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UNIVERSITY OF MASSACHUSETTS DARTMOUTH
DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
ECE 201
CIRCUIT THEORY I
EXTENDING THE RANGE OF AN ANALOG AMMETER
INTRODUCTION
The basic instrument used to measure electrical quantities is some form of analog
(moving pointer) ammeter. A typical d’Arsenval analog meter movement is shown below in
Figure 1.
Figure 1. A typical d’Arsenval analog meter movement
By adding circuit components that are external to the meter, we can construct instruments such
as multi-range ammeters, voltmeters, and ohmmeters. It turns out that every meter movement
displays some amount of resistance (the meter resistance, Rmeter ). In order to design additional
configurations, we need to know the meter resistance.
PRELAB ACTIVITY
Devise a NON-DESTRUCTIVE scheme to measure the resistance of the 0-1mA meter
that is provided in the lab. You may make use of an external power supply (or battery) and other
circuit components such as resistors to make your measurement. Design your measurement
scheme so that no more than 1 mA will flow through the meter! YOU CAN NOT USE THE
MULTIMETER AS AN OHMMETER TO MAKE THIS MEASUREMENT!
Have your measurement scheme approved by the instructor BEFORE you proceed
with the actual measurement!
EXTENDED-RANGE AMMETER
The basic meter movement can measure from 0 to 1 mA full-scale. In order to extend the range
of the meter, some external circuitry must be added. If the current to be measured exceeds the
full-scale current of the meter movement, a parallel, or “shunt” branch must be provided.
Consider the case shown below where the desired full-scale Load current to be measured is 5
mA. Assume that the meter resistance is 100 Ω.
1m AMeter
ILoad
A
+
0.000
Imeter
ILoad
DC 100
Vsource
5V
RLoad
1k
Rshunt
25
Ishunt
Figure 2. Circuit for a 0–1 mA meter to measure a 5 mA full-scale current
The Load current is 5 mA (if the meter resistance is equal to 0). When the shunt resistor is
properly chosen, current division causes 1 mA to flow through the meter and 4 mA to flow through
the shunt resistor.
Since the meter movement and the shunt resistor are in parallel, they have the same voltage drop
across them, so, if the resistance of the meter is known, we can determine the value of the shunt
resistor from the following:
V
=I
R
=I
R
meter meter meter shunt shunt
I
R
R
= meter meter
shunt
I
shunt
where
Imeter is the full-scale current of the meter
and
Ishunt = ILoad - Imeter
The equivalent resistance of the meter circuit is now the parallel combination of the meter
resistance Rmeter and the shunt resistor Rshunt.
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LAB DESIGN PROBLEM
Using Multisim, each group will model their given meter and design an external circuit
that can be used to convert the 0-1mA meter to measure a full-scale current of 10mA. Run a
simulation of your design. When you are satisfied that your design meets the specifications,
show it to the instructor.
TESTING / CALIBRATING YOUR CIRCUIT
Construct the meter circuit that you designed and test it against a “standard” meter
(provided in the lab). Using the power supply/resistor combination shown in Figure 2, connect
your “standard” meter and your “meter under test” in series with the A-B terminals.
Make measurements necessary to sketch a "calibration curve" which plots the current
measured by your meter circuit against the current measured by the “standard” digital multimeter.
Use EXCEL to create a data table and to plot the calibration curve. Compare your
measurements with the expected measurements.
CurrentLim it
A
100 Ohm
0.000
STANDARD
A DC 1e-009Ohm
0.000
MeterUnderTest
A DC 1e-009Ohm
+
VariableVoltage
12 V
+
B
Figure 2. Calibration Setup.
RESULTS / CONCLUSIONS / REPORT (1 per GROUP)
For your designed ammeter,
Submit a copy of the EXCEL data table and calibration curve.
Determine the "equivalent" resistance of your meter circuit.
Comment about the desired resistance (or range of resistance) of a practical ammeter
and give a reason for your conclusion.
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