E11
‐
page
1
BASIC D. C. MEASUREMENTS
OBJECTIVE
This experiment is designed to give you practice in measuring current, voltage, resistance and in using Ohm's
law. It should also help you understand the rules to find the equivalent series and parallel resistance.
Equipment Required:
•
•
•
•
•
one 0-24 V DC variable power supply
2 digital multimeters (on the top panel of the power supply)
five 0.5 watt carbon resistors with nominal values:
 R1 = 1.0 KΩ ± 5%
 R2 = 3.9 KΩ ± 5%
 R3 = 2.7 KΩ ± 5%
 R4 = 2.2 KΩ ± 5%
 R5 = 1.8 KΩ ± 5%
one board with banana plug sockets (four of them labeled A, B, C, D)
connecting cables as needed.
PART 1 - USING AN OHMMETER
This part will show you how to use the ohmmeter function of your multimeters to measure the resistance of the
five carbon resistors.
CAUTION: The ohmmeter must never be used when the resistors are connected to a source of power as the
internal circuitry of the meter (which in ohmmeter mode includes its own battery), could be seriously damaged
if connected to another power source.
Procedure:
1.
2.
3.
4.
Set the function selector switch on the multimeter to KΩ (kilo-ohms).
Select each resistor in turn and connect the meter across it.
Read off the resistance value from the meter’s display.
Reproduce the following table in your report and complete it.
Table 1: Resistors to be used in this experiment
Resistor
Nominal
Value (kΩ)
R1
1.0 KΩ ± 5%
R2
3.9 KΩ ± 5%
R3
2.7 KΩ ± 5%
R4
2.2 KΩ ± 5%
R5
1.8 KΩ ± 5%
Range of
Value
Ohmmeter
reading
(kΩ)
(kΩ)
(0.950, 1.050)
Within
tolerance?
(Yes or
No)
E11
‐
page
2
PART 2 - VOLTMETER-AMMETER METHOD OF RESISTANCE MEASUREMENT
Circuit Diagram:
Procedure:
1. Set up the circuit shown using the board to make the connections. On the top panel of the power supply
there are two multimeters. You should use one of them as the milliammeter and the other as the
voltmeter, although you may have already seen in part 1, both can be used as ohmmeters.
2. For the milliammeter turn the function-selector switch to DC mA and for the voltmeter set the selector
switch to DC V (You will be using a maximum of around 20V DC.).
3. Start with the power supply output control fully counterclockwise (zero output) and gradually turn it
clockwise. Record the corresponding current and voltage readings at 10 different voltages up to a
maximum current of 5 mA in the following table:
Table 2: Voltage and Current values for the R2 = 3.9 kΩ resistor
No.
V - Voltage (V)
I - Current (mA)
1
2
3
4
5
6
7
8
9
10
4. Turn the power supply control down and disconnect the voltmeter. Turn the power back up again until the
current is exactly 5 mA. Measure the voltage across the milliammeter. Use these current and voltage
readings to calculate the resistance of the milliammeter RmA.
Data Analysis:
5. Use the values in Table 2 and plot V (y - axis) against I (x - axis). Calculate the value of R2 from the slope
of the graph and compare this value of R2with the “ohmmeter read” value for R2 obtained in step 1.
6. Compare now the slope of graph with the sum of the “ohmmeter read” value for R2 and the milliammeter
resistance RmA calculated in step 4.
E11
‐
page
3
PART 3 - SERIES NETWORK
Circuit Diagram:
Procedure:
1. You will be using the 5 resistors as selected in Part 1.
2. Connect a network of 2 resistors on the board between AD and DC as directed in the table below. When
you disconnect any of the two resistors the ammeter reading becomes zero, while the voltmeter reading is
the same (or very close, but not zero!; in fact the voltmeter reading might be higher when one of the
resistors is disconnected!). Can you explain why this is happening?
3. Turn up the power supply until the voltmeter reads 10.0 V and record the values of the current and
voltage in the following table:
Table 3: Voltage, Current and Resistance Values for the SERIES network
No.
Resistors
Connected
1
R1 and R4
2
R2 and R3
3
R1 and R2
4
R3 and R4
5
R1 and R5
6
R2 and R5
Circuit Diagram
VVoltage
(V)
I - Current
(mA)
Res –
Equivalent
Series
Resistance
(KΩ)
Ohmmeter
reading Res
(KΩ)
E11
‐
page
4
4. Use Ohm’s law to calculate Res for each series combination and record these values in the table above.
5. Turn off and disconnect the power supply, the ammeter and the voltmeter and measure the resistance Res
between A and C using the ohmmeter for all six series combinations. Record them in the table in the
“Ohmmeter Reading Res” column.
Data Analysis:
6. Compare the last two columns. Can you draw any conclusions based on the values in these two columns?
PART 3 - PARALLEL NETWORK
Circuit Diagram:
Procedure:
1. You will be using the same five resistors (R1, R2, R3, R4 and R5) as specified in Part 2.
2. Connect the network of resistors on the board as directed in Table 4 below.
3. In each case, turn up the power supply until the voltmeter reads 10.0 V. Record the current and voltage in
Table 4 and calculate Rep using Ohm's Law.
E11
‐
page
5
Table 4: Voltage, Current And Resistance Values For The PARALLEL Network
No.
Resistors
Connected
1
AD = R1
AB = R2
(CD, BC =
patch wires)
2
AD = R1
AB = R2
AC = R5
(CD, BC =
patch wires)
3
AD = R3
AB = R4
(CD, BC =
patch wires)
4
AD = R3
AB = R4
AC = R5
(CD, BC =
patch wires)
Circuit Diagram
VVoltage
(V)
ICurrent
(mA)
Rep –
Equivalent
Parallel
Resistance
(KΩ)
Ohmmeter
Reading Rep
(KΩ)
4. Turn off and disconnect the power supply, ammeter and voltmeter and measure the resistance Rep between
A and C using the ohmmeter for all 4 parallel combinations in the table above and complete the
“Ohmmeter Reading Rep” column. Compare the values in the last three columns.
Data Analysis:
5. Compare the last two columns. Can you draw any conclusions based on the values in these two columns?