveforms with the oscilloscope.
EQUIPMENT AND MATERIALS REQUIRED
Oscilloscope with 2 probes
2 each DMM’S and VOM’s
Powered Protoboard
Audio Signal Generator
Frequency Counter
Resistor, ½W, 5%, 1.5kΩ
Capacitor, 35V, 10%, 0.1µF
INTRODUCTION
There is a wide variety of test equipment available to the technician, which provides information on
circuit operation. All test equipment, used to measure voltage in a circuit, must have a high input
impedance to prevent the test equipment from loading the circuit under test and thereby provide
inaccurate measurements. The VOM usually has a moderately high input impedance while the DMM
and oscilloscope have a high input impedance of 1MΩ or higher. This high input impedance assures
the measurements will be accurate since loading does not occur.
The VOM and DMM are used to measure ac currents and voltages in the same way they were used in
the ELEC 111 course experiments. Since the polarity of the waveform is constantly changing, there is
no polarity to consider when making ac voltage and current measurements.
The oscilloscope provides a visual representation of the voltage at any point in the circuit on a CRT.
As a result, the technician may view the display and determine the shape of the waveform, the
amplitude of the waveform, the frequency of the waveform, the phase relationships between voltage
waveforms at various points in the circuit, or the duration of some event lasting one or more cycles.
An ac voltage is one that is continuously changing in magnitude and direction, with respect to time.
These ac signals have a frequency of repetition measured in Hertz (cycles per second), which is the
number of complete waveforms created in one second. The symbol for frequency is f, and the unit of
measurement is the Hertz (Hz). Reference may also be made to the amount of time required for two
alternations, which make up one cycle, of the signal, which is called the period of the waveform. The
period of the waveform uses the symbol T, and has the second as the unit of measurement. There is an
inverse relationship between the frequency and period of the waveform. This may be expressed as:
1
f= T
<5 − 1>
The analog waveform used to carry information in an ac circuit is the sinewave. These sinusoidal
signals may be symmetrical above and below a zero volt reference, or they may be riding on a dc level.
Page 1
ELEC 103 Unit 5 The Oscilloscope
The amplitude of a signal is the height of the signal above or below the reference. In most cases, the
reference used is zero volts or ground.
The polarity of a signal refers to the waveform’s value, with respect to the common level in the
system, which is usually ground or zero volts. All parts of the signal that rise above the zero volt level
have a positive voltage polarity, and all parts of the signal that fall below the zero volt level have a
negative voltage polarity.
The sinewave in Figure 5 − 1 has equal excursions above
and below the zero volt reference. The maximum positive
level from the zero volt reference is called the positive
peak voltage, VPP, and the maximum negative level from
the zero volt reference is called the negative peak voltage,
−VP. Since the variation above and below the reference is
the same, the magnitude of VP is equal to −VP. While this
is the case for Figure 5 − 1, it is not required. An ac
information signal may also ride on a dc level. When the
dc level is such that there is no excursion about the zero
volt reference, but is always either above or below zero
volts, this signal is no longer an ac waveform but a
pulsating dc voltage. The amplitude is defined as the
voltage level between the positive and negative peaks,
FIGURE 5 − 1
and is called the peak−to−peak voltage, or VPP. When the
magnitude of VP is equal to the magnitude of −VP, we may determine VPP from the equation:
VPP = 2VP
The amplitude of the ac signal may be measured and defined in different formats. The instantaneous
voltage is the value of the voltage at a particular time. The symbol v is used to denote an instantaneous
voltage, and the unit of measurement is the Volt. The time at which we wish to determine the
instantaneous voltage may be θ (theta) and is measured in either degrees or radians, or time ( t )
measured in seconds. Both θ and t must be measured with respect to some defined reference of the
signal. This is normally when the signal is at zero volts and rising, or zero degrees. When the
instantaneous voltage is determined using t, the time must first be multiplied by 2 π f, which is the
angular velocity (also known as ω) of the voltage. The units of measurement for ω is rad/s (radians per
second). Since the voltage is sinusoidal, the instantaneous voltage may be determined from the
equations:
v = VP Sin θ
v = VP Sin (2 π f t)
Another measurement to define an ac voltage or current is the effective or Root−Mean−Square (rms)
voltage. This measurement is important, since the rms voltage or current has the same heating value as
the corresponding dc voltage or current. The symbol used to denote an rms voltage measurement is
Vrms, and is measured in Volts. Current measurements use the symbol Irms, and have the Ampere as the
unit of measurement. The VOM and DMM provide rms voltage and current measurements. The rms
voltage may be calculated from the peak voltage using the equation:
Vrms =
VP
2
which equals Vrms = 0.707 Vrms
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ELEC 103 Unit 5 The Oscilloscope
The final measurement, used to refer to an ac voltage or current, is the average value. The average
value for an ac voltage is the average of all the voltage levels for one−half of a cycle of an ac voltage.
