EET-112: Elementary Electronics
UNIT 2: COMMON LABORATORY INSTRUMENTS
DC Power Supply, Function Generator, Multimeter, and Oscilloscope
Revised by Mahbubur Syed Dec’ 04
2.1 DC Power Supplies and Function Generator
DC power supplies and function generators are used in electronics laboratories to supply electrical
signals/power to circuits. The DC power supply provides a constant signal (either constant voltage or
constant current) to a circuit. The signal does not change with time. In contrast, the function generator is
a source of signals that change with time and are said to be “functions of time” or "their value depends
upon the value of time.” A typical function generator can put out waveforms that repeat over and over
again in three patterns: sinusoidal, square wave and triangular. These waveforms are illustrated later in
this experiment. The title 'function generator" stems from the ability to put out several waveforms or
"functions of time". Waveforms that are referred to as AC, or alternating current, are sinusoidal in form.
Pure musical tones are sinusoidal waveforms.
If one stands back from the terminology used in electronics it becomes apparent that "power supplies”
and “function generator” are really power converters. They take the 120 volt AC power available from
normal electrical outlets and convert it to other forms of electrical signals which are useful in such places
as electronics laboratories.
Voltmeters
Voltmeters measure the voltage (electric potential) difference between two points in a circuit. Thus
voltmeters are placed with one probe at one location and the second probe at a second location. The
path through the voltmeter represents an alternative (parallel) path between the same two points in the
circuit. If this alternative path was “too easy", then the current would preferentially flow through the
meter instead of through the circuit. The results obtained would represent an altered circuit, not the
original circuit. To avoid this problem voltmeters are designed to appear to be a “very difficult” alternative
path for the current. This difficult path is said to have “high resistance” or “high impedance.” This means
there is high resistance/impedance to the flow of electricity.
Ammeters
Ammeters (current meters that measure current in amperes) measure the rate of flow of electrical charge
in an electrical conductor. They could also be called current meters, or ampere meters, but tradition has
caused their title to be “ammeters” instead. In order to measure current, the ammeter must be placed
directly in the path of the moving charge (current). The circuit is “opened” (open = circuit path for
current is broken) and the ammeter is placed in the break in the conductor. This is called “series”
position. The difference between “parallel” and “series” should be clear after completion of this course. If
the ammeter is in the path of the current then the resistance/impedance of the ammeter must be as low
as possible to avoid altering the circuit during measurement
The terms “open circuit” and “closed circuit" are often used in electrical / electronic circuit activities. The
term “open circuit” was defined above. A “closed circuit” is one in which the path for current flow is
continuous, or not broken. The words ”open” and "closed” for circuits carry opposite meanings than one
would apply if the words were applied to a door.
Multimeter and Oscilloscope
The multimeter and oscilloscope are two of the most important and most common measuring instruments
found on the bench in an electronics laboratory. A laboratory 'bench” is a workbench.
Although it will not be apparent in these experiments, every meter must extract some current from the
electrical circuit in order to determine the value of the parameter (voltage, current, resistance, etc.) being
measured. To minimize disturbance of the circuit as a result of the measurement meters must be
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EET-112: Elementary Electronics
designed for the type of measurements being made.
Multimeters can be built to measure both voltage and current. Setting of switches and/or changing of
input leads is done to modify the resistance/impedance from high impedance for voltage measurements
to low impedance for current measurements.
Multimeters can also be used to measure resistance. To do this the meter acts as a DC power supply and
send out voltage. The current that flows in the circuit because of the DC voltage is then detected. The
value of this current can be used with the known voltage and Ohm's law to determine the value of
resistance. Ohm's Law will be explained in Unit 3 of this course.
Oscilloscopes are designed to have very high impedances and thus are used only to detect voltage
signals. Oscilloscopes are very valuable instruments because they can display the waveform and
magnitude (size) of the electrical signal if it repeats itself time after time. It is also possible to take the
information from the screen of an oscilloscope and convert it to frequency. No other single laboratory
instrument can simultaneously provide all this information (shape of signal, magnitude of signal and
frequency of signal).
