EXPERIMENT 10:
Introduction to Electrical Circuits
Read the entire experiment and organize time, materials, and work space before beginning.
Remember to review the safety sections and wear goggles when appropriate
Objectives: To build and understand the principles of a simple electric circuit, and
To learn to use the various functions of a digital multimeter (DMM),
including ammeter, voltmeter, and ohmmeter.
Materials: Student Provides: Computer with spreadsheet software
From LabPaq: Digital multimeter, DMM
5 Jumper cables
3 1.5V Batteries with holders
100 Ħ Resistor
Discussion and Review: Reading and understanding a circuit diagram and then
building an electric circuit from it is a skill similar to reading a construction blue print. In
this laboratory exercise, wefll become familiar with the basic terms and symbols needed
to build electric circuits. Some of the most common symbols used in the circuit diagrams
you will be using are shown below.
Name Symbol(s) Example
DC Power (Battery) Flashlight batteries
VDC
AC Power, VAC Wall power outlet
Resistor, R Heating element, lamp, etc
Lamp Light bulb
On/Off switch Light switch
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Name Symbol(s) Example
Wire Any wire
Capacitor Current storage
Diode Digital thermometer
Inductor Wire coil
Meter Symbols: We will be using a digital multimeter (DMM) which can be used as a
voltmeter to measure voltage, an ammeter to measure current, and an ohmmeter to
measure resistance. The symbols for individual meters are:
Voltmeter to measure voltage
Ammeter to measure current
Circuit Drawings: The illustrations in this manual reflect the way
circuits are traditionally drawn to show how components are
connected. Your actual circuits will not look as nice and neat as the
diagrams since connecting cables will not
be in perfectly straight lines and angles. At
right is a photo of how an actual circuit
might look. The left photo shows how two
sets of jumper cables are connected to one
resister in parallel.
Reading and understanding the color codes of resistors: To calculate the value of a
resistor, use the color-coded stripes on the resistor and the following procedures, plus
the table on the following page:
1. Turn the resistor so that the gold or silver stripe is at the right end of the resistor.
2. Look at the color of the first two stripes on the left end. These correspond to the
first two digits of the resistor value. Use the following table to determine the first
two digits.
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3. Look at the third stripe from the left. This corresponds to a multiplication value.
Find the value using the table below. Multiply the two-digit number from Step 2
by the number from Step 3. This is the value of the resistor in ohms.
4. The fourth stripe indicates the accuracy of the resistor. A gold stripe means the
value of the resistor may vary within 5% from the value given by the stripes. A
silver stripe means the value of the resistor may vary within 10% from the value
given by the stripes.
Resistor Color Codes: Read the code with gold or silver stripe on right end.
With a little practice you soon will be able to quickly determine the value of a resistor
by just a glance at the color -coded stripes. Assume you are given a resistor whose
stripes are colored from left to right as brown, black, orange, and gold. To find the
resistance value:
.. Turn the resister to where its gold stripe is on the right.
.. The first stripe on the left is brown, which has a value of 1. The second stripe
from the left is black, which has a value of 0. Since the first two digits of the
resistance value are 1 and 0, this means this value is 10.
.. The third stripe is orange, which means to multiply the previous value by
1,000.
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.. Thus the value of the resistance 10 time 1000 or 10 x 1000 = 10,000 ohms.
10,000 ohms can also be expressed as 10 kilohms or 10 k Ħ ohms.
.. The stripe in Step 1 is gold; this means the actual value of the resistor may
vary by 5%. Since 5% of 10,000 = 0.05 x 10,000 = 500, the actual value in
our example will be somewhere between 9,500 ohms and 10,500 ohms.
DIGITAL MULTIMETER OPERATING INSTRUCTIONS: It is important that you read
and understand the following instructions plus pay attention to the special
cautions noted below or you could damage the multimeter and/or blow a fuse.
Replacement fuses can be purchased at electronics stores. Direct any use
specific questions to your course instructor.
