Lab 1: DC Multimeter Measurements
Objective
The main goal of this experiment is for you to learn how to construct circuits on a breadboard and
use a multimeter for measurements. Along the way, you should also come to understand the resistor
color code, Ohm’s Law, and the distinction between series and parallel circuit elements.
Concepts: Voltage, Current, Resistance, and Ohm’s Law
DC (Direct Current) voltage and current do not vary with time. Voltage is an energy difference
between two points. When a voltage source is included in a closed path with resistors, a circuit is
completed, and current flows. Current is electron flow, or moving charge. When current flows across
a resistor, which impedes current flow, voltage drop occurs across the resistor. This relationship
between voltage (V), current (I), and resistance (R) is known as Ohm’s Law:
V = IR
(1)
Thus, if two of the above quantities are known, the third can be calculated.
In the experiment, you will use a multimeter to measure resistance, voltage, and current. The
experimental results will then be compared against Ohm’s Law.
Equipment and Components
This experiment requires the use of the Agilent E3630A Power Supply and Fluke Digital Multimeter
(DMM), as well as a breadboard, 22 AWG wire, and resistors with nominal values of 1.5 k, 2.2 k,
3.3 k, and 4.7 k.
Operation of the Agilent E3630A Power Supply
To power your circuit, you‘ll use the Agilent E3630A, shown in Figure 1. You can use either the
+6V, +20V or the -20V supplies by pressing the button on the button labeled Meter. The red lead
goes to the selected supply (+6V, +20V,-20V), which is connected in the lower-right side labeled
output. This terminal provides a voltage source to the circuit. The COM terminal is considered the 0
V (ground) reference voltages. Always begin by turning the knob for the selected supply to the
counter-clockwise stop so that voltage begins at 0 V. These Knobs can be located in the upper-right
side of the power supply under the label Voltage Adjust.
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The power supply should always be turned off until a circuit is complete and ready for use.
Once a circuit is complete, the supply can be turned on, and the voltage can be adjusted by turning
the voltage knobs slowly clockwise until the desired voltage is reached.
Figure 1: Agilent E3630A front panel
Operation of the Fluke DMM
The multimeter you’ll be using in this lab is the Fluke 189 DMM, shown in Figure 2. A multimeter is
a device used to take measurements of electrical quantities such as resistance, current, and voltage.
DMM stands for Digital Multi Meter.
To use the DMM as a voltmeter to measure voltage, insert a red wire in the jack labeled V and a
black wire in the jack labeled COM. Set the meter knob to the voltage setting. Voltmeters have very
high resistance that typically exceeds 1 MΩ, so when making voltage measurements, be sure the
voltmeter is connected in parallel with the circuit component(s) across which voltage is measured. A
common mistake is to connect the voltmeter in series with the circuit components. This error would
add a 1MΩ series resistance to the circuit and drastically change the circuit parameters.
For use as an ammeter to measure current, insert a red wire in the 10 A max fused input, and black
wire in the jack labeled COM. Turn the knob to measure current, and select a current range (mA or
(A) dependent on the expected current measured. For this experiment the 20m range is the most
appropriate. Ammeters have very low resistance that typically is less then 0.5 Ω. A common mistake
is to connect the ammeter in parallel with the circuit components. This error would effectively cause
a short circuit, altering the circuit parameters, and possibly damaging the ammeter. When making
current measurements, make certain the ammeter is connected in series with the circuit components
through which current is measured. Never connect an ammeter directly across a power supply as
it will cause a short circuit and will certainly damage the meter.