Manuals for the instruments will be available from the laboratory instructors in the Trafton S-189. To
check out a manual give your student identification card to the laboratory instructor. These manuals are
not to be removed from Trafton S-189.
2.2 Unit 2 Laboratory: Setting the output voltage of a DC POWER SUPPLY
1) Locate both a DC POWER SUPPLY and a DIGITAL MULTIMETER (DMM) on the shelf above the
laboratory bench. The term “DUAL” in the title of the DC power supply means the power supply has
two parallel outputs. Only one output will be used in these first experiments. Both the DC power
supply and the DMM are already connected to the electrical power system in the room.
2) Connect the DC power supply and the DMM using electrical “cables” or “conductors”, or “leads” with
“banana plugs” at each end. These "leads” are located in racks in the laboratory. The leads from the
DC POWER SUPPLY should be connected to the jacks labeled “V – Ω” and “COM” on the DMM. The
abbreviation, COM, stands for “common.” The Greek letter Ω (omega), that looks like a horseshoe
with the open side down, is used for ohms - a measure of resistance.
4) Set the DMM to measure DC voltages:
a) Depress the DC voltage mode/function switch which is indicated by 2 parallel horizontal lines “=”
(symbol for DC) and the letter “V” (for voltage) adjacent to the switch. Note that the symbol “~ “
is used for Alternating current (AC)
b) Connect one electrical lead from JACK-V (volts)- Ω (Omega).
c) Connect a second electrical lead from JACK-COM (common).
5) Set the DMM to autoranging by depressing the AUTO switch. The meter will then automatically select
conditions to measure the magnitude/amplitude of the signal input to the meter. As an alternative,
one may select a particular measurement range if knowledge of the maximum signal to be measured
is available.
6) "Energize”, or "turn on”, or ”power up” both the DC power supply and the DMM.
7) Using the reading on the DMM adjust the output of the DC power supply to 5 volts. Turn off the power
supply.
2.3 Unit 2 Laboratory: Preparation of the experimental circuit
1) Locate an experimental circuit labeled 'Units 2 & 3 & 4”. This same circuit will be used for laboratory
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EET-112: Elementary Electronics
exercises in Units 2, 3 and 4. This circuit diagram is shown in Figure 2-1.
2) The experimental circuit contains only 3 resistors (each of 1000 ohms) and several banana jacks for
connections from either a DC power supply or a function generator and metering can be conveniently
connected to the circuit. The resistors can be seen through the transparent plastic top of the
experimental circuit. Also seen through the top is either the presence or absence of electrical
connectors between the banana jacks. In many of the experiments to follow the student will be asked
to insert electrical leads between jacks (that are not connected by conductors beneath the
transparent top) to allow electrical current to pass between the jacks without resistance/impedance.
A lead in such a position is called a “jumper”.
Figure 2-1 - Circuit diagram of the equipment to be used for UNITS 2. 3 and 4
No current can flow through the resistors between JACKS B3-B4, JACKS C3-C4 and D3-D4, because
the lack of jumpers between JACKS B1-B2, JACKS C1-C2 and D1-D2 leaves these paths as open
circuits.
3) Connect jumpers between:
a) JACKS B1-B2. This jumper allows current flow through JACKS A1-B1-B2-B3-B4-C1-D1, but not any
further to close a loop to the source of the power supply. Note that permanent jumpers between
JACKS B2-B3 and B4-C1-D1 are part of the circuit.
b) JACKS C1-C4. This jumper provides a short circuit path (a bypass with no resistance) between
JACKS C1-C4-A2. This jumper also is a short circuit path between JACKS D1-D4 since the C1-C4
branch is parallel with the D1-D4 branch. A branch in a circuit is a series connection wherein all
components carry the same current.