Digital Multimeters (DMM): It is important to familiarize yourself with the DMMfs
operations now so you can take accurate measurements without damaging the meter.
Multimeters are so called because they can measure three different qualities of a circuit.
These qualities, their symbols, and their basic units of measurement are summarized in
the table below: A different model of multimeter may be included in your LabPaqs, so
generic DMM operating instruction as well as those specific to the Cen-Tech DMM are
included here.
Regardless of the DMM model in your
LabPaq, you should thoroughly review
its accompanying instructions in
addition to the ones discussed below.
Lead Wires (Cen-Tech): Lead wires must be
connected correctly. The black lead is normally
connected to the bottom terminal labeled COM for
common which is also called ground. The red lead
must be connected to the corresponding terminal
for what you want to measure. For voltage,
resistance and low DC current, use the middle
terminal labeled VĦmA. (V=volts, Ħ=resistance in
ohms, mA=milli-ampere).
For DC current above 200mA, use the top 10ADC
terminal. Detailed instructions as to when to use
the middle vs. the upper terminal for the red lead
are given for each of the specific measurement
instructions below. Always read instructions
carefully to be certain you plug the leads into the
correct terminals for the appropriate quantity of
what you want to measure.
Symbol: V I and A R
Measurement:
Units:
Voltage
Volt
Current
Ampere
Resistance
Ohm or Ħ
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Basic Operations: The CEN-TECH digital multimeter (DMM) has a circular range dial
knob and a separate On-OFF switch. The central dial must be in the appropriate
position for the operation you want to perform.
The dial has the following positions starting with DCV and going clockwise:
.. DCV – To measure DC voltage: settings 200mV, 2000mV(2V); 20V, 200V, 1000V
.. ACV To measure AC voltage: settings: 200 & 750V
.. 1.5V(4.0mA) & 9V(25mA) – To measure battery charge for 1.5V & 9V
batteries only
.. DCA . To measure DC current . settings: 200ƒÊA, 2000 ƒÊA (=2mA); 20mA, and
200mA (= 0.2A)
.. 10A . To measure DC current greater than 200mA
.. hFE . To measure transistor values
.. To measure diode voltage drop
.. Ħ РTo measure resistance . settings: 200 Ħ, 2000 Ħ, 20K Ħ, 200K Ħ, 2000K Ħ
Use of the DMM as a DC Voltmeter: To measure voltage (V) difference, the DMM
leads are connected to the ends of the component(s) while the circuit is energized.
Connect the positive red lead close to the + end of the battery and the negative black
lead to the . end of the battery.
1. Turn the center dial to the appropriate DCV setting. The setting selected must be
higher than the quantity of expected volts or the DMM fuse may blow out! For a
1.5V system, set at the 2V setting, for a 9V system set it at the 20V setting, etc.
If you do not know the range of your value, start with the highest range and
switch down to lower ranges as necessary. This will prevent damage to the meter
that might occur if you select a range too low for the voltage you are measuring.
2. Plug the red cable lead into the center VĦmA jack. Plug the black cable lead into
the bottom COM jack.
3. Switch the multimeter on via the ON-OFF switch.
4. To measure the voltage, carefully touch the appropriate points in the circuit with
the tips of the multimeterfs probes.
5. Read and record the measurement.
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6. When testing is complete, turn off the DMM; remove the test leads, and store
your DMM.
Use of the DMM as an Ohmmeter: To measure the resistance (R) of a component
such as a resistor, the component must be disconnected from the circuit. You will get
an incorrect measurement if the component is in the circuit. You may also damage the
meter if the component is in the circuit and the circuit is also energized. This is the only
DMM reading that requires the circuit to be disconnected. You may measure circuit
resistance up to 2000K ohms.
1. Turn the range selector switch to an appropriate Ħ setting higher than the
expected ohms. For a 100 Ħ resistor, set the range switch on the 2000 Ħ
setting, etc.
