Instrumentation and Measurement (EET 204)
Laboratory Module
EXPERIMENT 3
BASIC VOLTMETER DESIGN
Objective:
To design and build a basic dc voltmeter within 5% accuracy as accepted to standard.
Equipment:
1 DC power supply
1 potentiometer, 20kΩ
1 panel meter, 0 – to 100mA
1 digital multimeter
Selected resistors, as required
Introduction:
Voltmeters are one of the most frequently used pieces of test equipment in the electronics
industry. The basic dc voltmeter uses a current-sensitive meter movement with the meter face
calibrated in voltage units. A typical laboratory-quality voltmeter uses a 50µA meter movement.
For this meter movement to be used as a voltmeter, it is necessary to put a resistor called a
multiplier in series with the meter movement. Design a voltmeter as described in procedure
below.
Procedure:
Determine the internal resistance of the 100mA meter movement by each of the following
three methods.
1. Resistance measurement method
a) Connect your multimeter across the meter movement terminals to measure the internal
resistance.
b) Record the resistance as Rm1 in the table given.
2. Variable-resistor method. (Fig.3.1)
a) Set the 20kΩ potentiometer to maximum and the voltage source to minimum.
b) Slowly increase the voltage to 1V.
c) Slowly decrease the resistance of the potentiometer until a full-scale reading is
obtained on the meter.
d) Carefully disconnect the potentiometer and measure the value of the potentiometer
resistance (using a digital multimeter) that was in the circuit. Record this as R1 in table.
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Instrumentation and Measurement (EET 204)
Laboratory Module
e) Reconnect the circuit and slowly vary the potentiometer until the meter reads exactly
half scale. Record this as R2 in Table 3.1.
f) Calculate the internal resistance of the meter movement by the equation;
Rm2 = R2 – 2R1
20kΩ
E = 1V
-
I
+
0 − 100mA
Figure 3.1: Variable-resistor method
3. Potentiometer method (Fig.3.2)
a) Set the 20kΩ potentiometer to maximum and the voltage source to minimum.
b) Slowly increase the voltage to 1V.
c) Slowly decrease the resistance of the potentiometer until a full-scale reading is
obtained on the meter under test.
d) Calculate the internal resistance, Rm3, of the meter under test using the millivolt and
microampmeter readings. Record the results in the data table.
20kΩ
E = 1V
Voltmeter
Panel Meter
under test
I
E
Figure 3.2: Potentiometer method
4. Calculate the average internal resistance of the meter under test as;
R + R m2 + R m3
R mave = m1
3
5. Using the test meter movement, design a voltmeter having the following five ranges.
a) 0 to 1V
b) 0 to 5V
c) 0 to 10V
d) 0 to 15V
e) 0 to 20V
Use the Rmave of meter resistance in the calculations for multipliers.
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Instrumentation and Measurement (EET 204)
Laboratory Module
6. Construct the experimental circuit shown in Fig. 3.3. Use fixed composition resistors for
the multipliers.
I
0 − 100 mA , Rm
Rs1
1V
Rs2
5V
Rs3
10V
Rs4
15V
Rs5
20V
Common
Figure 3.3: Experimental Circuit
7. Apply voltage according to the ranges to each range of the experimental circuit. Wait
until the needle stop to move. Observe the value of scale deflection as E2. Using the
ratio formula, calculate the absolute value for E2 and record in the table.
E1 is the expected value for full-scale deflection on meter movement. As example, for
5V range, the full-scale deflection is 5V. Therefore, E1 = 5V.
8. Calculate and record the percent of error of each scale on your meter.
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Instrumentation and Measurement (EET 204)
Laboratory Module
Name:______________________________ Matrix no.:_____________ Date:_________
RESULT
Table 3.1 : Meter Movement Data
Resistance
Measurement
Method
Rm1 =
Variable Resistor
Method
Potentiometer
Method
R1 =
E (mV) =
R2 =
I (mA) =
Rm2 =
Rm3 =
Internal Resistance
of Meter
Rmave =
Table 3.2 : Voltmeter Data
Volts Scale
Multiplier
Resistor
E1
E2
Percent Error
0 -1
0–5
0 – 10
0 - 15
0 - 20
Instructor Approval: …………………………….
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Date: ………………
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Instrumentation and Measurement (EET 204)
Laboratory Module
Name:______________________________ Matrix no.:_____________ Date:_________
CALCULATION
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Instrumentation and Measurement (EET 204)
Laboratory Module
Name:______________________________ Matrix no.:_____________ Date:_________
DISCUSSION:
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CONCLUSION:
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Instructor Approval: …………………………….
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Instrumentation and Measurement (EET 204)
Laboratory Module
Name:______________________________ Matrix no.:_____________ Date:_________
PROBLEMS
1. A permanent-magnet moving coil (PMMC) instrument with a 900Ω coil resistance has a
full scale deflection of 75µA will be used as dc voltmeter. Determine the individual
multiplier resistance to give a full scale deflection when a
a) 100V
b) 30V
c) 5V
Instructor Approval: …………………………….
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