As a result of the connection of the jumpers only the resistor between JACKS B3-B4 is now
included in a closed (complete) circuit between JACKS A1-A2. In addition the parallel path of no
resistance between JACKS C1-C4 would cause all current to flow there even if there were jumpers
between JACKS C1-C2 and D1-D2. The current will always prefer the path of least resistance.
4) Turn off the DC power supply. Connect the leads from the output of the DC power supply to JACKS A1A2 of the experimental circuit. These were the leads connected to the DMM in Section 2.2 above.
2.4 Unit 2 Laboratory: Measuring DC voltage with a Digital Multimeter (DMM)
1) Turn on the DC power supply. Set the DMM to measure DC voltages (it should be in this mode
already from Section 2.2 above).
2) Connect leads from:
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EET-112: Elementary Electronics
a) JACK-V (volts)/ Ω (Omega) on the DMM to JACK B3 in the experimental circuit
b) JACK-COM on the DMM to JACK B4 in the experimental circuit
These connections will place the DMM in parallel with the 1000 ohm resistor which is mounted
between JACKS B3-B4 in the experimental circuit.
3) Observe the voltage reading on the DMM. If the DMM is in the autoranging mode change it to a
particular range that has an upper value greater than the value read.
4) Reverse the connections of either both leads at the meter or both leads at the circuit (do not change
the leads at both the meter and the circuit) and note the change in meter reading. There should be
change of polarity (+ to -), but no change in magnitude (size).
5) Change the output of the DC power supply to 3V and repeat the measurements described in Steps 3)
and 4) above.
6) If you reversed the meter leads in Step 3) above try changing the circuit leads and observing the
results. Try changing both sets of leads simultaneously and note the results.
2.5 Unit 2 Laboratory: Measuring DC current with a Digital Multimeter (DMM)
1) Turn off the DC power supply connected to the circuit. Note that each time changes are made in the
circuit the DC power supply must be turned off before leads are removed or connected.
2) Set the DMM to measure DC current
a) Depress the DC current mode/function switch which is indicated by 2 parallel horizontal lines “=”
(indicating DC) and the letter “A” (indicating ampere-current) adjacent to the switch.
b) On the DMM move the input lead from the JACK-V/Ω to the JACK-A (current in amperes). Do not
change the lead to JACK COM.
c) The autoranging mode is not available for current measurements. Set the range to 10 millimaperes
(10mA).
3) Remove the “jumper” lead between JACKS B1-B2 in the experimental circuit
4) Connect leads from:
a) JACK-A (amperes) on the DMM to JACK B1 on the experimental circuit
b) JACK-COM on the DMM to JACK B2 on the experimental circuit These connections will place the
meter in series with the 1000 resistor mounted between JACKS B3-B4 in the experimental circuit
All the current that goes through the resistor must go through and will be measured by the
meter.
5) Turn on the DC power supply which should have been left at a setting of 3V.
6) Observe the current reading including both magnitude and polarity.
7) Turn off the DC power supply and reverse the connections of either both leads at the meter or both
leads at the circuit (do not change the leads at both the meter and the circuit. It is very important to
note that the leads from the voltmeter could be disconnected and then reconnected with the circuit
still energized. This should never be one when using an ammeter. Turn the DC power supply back on
and observe the current reading, especially any changes from the initial reading. There should be
change of polarity (+ to -), but no change in magnitude (size).
8) Without changing any connections in the circuit/meter change the output of the DC power supply to 5
volts, repeat Steps 4-6 described above, and observe the current readings.
9) Turn off the DC power supply and the DMM and remove all the electrical leads between the DC power
supply and meter and the circuit.
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EET-112: Elementary Electronics
2.6 Unit 2 Laboratory: Setting the Output of a Function Generator
1) Locate a FUNCTION GENERATOR on the shelf above the laboratory bench. The name, “function
generator”, is applied to power supplies that can produce waveforms that vary in time in a regular,
repeating fashion. In this experiment you will observe three types of waveforms: sinusoidal, square
and triangular.