2. Plug the red cable lead into the center VĦmA jack. Plug the black cable lead into
the bottom COM jack.
3. Switch on the multimeter via the On-OFF switch.
4. Touch the test leads together. The meter should read g0h Ħ, (Ohms)
5. Carefully touch the appropriate points in the circuit with the tips of the probes to
measure the resistance.
6. Read measurement.
7. If the reading is g1h, set the range selector switch to the next higher Ohm (Ħ)
position.
8. When testing is complete, turn off the DMM; remove the test leads and store your
DMM.
Use of the DMM as an Ammeter (Current meter): To measure current (I) the leads of
the meter must be connected into the circuit. Wherever the meter is inserted into the
circuit make certain that the red lead is closest to the + end of the battery along the
circuit and that the black lead is closest to – end of the battery. It is very important that
the multimeter be used in series as part of the circuit when measuring current instead of
in parallel outside of the circuit as when measuring voltage differences. Improper use
may damage the meter and blow the fuse making the multimeter inoperable.
1. Turn the range Selector Switch to the 10 A (amperes) position. Always start with
the highest range if the amperage is unknown.
2. Plug the red cable lead into the top10 A jack. Plug the black cable lead into the
bottom COM jack.
3. Switch the multimeter on via the On-OFF switch.
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4. Insert the multimeter in series with the circuit to be tested.
5. Read measurement. If the reading is less than .2 A switch the red cable lead to
the center VĦmA jack and set the range selector switch to the 200mA.
6. Read and record measurement. If you need a current reading in Amps instead of
milliAmperes, simply divide the mA reading by 1000.
7. When testing is complete, turn off the DMM; remove the test leads and store your
DMM.
Here is an example of how easily you can blow a fuse if your DMM is used incorrectly.
Assume you started in the 200mA (0.2A) position and used the DMM as a current meter
for a circuit with a 1.5V battery and a 1ohm resistor; you would blow its fuse
immediately!
Ohmfs law, V = IR, is also stated as I = V/R. Thus I = 1.5V/1Ħ = 1.5A or 1,500 mA.
This is 7.5 times the limit of the 0.2A setting. For this example circuit you would need to
use the 10A setting.
Always turn off your DMM when you have completed your measurements by moving
the switch to the goffh position. Otherwise the DMM battery will be used up prematurely
and have to be replaced.
Maintenance:
.. Remove battery if not in use for long periods.
.. Store unit in dry location
.. Other than the battery and the fuse, this DMM has no replaceable parts.
.. Repairs should be done by a qualified technician.
Battery/fuse Replacement:
.. Remove the test leads form the multimeter
.. Turn the unit over and remove both screws with Philips screwdriver.
.. Remove the back cover
.. Remove the battery or fuse and replace with a new 9V battery or 250mA fastacting
fuse.
.. Replace cover and retighten screws.
Your DMM may also be used to make AC Voltage Measurements; Transistor (hFE)
measurements, battery charge measurements, and diode measurements. However,
these measurements will not be used in any of the physics experiments in this LabPaq.
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PROCEDURES: Set up the following data table to use for these experiments:
DATA TABLE:
Resistance based on color bands: ____Ħ; % uncertainty ____ (from color band)
DMM-Measured Resistance ______ Ħ
Measured V
(V)
Measured Current
(A)
Calculated R = V/I
1.5V battery
3V battery
4.5V battery
Part 1: Before assembling the following circuits, set the DMM as an ohmmeter. Slide
the function switch to Ħ and the dial range to 2 kĦ. Check the resistance of the resistor
by touching the two DMM leads to the two wires extending from each side of the
resistor. Record this value.
A. Your first circuit will consist of a 1.5V battery in its holder and a 100-ohm resistor.
You will use 3 separate jumper cables to set up the circuit as shown below in the
illustration below.
1. Set the DMM as an ammeter.
Slide the function switch to A
and the dial range to mA.