This measurement is important since it corresponds to the deflection of the pointer for an analog ac
meter, even though the ac meter scale is calibrated to provide rms readings. The relationship between
the peak and average voltage of a sinusoidal ac voltage or current is:
2VP
VPP
=
= 0.6366 VP
π
π
Remember that VPP, VP, Vrms, and Vav values are equivalent methods to define the same voltage. The
type of measurement made is determined by the equipment used to make the measurement. A VOM
would indicate 120Vrms supplied to the home, while an oscilloscope would indicate the same voltage as
169.71VP, and 339.41VPP. To find the relationship between the rms and average voltages recall:
Vav =
VP =
2 Vrms
and
⎛ π⎞
VP = Vav ⎜ ⎟
⎝ 2⎠
Therefore, if we equate these two readings we obtain:
⎛ π ⎞
⎛π⎞
2 Vrms = Vav ⎜ ⎟ and Vrms = Vav ⎜
⎟
2
⎝ ⎠
⎝ 2 2 ⎠
Since π, 2, and 2 2 are constants, we may solve for the constants in the previous equation and obtain:
Vrms = 1.1107Vav and Vav = 0.9 Vrms
The oscilloscope is used to measure the amplitude of a voltage, displayed on the vertical axis, against
time, which is displayed on the horizontal axis. The vertical and horizontal axis have marked
graduated scales, in each direction, calibrated into major and minor divisions. A range switch, in
conjunction with the vertical amplifier, controls the vertical deflection of the electron beam in the
oscilloscope, while the time base and associated circuitry control the horizontal sweep speed.
The vertical gain of the scope is calibrated in volts per division. The signal is applied to the vertical
deflection circuitry of the scope that deflects an electron beam vertically on the face of the oscilloscope
according to the polarity and magnitude of the applied voltage. To determine the amplitude of the
voltage, count the number of divisions over which the beam is deflected, and multiply this value by the
gain setting. If the measurement is made between the zero volt reference, and the peak value, a peak
voltage is being measured, while a measurement between the positive and negative peaks is a
peak−to−peak voltage measurement. For Figure 5 − 1, if the beam is deflected 7.2 divisions between
the positive and negative peaks, and 3.6 major divisions between the zero volt reference and the
positive peak, with the vertical gain set to 5 V/div, the voltage represented by this deflection is:
VPP = 7.2 div x 5 V/div = 36VPP and VP = 3.6 div x 5 V/div = 18VP
The horizontal deflection on the oscilloscope is controlled by a sweep generator. The sweep generator
is used to move the beam from the left side of the CRT to the right side of the CRT. When the beam
reaches the right side of the CRT, it is swept back very rapidly and the process is repeated. The control
for the sweep speed is calibrated in seconds/division, and the horizontal axis is used to measure time.
Therefore, on the horizontal axis of the scope, you will be able to determine the period of the
waveform.
To determine the frequency of a waveform, with an oscilloscope, the period of one cycle is usually
determined. The sweep speed is adjusted until more than one complete cycle occupies the horizontal
axis of the oscilloscope. Then the number of divisions for one cycle is counted and multiplied by the
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ELEC 103 Unit 5 The Oscilloscope
sweep speed. For Figure 5 − 1, if the sweep speed is set to 0.5 ms/div and one cycle occupies 6.6
major divisions, the period for one cycle is:
T = 0.5 ms/div x 6.6 div = 3.30 ms
Since the frequency of a waveform is the inverse of the period, the frequency of the signal in Figure 5
− 1 is:
1
1
f = T = 3.3ms = 303 Hz.