Connections to the output of the function generator should be made between banana jacks labeled
600 OHM OUTPUT. Do not make connections to the banana jacks labeled PULSE OUTPUT.
A different type of lead will be required to make connections to the oscilloscope. This lead will have
banana jacks on two conductors at one end and a BNC connector (only one) at the other end. For the
BNC connector one conductor is found at the center of the connector and the cable to which they are
attached. Another conductor is found in the exterior cylinder of the connector which in turn is
connected to a shield layer on the exterior of the cable. Such connectors and cable are used to shield
the signals carried on the central conductor from external interference.
2) Locate one of the banana-to-BNC leads on the racks in the laboratory.
3) Select a sinusoidal waveform by depressing the SINUSOIDAL MODE SWITCH which is indicated by a
graphical symbol.
4) Select a frequency of approximately 1000 cycles per second (also called 1000 hertz, abbreviated Hz,
or 1K Hz = 1 kilo hertz) by depressing the 1K MODE SWITCH amongst the many range switches.
Hertz was a German scientist who did important early work with electrical circuits. Units of measure
in science are named for scientists who did important early work and who have since died.
5) Select a signal of medium size by positioning the AMPLITUDE KNOB at mid-range.
6) Do not turn on the function generator until the oscilloscope is prepared for measurements.
2.7 Measuring Time-Varying Voltage Waveforms with an Oscilloscope
2.7.1 General description of oscilloscope operation
An oscilloscope allows display of repeating waveforms. The heart of an oscilloscope is a cathode ray tube
(CRT) as shown in Figure 2-2.
Figure 2-2. Cathode Ray Tube
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EET-112: Elementary Electronics
At the back of the CRT is a very hot wire called a filament / cathode. The filament is so hot that electrons
are boiled out. The electrons (have negative electric charge) that have boiled outside the filament are
strongly attracted to the anodes which have high positive voltages (potentials) on them. The initial anode
has a hole through which some of the electrons pass very quickly, and the hole limits the escaping
electrons to only those which resemble a beam on their way to the front of the CRT. The electrons have
obtained their speed from the voltage difference between the filament and the anodes. The anodes
beyond the initial anode have voltages which "shape" the initial "fat & possibly irregular" electron beam
into a narrow, circular beam. When the electrons strike the back side of the display screen they cause
light to be emitted from the phosphor coating that is found there. Because the phosphor coating is thin,
light created on the back side of the phosphor can be seen through the phosphor. The rest of the front
screen is made from transparent glass so an observer can 'see' where the electrons are striking the
display screen. The pattern seen with no deflection plates would be a dot. However, in a CRT there are
deflection plates which influence the direction. A positive voltage on the deflection plates will attract the
negatively charged electrons in the beam. A negative voltage on the deflection plates will repel the
negatively charged electrons in the beam. By placing voltages which vary with time on the vertical
deflection plates the electron beam can be moved up and down in the same time varying pattern as the
voltages applied to the vertical deflection plates. If voltages were only applied to the vertical deflection
plates then the pattern on the front of the CRT would be only a vertical line. Time is included in the signal
by applying a ramp voltage to the horizontal deflection plates. The passing electron beam is first deflected
toward one horizontal plate and then toward the other by the ramp voltage over an interval of time that
can be selected by then user. After a short and predetermined interval of time is ended the voltages to
the horizontal deflection plates return to their original values, and the ramp change is repeated over and
over again. Thus for a given interval of time determined by the inputs to the horizontal deflection plates
there is a corresponding voltage signal applied to the vertical deflection plates. This results in a twodimensional pattern on the display screen.
