Download Voltmeter and Ammeter Loading Effects

EEE3406 Instrumentation & Measurements
│LABORATORY – Experiment 1│
│
Name
Date
Class
Class No.
Marks
Voltmeter and Ammeter Loading Effects
Objectives:
After completing this lab, you will be able to
use the sensitivity, S of an analog meter to determine the loading resistance of a
voltmeter,
calculate the loading effect of DMM and VOM when measuring voltage and current,
determine the input resistance of a voltmeter and ammeter,
measure and compare the loading effects of analog and digital voltmeters and ammters in
high-resistance and low-resistance circuits.
Equipment/Components Required
Digital multimeter (DMM)
Volt-ohm-milliammeter (VOM)
Variable DC power supply
Breadboard
Resistors : 4.7 Ω, 47 Ω, 3.9 kΩ, 5.6 kΩ, 3.9 MΩ, 5.6 MΩ
Ver
Author(s)
Date
1.0
YH WONG
1/07
For EEE3406 (PT)
1.1
YH WONG
7/07
Block for students’ name/class, date of experiment added.
Remark
Laboratory
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Experiment 1
1
PART A: BACKGROUND
When we use a voltmeter to measure the voltage in a circuit, we always assume its input
impedance is very large that it does not load the circuit and it will always indicate the correct
voltage in a circuit. Unfortunately this is not always the case.
5.6 MΩ
A
E = 10 V
V
B
Figure 1
Loading effect of a high resistance source
In order for any instrument to provide a measurement, it must take a small amount of energy
from the circuit under test, and use this energy to obtain a reading. While the amount of
energy taken from most circuits is virtually undetectable, this is not always so. Consider the
circuit as shown in Figure 1. Ideally, the internal resistance of a voltmeter is infinitely large,
resulting in no circuit current. The voltage appearing across the voltmeter will be 10 V
which means that the voltmeter provides a correct reading of 10.0 V.
However, all voltmeters have some internal resistance.
If the internal resistance of the
voltmeter was equal to the series resistance, 5.6 MΩ, then the voltage appearing across the
voltmeter would be half of the supply voltage, resulting in a reading of 5.0 V. If the internal
resistance of the voltmeter is even smaller, the voltmeter’s reading will be smaller too. The
degree to which a meter loads a circuit under test is called the loading effect and is
determined mathematically as:
loading effect =
2
Experiment 1
unloaded value − reading
× 100%
unloaded value
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EEE3406 Instrumentation & Measurements
A similar effect occurs when an ammeter is connected into a circuit. Consider the circuit in
Figure 2. When an ammeter is placed into the circuit to measure the current I, we expect
that the ammeter will indicate the correct current. This would be a correct assumption if the
ammeter had an internal resistance of zero ohms. However, since all ammeters have some
internal resistance, there will always be some loading effect on the circuit. The amount of
loading is dependent upon the resistance of the meter and on the equivalent circuit resistance.
I
4.7Ω
5.6
Ω
1V
E = 0.2V
Figure 2
A
Measuring current in a low-resistance circuit
If the ammeter has an internal resistance at least ten times smaller than the circuit resistance,
its loading effect will be relatively small. Conversely, if the resistance of the ammeter is
comparable with the circuit resistance, the loading effect will be large, a meaningless
measurement is resulted. Recall the loading effect for any meter is given as:
loading effect =
unloaded value − reading
× 100%
unloaded value
In order to know whether the internal resistance of the meters is comparable with the circuit
under test, you must know the meter’s internal resistance. The internal resistance of the
DMM and VOM for its ammeter operation can be found in their operation manual. Please
check these values from the operation manuals.
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Experiment 1
3
PART B:
PROCEDURES
1. Voltmeter Loading Effect
1.1
Refer to the manual of the DMM, obtain the internal resistance of the voltmeter and
record this value.
Rint = ______________Ω
1.2 The internal resistance of an analog meter is generally dependent on the voltage range
used.
In order to determine the internal resistance, the manufacturer provides a
specification called the sensitivity, S which has units of kΩ/V. The internal resistance is
then determined as the product of sensitivity and the voltage range of the meter as:
Rint = SVrange
Refer to the manual of the VOM, obtain the sensitivity of the meter. Determine the
correct range that you would use to measure a voltage of 10 V. Calculate and record
the internal resistance of your meter on this range.