When two or more waveforms of the same frequency are present at the same time in a circuit, it is
possible to define the time relationship between corresponding values of the waveform. This
relationship is called the phase or phase angle. The phase angle uses θ as the symbol of measurement,
and has either degrees or radians as the units of measurement. Two waveforms are in phase when both
waveforms begin at zero volts and rise or fall together. If both waveforms are zero volts at the same
time, but one waveform rises as the other falls, the waveforms are antiphase (θ is 180o or π rads out of
phase). Waveforms may also be 90o out of phase, which is referred to as phase quadrature.
The phase angle of one waveform in a circuit may
lead or lag the other. Assume waveform A in Figure
5 − 2 is the reference voltage for the circuit, usually
the supply voltage. Since waveform B is at zero
volts and rising, after waveform A is at zero volts
and rising, waveform B lags waveform A by some
phase angle θ. Using the same analogy, we may also
say that waveform A leads waveform B by the
phase angle θ. A leading phase angle is considered
positive, while a lagging phase angle is negative.
The phase angle may be any value between 0o and
180o (0 and π rads).
Figure 5 − 2 contains two signals of the same
frequency applied to the inputs of an oscilloscope.
FIGURE 5 − 2
The amplitude of each channel may be determined,
and each channel may have the vertical gain set to different levels. To determine the phase relationship
between the two waveforms. First, select the channel that contains the reference waveform. Assume
waveform A is the reference signal for Figure 5 − 2. The vertical gain is adjusted to provide maximum
deflection about the zero volt reference, and both signals must have the same zero volt reference on the
CRT. The sweep speed is adjusted to provide a minimum of 180o for each signal. This oscilloscope
has ten major divisions across the CRT. That breaks down to 18o per division. Notice, in Figure 5 − 2,
the horizontal divisions are usually calibrated in milliseconds per division or microseconds per
division and this scope display is no different. However, we will convert the horizontal directly into
degrees, using a ratio.
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ELEC 103 Unit 5 The Oscilloscope
Usually, the number of divisions across the horizontal are divided into
180o
180o, the number of degrees in the one alternation being displayed.
= α = 18o/ div
10div
That provides the number of degrees per division. If you measure the
number of divisions along the zero volt reference that waveform A and
⎛ 2div ⎞
waveform B are displaced from one another, you can calculate the
θ = 180o ⎜10div⎟
⎝
⎠
phase angle. The phase angle between waveform A and waveform B
may be calculated using a proportion, as shown to the left. Since one
θ = 36o
alternation is 180o and there are 10 divisions, a displacement of two
divisions between the waveforms equals a phase shift of 36o.
The oscilloscope may be used to measure the phase angle between two waveforms of the same
frequency by displaying a pattern known as the Lissajous Figure. Under this method, the time base or
sweep speed is disconnected from the horizontal deflection circuitry.
The reference signal is connected to the horizontal (X) deflection circuitry, an unknown signal is
connected to the vertical (Y) deflection circuitry, and the sweep speed selector is set to the X−Y mode.
With no signals applied, a dot will appear on the CRT.
Do not leave the CRT in this mode for too long at a high intensity or damage to the
phosphor coating on the CRT will occur.
The dot is adjusted so the dot appears at
the intersection of the horizontal and
vertical scales. The input signals are
applied, and a waveform similar to that of
Figure 5 − 3 will appear on the CRT.
Make certain the pattern is centered on
the zero axis and the amount of deflection
for A and B is measured. The phase
angle, θ, may be determined from the
equation:
A
θ = sin−1 B
FIGURE 5 − 3
NOTE
When connecting line−powered equipment to a circuit, make sure the ground terminals from
all devices are connected to the circuit ground as indicated on the schematic diagram.
PROCEDURE
1.
Measure the values for R and C and record the values in Table 5 − 1.
2.
Calculate and record the values for Table 5 − 2 and Table 5 − 3 using information in the tables.
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ELEC 103 Unit 5 The Oscilloscope
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Build the circuit of Figure 5 − 4. Make certain
the ground leads of the frequency counter (FC)
and the audio generator (VS) are connected to the
same point in the circuit. Connect one DMM
across the output terminals of the audio
generator, and insert the second DMM to
measure the current. Adjust the settings of the
DMM’s to their proper positions.
Connect a probe to the oscilloscope Channel 1
Figure 5 − 4
terminal. The other probe will be connected to
Channel 2 later in the experiment. Connect the ground lead, of the oscilloscope probe
connected to CH 1, to the same point in the circuit as the ground lead of the audio generator.