A modified version of a CRT is the heart of every television set. In a television set the electron beam is
swept horizontally across the screen over and over again in a very short interval. For each horizontal
sweep the beam is moved vertically one line width until the entire front surface of the viewing area has
been covered. The brightness of the display is determined by the number of electrons in the electron
beam at the moment it strikes the phosphor. The entire process is so swift that our eyes cannot see the
point of light moving, or even the individual horizontal sweeps. Our eyes integrate the display into a
complete picture covering the front of the display area. In fact the display is “refreshed” several times in
the interval required for our eyes to form an image.
For the measurements to be made in this laboratory a voltage proportional to time will be sent to the
horizontal deflection plates. That means the electron beam will describe a horizontal line on the front of
the oscilloscope if no signal is applied to the vertical deflection plates. Each division of centimeter size
shown on the screen of the CRT will represent the passage of an interval of time selected by the operator
of the oscilloscope and shown on the selection knob.
The next step will be to place a periodic (repeating) waveform (the kind available from a
function generator) on the vertical deflection plates. The oscilloscope is designed to detect the
magnitude and direction of change of the signal coming into the vertical deflection system. At a
particular point it will "trigger” the system to display the vertical signal while the horizontal
deflection plates display the passage to time. The result is a 'fixed* display which is really a
repeating display. The repeat time is normally so short that a human eye cannot see that the
display is constantly being refreshed.
2.7.2 Unit 2 Laboratory: Prepare the oscilloscope to make measurements.
1) Locate an oscilloscope on the shelf above the lab bench.
2) Connect the function generator to CHANNEL 1 of the oscilloscope using the lead with banana jacks at
one end (for connection to the function generator) and a BNC connector at the other end (for
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EET-112: Elementary Electronics
connection to the oscilloscope). The oscilloscope is a Dual Trace model meaning it can display two
separate signals simultaneously. For this experiment only one of the two channels will be used.
3) Set the MODE switch between the channel inputs to CHANNEL 1 so the input of Channel 1 will be
displayed on the horizontal axis of the display screen.
4) Set the VOLTS PER DIVISION KNOB for Channel 1 to the 0.5 volts per division position. This knob
determines how much the incoming signal will be magnified vertically as it is displayed on the screen
of the oscilloscope.
5) Set the TIME PER DIVISION KNOB to the 02 milliseconds (mS) per division position. This knob
determines how much the horizontal display of the incoming signal will be stretched or contracted on
the screen of the oscilloscope.
6) Turn on the function generator.
7) Turn on the oscilloscope. A display of a sinusoidal waveform should be seen on the screen of the
oscilloscope. If a clear sinusoidal waveform does not appear change the settings of the VOLTS/DIV
setting and/or the TIME/DIV setting. If no display occurs after making these changes contact the
laboratory instructor for help.
8) Once a proper display has been established, change separately the settings of the VOLTS/DIV switch
and then the TIME/DIV switch and note the changes in the display. Changing the setting on the
oscilloscope only changes the display of the signal coming from the function generator, but does not
change the signal itself.
9) Change separately the amplitude setting of the function generator and the frequency setting and note
the changes in the oscilloscope display. These changes actually modify the signal coming from the
function generator and this will alter the display seen on the oscilloscope screen.
10) Change the waveform output (sinusoidal, square, triangular) of the function generator to square
wave and triangular wave and note the changes in the oscilloscope display. These changes also modify
the signal coming from the function generator and this will alter the display seen on the oscilloscope
screen.
2.8 Unit 2 Laboratory: Measuring resistances with a Multimeter
To measure resistances a meter must be a DC power supply. A small and very exact voltage appears at
the terminals of the meter when it is put in the ohmmeter mode. You can measure this voltage by using
another voltmeter. Then the meter measures the current which passes through the circuit which is
external to the meter. Using Ohm's Law, which you will investigate in Units 3 and 4, the meter calculates
and then displays the value of resistance detected in the external circuit.
1) To measure resistance depress the multimeter mode switch K ohms.