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a
b
c
Figure 2: Fluke Digital Multimeter shown in three different measurement configurations
Prelab (25 points) – Due at the beginning of lab
1. In the discussion session, you were introduced to the color code by which the nominal values of
resistors are represented. The nominal values of the resistors you will use in the lab are listed on the
data sheet at the end of this document. Next to each value, write the color code. (10 points)
2. The Fluke DMM is shown in three different measurement configurations. For each setup a, b, and
c, in Figure 2, determine what electrical quantity the meter is set to measure, and write them in the
data sheet. Refer to the online user’s manual. (10 points)
3. You’ll be using a multimeter to measure voltage and current. Voltage through a circuit element
such as a resistor is always measured in parallel with the circuit element, while current is always
measured in series. Two circuit elements are in series with one another if there is no alternate path
between them for current to flow. If two circuit elements are in series, the same current flows through
them. If there is an alternate path between them, the elements are in parallel. Figures 1 and 2 show
examples of series and parallel circuit elements.
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R1
R1
1k
1k
V1
V1
R2 5Vdc
1k
5Vdc
DMM
R2
1k
0
0
Figure 3: Series resistive circuit
Figure 4: DMM in parallel with R2
In Figure 1, the voltage source, V1, is in series with the resistor R1, since there is no other path
between them. Likewise, R1 is in series with R2. In Figure 2, the voltage source is still in series with
R1. The DMM, is in parallel with R2, since current can be split between them. Resistor voltage is
measured with the DMM in parallel with a resistor, and current is measured with it in series. Using
Figures 1 and 2 as examples, draw the DMM in series with a resistor and voltage source on the
data sheet. (5 points)
Part 1: Resistance Measurements
Resistor values vary slightly from their nominal value. Since voltage and current are related to
resistance, it’s a good idea to measure the actual resistance value so you know exactly what you’re
working with. Manufacturers specify the percent error from which the actual value of a resistor can
deviate from the nominal value. This quantity is called the tolerance. Your first task is to measure the
resistance of the four resistors from the prelab and determine whether they fit within the
manufacturer’s specified tolerance.
1. A gold band represents 5% tolerance, a silver band represents 10% tolerance, and no band
represents 20% tolerance. Write the tolerance for each resistor in your data sheet.
2. Measure the resistance and enter the values in the data sheet. The resistor should not touch your
fingers or be part of a circuit when you measure it. The resistor leads should only touch the
DMM terminals.
Calculate and enter the percent error for each resistor using the following formula:
Percent Error = ((Nominal value– Measured value) / Nominal value) x 100%
(2)
On the data sheet, state whether each resistor meets the manufacturer’s tolerance specifications.
3. On the data sheet, give the reason why your fingers can’t touch the resistor when measuring it.
Part 2: Voltage and Current Measurements for Single-Resistor Circuit
s
Our first circuit will be a simple series circuit in which the source voltage, V s, the resistor voltage,
VR1, and their corresponding current will be measured. The current will then be checked using
Ohm’s Law. First assemble the circuit shown in Figure 5.
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+
Lodestar
8102
Vs
8Vdc
R1
4.7 k
-
0
Figure 5: Series circuit for measurements
Use the DMM to make the following measurements and write the values in the data sheet.
1. For the circuit in Figure 5, use the DMM to measure Vs and write this value on the data sheet.
This value is the source voltage. Since the meter on the power supply is difficult to read
accurately, it is a good idea to always measure the source voltage so that you know the circuit
input. If the voltage value was not exactly 8 V, use the FINE knob and the DMM to get as close
to 8 V as possible.
2. Reconfigure the meter to measure voltage instead of resistance. Measure the voltage across the
resistor, as shown in Figure 6. The multimeter should be in parallel with the resistor for use as a
voltmeter. This is because the voltmeter has a very high input resistance, and current tends
toward the path of least resistance, and ideally we want all the current to pass through the
resistor.
Figure 6: DMM in parallel to measure voltage
Figure 7: DMM in series to measure current
3. Measure the current through the resistor as shown in Figure 7. Remember that the multimeter
must be in series with the resistor to measure current, since ammeters (current meters) have low
input resistance. We connect the DMM in series so that the current passing through the meter and
the resistor are the same. Placing the DMM in parallel would short the circuit. This could blow a
fuse in the meter, and you’d wait around while someone fixes it.
4. Since the voltage source and resistor are the only circuit elements, all the voltage in the circuit is
going across the one resistor. Given Ohm’s Law in equation (1), use your measured values of
voltage and current to calculate the resistance.
