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.
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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.
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ELEC 103 LABORATORY EXERCISE 3 CURRENT MEASUREMENTS
STEP 3:
− Schematic −
Current = __________
Range = __________
STEP 5:
− Schematic −
Current = __________
Range = __________
STEP 7:
− Schematic −
Current = __________
Range = __________
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ELEC 103 LABORATORY EXERCISE 3 CURRENT MEASUREMENTS
STEP 9:
− Schematic −
Current = __________
Range = __________
STEP 12:
− Schematic −
Current = __________
Range = __________
STEP 13:
− Schematic −
Current = __________
Range = __________
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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?
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