2) Set the RANGE SWITCH to AUTO.
3) Be certain that no power supply is connected to the experimental circuit
4) Connect leads from:
a) JACK-V/Ω (Omega) on the DMM to JACK B3 in the experimental circuit
b) JACK-COMMON on the DMM to JACK B4 in the experimental circuit
5) Note the reading on the DMM. It should be about 1000 ohms. Most likely the reading will not be
exactly 1000 ohms for the resistors used in the laboratory have a manufacturing tolerance of +/- 5%.
This means the resistors labeled 1000 ohms can have values from 1050 ohms [1000 + (5% of 1000 =
50)] to 950 ohms [1000 - (5% of 1000 = 50)].
6) Disconnect any power supply or function generator connected to JACKS A1-A2. Then set jumpers only
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EET-112: Elementary Electronics
between JACKS B1-B2 and C1-C2. These jumpers modify the circuit such that the 1000 ohms resistors
between JACKS B3-B4 and JACKS C3-C4 are in series and are the only two resistances effectively in
the circuit. The resistor between JACKS D3-D4 is not included because the lack of a jumper between
JACKS D1-D2 leaves this branch as an open circuit. The total resistance of resistors in series, known as
the effective resistance (Re), is the sum of the individual resistances. Connect the DMM from JACKS B3
to C4 and note the reading which should be approximately 2000 ohms, given by the following equation
(eqn. 2-1).
Re = RB3-B4 + RC3-C4, where Re is effective (total) resistance, RB3-B4 is the resistance between
JACK B3-B4 and RC3-C4 is the resistance between the JACK C3-C4. (eqn.2-1)
7) Insert a jumper between JACKS D1-D2. This jumper modifies the circuit such that the 1000 resistors
between JACKS C3-C4 and D3-D4 are in parallel. It can be shown that if two resistances (R1 and R2)
are in parallel, then the resultant effective resistance (Re) have the following relationship shown in
equation 2-2.
1/Re = 1/R1 + 1/R2 , which means, Re = (R1* R2 )/(R1 + R2)
(eqn. 2-2)
Connect the DMM between JACKS C3/D3 and C4/D4 and note the reading. It should be approximately
500 ohms. This value may be calculated by replacing the resistances between JACKS C3-C4 (RC3-C4)
and JACK D3-D4 (RD3-D4) as shown below.
Total Resistance = (RC3-C4* RD3-D4)/ (RC3-C4+ RD3-D4) = (1000*1000)/(1000+1000) = 500 ohms
The measurement reading will probably not be exactly 500 ohms because the two “1000” ohm
resistors are probably not exactly equal to 1000 ohms. For this measurement the 1000 ohm resistor
between JACKS B3-B4 is not considered because the open circuit between JACKS A1-A2 makes this
branch an open circuit.
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SPECIAL NOTES:
-
Note that for all subsequent experiments it will be necessary that you MUST know how
to obtain proper output voltage from a DC power supply, generate appropriate signal
from a signal generator and be able to make measurement with a digital and/or analog
multimeter and oscilloscope.
-
Before connecting any output to a multimeter for measurement, YOU MUST change the
mode of operation of the multimeter along with the range (ammeter (A), voltmeter
(V/Ω, autoranging or proper range switch and “=” or “~ “ ), which must be higher than
the anticipated measuring value. Make the necessary changes with connections to the
meter required by the mode of operation (for current measurement to JACK with A, for
voltage or resistance measurements to JACK with V/Ω symbols).
-
For resistance measurement the circuit must be disconnected from any power supply.
-
You must also note that the POWER SUPPLY and/or THE FUNCTION GENERATOR MUST BE
TURNED OFF each time:
o
o
IT IS CONNECTED OR DISCONNECTED FROM A CIRCUIT, OR IF
AN AMMETER IS INSERTED INTO OR REMOVED FROM A CIRCUIT.
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