Vrange
S
Rint
1.3 Connect the circuit of Figure 3. Place the DMM voltmeter directly across the terminals
of the voltage source and adjust the voltage source for exactly 10 V which is the
unloaded voltage between terminals A and B.
VAB (unloaded) = ___________________V
5.6 kΩ
A
E = 10 V
V
B
Figure 3
4
Experiment 1
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EEE3406 Instrumentation & Measurements
1.4 Remove the DMM and connect it between terminals A and B.
appearing between these terminals.
Measure the voltage
VAB (measured) = ___________________V
1.5 Calculate the loading effect of the DMM.
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
1.6 Replace the DMM with a VOM. Set the Vrange of the VOM to appropriate range to
measure 10 V. Measure the voltage appearing between these terminals.
VAB (measured) = ___________________V
1.7 Calculate the loading effect of the VOM.
____________________________________________________________
_____________________________________________________________
___________________________________________________________
____________________________________________________________
____________________________________________________________
1.8 Based on the test circuit, calculate the internal resistance of the VOM.
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
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Experiment 1
5
1.9
Refer to Figure 4. Calculate the voltage across each of the resistors using the voltage
divider rule.
V1 (unloaded)
V2 (unloaded)
+ V1
-
R1
5.6kΩ
+
R2
3.9kΩ
E = 10V
Figure 4
V2
-
Voltmeter loading effect of a low-resistance circuit
1.10 Connect the circuit of Figure 4 and measure the voltages V1 and V2 with both the
DMM and VOM voltmeters.
Reading
V1
V2
DMM
VOM
1.11 Calculate the corresponding loading effect of the DMM and the VOM.
Loading effect
V1
V2
DMM
VOM
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Experiment 1
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EEE3406 Instrumentation & Measurements
1.12 Replace the resistors 5.6 kΩ and 3.9 kΩ with 5.6 MΩ and 3.9 MΩ in Figure 4.
Calculate the voltage across each of the resistors and measure the voltage across each of
the resistors with DMM and VOM.
V1 (unloaded)
V2 (unloaded)
Reading
V1
V2
DMM
VOM
1.13 Calculate the corresponding loading effect of the DMM and the VOM.
Loading effect
V1
V2
DMM
VOM
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Experiment 1
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2.
Ammeter Loading Effect
Use a DMM ohmmeter to measure the actual resistance of a 4.7 Ω resistor. Use a
DMM voltmeter to adjust the voltage source to 0.3 V. Calculate the unloaded current of
the circuit in Figure 5.
Actual resistance of the 4.7 Ω resistor = ____________Ω
Unloaded current , I = _______________A
4.7 Ω
E = 0.3 V
A
Figure 5
Connect the circuit of Figure 5. Use the DMM as ammeter first and record the meter’s
reading. Repeat with a VOM ammeter. (Ensure that the ammeter is on the correct range
and polarity to measure the current).
Meter
Reading
I (DMM)
I (VOM)
Calculate the loading effect of the two ammeters.
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
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Experiment 1
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EEE3406 Instrumentation & Measurements
2.4 Calculate the internal resistance of each of the ammeters
____________________________________________________________
____________________________________________________________
____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
2.5 Replace the 4.7 Ω resistor with a 47 Ω resistor and adjust the voltage of the DC supply to
3 V. Measure the current of the circuit with the DMM and VOM ammeters and
calculate the corresponding loading effect.
R (47 Ω)
Loading Effect
I (unloaded)
I (DMM)
I (VOM)
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Experiment 1
9
Part C – Discussion
1.
Which voltmeter (DMM or VOM) has the higher loading effect? Why?
____________________________________________________________
____________________________________________________________
____________________________________________________________
2.
How does the loading effect of a voltmeter in a low-resistance circuit compare to the
loading effect in a high-resistance circuit?
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
3.
Which ammeter, DMM or VOM, has the higher loading effect? Why?
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
4.
How does the loading effect of an ammeter in a low-resistance circuit compare to the
loading effect in a high-resistance circuit?
____________________________________________________________
____________________________________________________________
_____________________________________________________________
___________________________________________________________
____________________________________________________________
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Experiment 1
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EEE3406 Instrumentation & Measurements
PART D : SUMMARY
In about 100 words, summarize what you have learnt from this experiment.
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Experiment 1
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