Connect the other lead of the oscilloscope probe to measure the voltage across the resistor.
Set the oscilloscope to trigger internally on Channel 1. Make sure all controls are set to their
calibrated position. Check all circuit connections.
Apply power to all the equipment. Adjust the frequency of the audio generator until the
frequency counter reads 500 Hz. Adjust the voltage output of the audio generator until the
DMM reads 3V.
Adjust the sweep speed to obtain a display similar to Figure 5 − 1. It would probably be better
to view two or three cycles instead of the single cycle displayed in Figure 5 − 1.
Slide the selector under the CH 1 vertical gain control, marked AC−GND−DC, to the GND
position. Adjust the CH 1 position control to place the trace in the center of the CRT. Move the
CH 1 selector to AC, and adjust the vertical gain until the amplitude of the signal looks similar
to Figure 5 − 1. Use the CH 1 selector to make sure the vertical position of the trace is centered
on the CRT.
Measure the period of the waveform with the oscilloscope and record the data in Table 5 − 1.
Using the data from step 9, calculate the frequency of the waveform and record the results as a
measured value in Table 5 − 1.
Measure the applied voltage (Vrms) with the DMM and record the results in Table 5 − 2 on the
appropriate line.
Measure the total circuit current (Irms) with the current DMM and record the results in Table 5
− 3 on the appropriate line.
Measure VP and VPP with the oscilloscope, and record the data in Table 5 − 2.
From the data in step 13, calculate IP by dividing VP by R and record the result as the measured
value in Table 5 − 3. Repeat for IPP.
Adjust the voltage output of the audio generator until VP on the oscilloscope reads 7V. Then,
repeat steps 11 to 14.
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ELEC 103 Unit 5 The Oscilloscope
16.
17.
18.
19.
20.
21.
22.
23.
Adjust the voltage output of the audio generator until
VPP on the oscilloscope reads 20V. Then, repeat steps
11 to 14.
Remove power. Build the circuit in Figure 5 − 5.
Connect CH 1 of the oscilloscope across the audio
generator. Connect the second probe to the CH 2 input
of the oscilloscope and then connect CH 2 across the
resistor as shown in Figure 5 − 5.
FIGURE 5 − 5
Apply power to all equipment. Adjust the frequency of
the audio generator to 1000Hz, and the voltage output to 5Vrms. Place the CH 1 and CH 2
AC−GND−DC selectors to GND and center the traces on the CRT. Return the CH 1 and CH 2
selectors to the AC position.
Adjust the oscilloscope, as necessary, to obtain a display similar to Figure 5 − 2. Measure and
record the readings for the Number of Divisions for 180o and the number of divisions the two
waveforms are displaced from one another along the zero axis, where they cross the zero axis
going positive, and enter the data in Table 5 − 4.
Calculate the Phase Angle, and record the result in Table 5 − 4.
Set the sweep speed to the X−Y position. Place the CH 1 and CH 2 AC−GND−DC selectors to
GND and center the dot on the CRT. Return the CH 1 and CH 2 selectors to the AC position.
Adjust the oscilloscope as necessary to obtain a display similar to Figure 5 − 3.This is best
accomplished by adjusting the amplitude of both channels for the same deflection, one channel
at a time. Disconnect channel one by moving the AC−GND−DC selector to GND and adjust
channel two for a specific amplitude. Then disconnect channel two and adjust channel one to
the same amplitude. After both channels are adjusted for the same number of divisions
deflection, activate both channels and record the readings for the Number of Divisions for
distance A and for distance B in Table 5 − 5.
Calculate the Sine θ and the Phase Angle. Record the results in Table 5 − 5.
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ELEC 103 Unit 5 The Oscilloscope
DATA TABLES
Value
R
C
f
T
Rated
1.5kΩ
0.1µF
500 Hz
2 ms
Measured
TABLE 5 − 1
Voltage
Vrms
Calculated
3V
VP
VPP
Measured
Calculated
5V
Measured
Calculated
15V
Measured
TABLE 5 − 2
Current
Calculated
Irms
IP
IPP
2mA
Measured
Calculated
4.67mA
Measured
Calculated
13.33mA
Measured
TABLE 5 − 3
Number of Divisions from
0o to 180o
Waveform A to B
Phase Angle
TABLE 5 − 4
Number of Divisions from
A
B
Sine θ
TABLE 5 − 5
Page 8
Phase Angle