2. Connect the first jumper cable
(1) to the negative end of the
battery holder and to the 100 Ħ resistor. Do this by simply opening the jaw of
an alligator clip at one end of a jumper cable and firmly clasping that jaw
around the metal tail, wire, or extender of the item to be connected into the
circuit. Metal must touch metal in all connections.
3. Connect the second jumper cable (2) to the other end of the resistor and the
black lead of the DMM.
4. Connect the third jumper cable to (3) the red lead of the DMM and the
positive end of the battery holder.
Remember that it is very important when measuring current to use the
multimeter in series, which means that it is inside and part of the circuit
as shown above. When measuring voltage differences the meter must
be in parallel, which means it is outside of the circuit as shown in B.
below. If you confuse these procedures you will blow out the DMMfs
fuse!
5. Take the mA reading and record it in the data table.
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6. You measured the current at only one point, call it point gP.h Is the current the
same everywhere in a simple circuit like this? To find out, rearrange the
jumper cables and meter to measure the current at a second point, call it
point gQ.h Discuss your findings in your conclusions.
B. Remove the DMM from the above circuit and close the circuit by connecting the
jumper cablesf alligator clips that previously connected the DMM within the
circuit.
1. Set the DMM as a voltmeter. Slide function switch to V , and the range dial
to 2V.
2. Set up the DMM as a voltmeter, which requires that it be parallel to and
outside of the main circuit as shown in the next illustration. Touch the DMMfs
positive red lead to the jumper cable connection at the positive end of the
battery holder (A) and touch the DMMfs negative black lead to the jumper
cable connection at the negative end of the
battery holder (B). Now take the V reading
between points A and B and record in the data
table.
3. Reverse the DMM leads at points A and
B by moving the black lead to point A
and the red lead to point B. Observe,
then record and explain your
observation.
4. Reposition the voltmeter to take a voltage reading between A and C, first with
the leads in one position and then with the leads reversed. Record and
explain these voltage readings.
5. Reposition the voltmeter to take a voltage reading between C and D, first with
the leads in one position and then with the leads reversed. Record and
explain these voltage readings. Note: You will want to thoroughly discuss
these observations in your lab report summary.
Part 2:
A. Again set up the DMM as an
ammeter within a circuit as in the
previous Part 1 A-1, but this time you
will add a second 1.5V battery in
series with the first 1.5V battery as
shown at right.
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1. To do this you will simply insert a second battery holder and jumper cable
next to the original battery holder so that two batteries are in the circuit
between the ammeter and the resister.
2. When the circuit is again complete, take the mA reading and record in the
data table.
B. Remove the DMM from the circuit shown in Part 2A above and close that circuit.
1. Set up the DMM as a voltmeter
parallel to the circuit with the leads
attached around both batteries as
shown at right.
2. Set the function switch on V and
the range dial to 20V.
3. Take a V reading and record in the data table.
Part 3:
A. Again set up the DMM as an ammeter as in Part 2A, but this time add a third
1.5V battery in series with the other two 1.5V batteries.
1. To do this you will simply insert a third battery holder and jumper cable next to
the original battery holder so that three batteries are now in the circuit
between the ammeter and the resister.
2. When the circuit is again complete, take the mA reading and record in the
data table.
B. Remove the DMM from the three batteries and the resistor connected in Part 3A
above and close that circuit.
1. Set up the DMM as a voltmeter parallel to the circuit with the leads attached
around all three batteries.
2. Set the function switch to V and the range dial to 20V.
3. Take a V reading and record in the data table.
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Calculations and Graphing: For each of the three previous procedures calculate the
resistance from the measured current and voltage: R = V/I.
1. Use an xy scatter graph to graph voltage on the y-axis versus current on the
x-axis.
2. Use the linear fit trendline function of ExcelR to add the slope of the line to the
graph.
3. What is the significance of the slope?
4. How do the graph and the slope of the line relate to Ohmfs law?
5. Can this series of experiments be considered a verification of Ohmfs law?
Why or why not?
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