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5. Measure the voltage across the resistor again, but disconnect one end of the resistor while still
measuring the voltage. One end of the multimeter should be connected to the end of the resistor
that’s connected to the circuit, and the other end should be connected to the point on the circuit
board where you disconnected the resistor. Enter the value in the data sheet. This value is known
as the open-circuit voltage, Voc.
6. Now do the same thing with current. Measure the current, and without changing anything else in
the circuit, disconnect one side of the resistor. Write the value of the open-circuit current in the
data sheet.
What you should observe from steps 5 and 6 is that current needs a closed electrical path to flow. In
this sense, it can be thought of as analogous to water flowing through a pipe. Voltage, however, can
exist in an open circuit, since it is an energy difference between two points. In this sense, you can
think of voltage as the pressure in a water pipe that exists when the faucet is turned off.
7. Compare your measured and calculated values of I and include your observations in your answer
to question 4 on your data sheet.
Part 3: Voltage and Current Measurements for Four-Resistor Circuit
.
This last part of the lab is to gain get some practice taking voltage and current measurements, and
experimentally determine resistance values using Ohm’s Law. First build the circuit shown in Figure 8.
Figure 8: Series circuit for measurements
1. As always, measure the source voltage and adjust it to be as close to the nominal value (in this
case 10 V) as possible.
2. Answer question 5 on the data sheet. Refer to Question 3 of the Prelab on page 3 for hints.
3. Measure the voltage across each resistor and write the values on the data sheet. To check if your
voltage values are correct, use Kirchhoff’s Voltage Law (KVL). It states that the algebraic sum of
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the voltages around each loop of a circuit must equal zero. This will be the case if your
measurements are right.
4. Measure the current through each resistor and write those values on the data sheet. You can
check your measurements by Kirchhoff’s Current Law (KCL). It says that the current into a node
(a point on a circuit where two or more circuit elements meet) is equal to current out of a node.
Another way of putting it is that all the currents into a node (taking, say current out being positive
and current in being negative – although you can also do it the opposite way, with current out
being defined as negative and incoming current being positive – either way, you’ll get the same
result) algebraically sum to zero. Kirchhoff’s Laws are based on conservation of energy, and
we’ll be looking at them again in the next lab.
5. Use Ohm’s Law to calculate the resistance for each resistor and enter the values in the data sheet.
6. Lastly, calculate the percent error between your calculated resistance values and the nominal
values.
When you’re finished, turn in the data sheets (pages 8 and 9) to your TA.
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Lab 1: DC Resistive Measurements Data Sheet
Name_____________________________
Section________
Prelab (due at the beginning of lab).
.
1. Next to each value, write the corresponding color code.
1.5 k _______________________________
2.2 k _______________________________
3.3 k _______________________________
4.7 k _______________________________
Figure 2a is set to measure _________________________
Figure 2b is set to measure _________________________
Figure 2c is set to measure _________________________
3. In the space below, draw the multimeter in series with a voltage source and resistor.
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Part 1: Resistance Measurements
1. Color codes, measurements, and percent error
Nominal
Resistance
Color Code
()
.
Nominal
Tolerance
(%)
Measured
Resistance
()
Percent
Error (%)
Within
Manufacturer’s
Spec (y/n)
1500
2200
3300
4700
2. Why can’t your fingers be touching the resistor when measuring it?
Part 2: Voltage and Current Measurements for Single-Resistor Circuit
3. Voltage and current values
VS (V)
VR1 (V)
IR1 (mA)
V/I ()
VOC (V)
4. How does the value of V/I compare to the value you measured in Part 1? Give reasons for the
similarities or differences.
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IOC (mA)
Part 3: Voltage and Current Measurements for Four-Resistor Circuit
5. Which two resistors should have the same current through them? Why?
6. For each resistor below, list the voltage, current, value of V/I, and percent error.
1.5 k
2.2 k
V (V)
I (mA)
V/I ()
R Percent
Error (%)
10
3.3 k
4.7 k