Introduction The Direct Current Multimeter Multimeter Project is an investigation to take a 100 micro Amp D’Arsenal movement and use it as an Ammeter, Ammeter, Voltmeter and Ohmmeter. This meter movement takes a small electric current that flows across a magnetic coil, then using that current to rotate a needle displayed on a gauge. This movement can only measure a max of 100 micro Amp therefore the current must be limited in a certain fashion, in ord er to produce a useable Ammeter, A mmeter, Voltmeter and Ohmmeter. The D’Arsenal movement has an internal resistance of 3.699K Ω. The first quarter of this project is dedicated to creating a single range Ammeter and Voltmeter. The second quarter will be used used to expand both meters to measure measure a larger scale. The third quarter will be used to design and build an Ohmmeter. Finally in the last section of the project all three meters will be combined into one circuit.
DC Ammeter The D’Arsenal movement only allows for a maximum current reading of 0.100 mA. In order to extend the range to the required 0 – 0 – 5mA 5mA and 0 – 0 – 10mA 10mA a resistor must be placed p laced in parallel with the meter movement known as the shunt shunt resistor. This resistor absorbs absorbs the remaining current that that cannot be handled by the movement. Figure 10 Ammeter Design
The implementation of the dual range ammeter requires three resistors for the 0 – 0 – 5mA 5mA range. A 22 Ω, 33Ω, and a second 33Ω, measured with the digital multimeter at 22.37Ω, 32.80Ω, and 32.90Ω re 32.90Ω respectively. spectively. For the the 0 – 0 – 10mA 10mA range two resistors a 39Ω and 3.3Ω resistor were used measured with the digital multimeter at 38.6Ω and 3.37Ω respectively. 3.37Ω respectively. A double pull double throw switch will integrate the two ranges for the meter together. Multisim Schematic
Figure 11 Ammeter Schematic
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DC Multimeter
Josh Osborne
Test Results
Table 1 Ammeter Results Range Input (mA)
0 – 5mA – 5mA
0 2.5 5 0 5 10
0 – 10mA – 10mA
D’Arsenal
D’Arsenal
Expected Reading (μA) 0 50 100 0 50 100
Actual Reading (μA) 0 51 100 0 48 98
Actual Measurement (mA) 0 2.55 5 0 4.8 9.8
Percent Error (%)
0 2 0 0 4 2
DC Voltmeter The D’Arsenal Meter Movement is is also capable of measuring voltage. Much like the ammeter the D’Arsenal movement has a very low max voltage. In order to accomplish the required ranges of 0 – 0 – 1V 1V and 0 – 0 – 10V, 10V, a resistor must be put p ut in series with the movement in order to limit the amount of current passing through. Figure 12 Voltmeter Design
R series series Calculation Equation
Calculations 0 – 1V 1V Range
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DC Multimeter
Josh Osborne
0 – 10V 10V Range
= 96.3K Ω
Implementation Similar to the ammeter the voltmeter uses a double pull double throw switch to change chan ge between the 0 – 0 – 1V 1V and 0 – 0 – 10v 10v ranges. For the 0 – 1V 1V range a 2.7K Ω and a 3.3K Ω resistor were used measured at 2.695K Ω and 3.342K Ω respectively. respectively. For the the 0 – 0 – 10V 10V range an 82K Ω resistor is was used measured at 81.79K Ω Multisim Schematic Figure 13 Vo ltmeter Schematic
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DC Multimeter
Josh Osborne
Test Results
Table 2 Voltmeter Results Range Input (V)
0 – 1V – 1V
0 0.5 1 0 5 10
0 – 10V – 10V
D’Arsenal
D’Arsenal
Expected Reading (μA) 0 50 100 0 50 100
Actual Reading (μA) 0 47 97 0 48 98
Actual Measurement (V) 0 0.5 5 0 4.8 9.8
Percent Error (%)
0 6 3 0 4 2
DC Ohmmeter When measuring resistance with a D’Arsenal Meter Movement 0 ohm is at full scale deflection and infinite ohm is at zero deflection. Figure 14 Ohmmeter Design
R series series Calculation Equation
Calculations
= 25.8K Ω
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DC Multimeter
Josh Osborne
Implementation
For the ohmmeter a 22K Ω resistor is used for the series resistor measured at 21.7K Ω. A 1K Ω potentiometer is used as the zero-adjust. A 3V DC voltage is provided from a power supply to power the ohmmeter
DC Multimeter The DC Multimeter used a 5 position p osition rotary switch to switch between the 2 ranges of the ammeter, the 2 ranges of the voltmeter and the ohmmeter. A double pull double throw switch switch is used to allow the shunt resistors of the ammeter to be placed in parallel, and for the ohmmeter to be used.
0 mA 2.5 mA 5 mA 0 mA 4.6 mA 9.6 mA 0V 0.46 V 0.95 V 0V 4.7 V 9.9 V 0Ω
0 0 0 0 8 4 0 8 5 0 6 1 NA
1K Ω 10K Ω 47K Ω 100K Ω 330K Ω 470K Ω 1M Ω
DC Multimeter
Josh Osborne
Figure 17 Ohmmeter Graph 120
100
) A
μ ( g 80 n i d a e r 60 r e t e 40 m m A
20
0 0
200000
400000
600000
800000
10 0 000 0
12 1 2 000 00
Resistance ( Ω)
Figure 18 Multimeter
Conclusion The DC Multimeter project was a success, both in theory and in the implementation. By designing a fully functional multimeter from basic electrical components it showed that you can measure almost any value of amp, volt or ohm with a simple 100 μA D’Arsenal movement. The schematics were built on Multisim and tested on a breadboard.
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DC Multimeter
Josh Osborne
Every calculation and percent error was documented along the way in a log book. The errors from this meter were due to the inaccuracy inaccurac y of using a D’Arsenal movement, as well as the inconsistency of the resistors. resistors. This meter demonstrates how current day meters that use digital signals are far more more accurate. From this project, knowledge has been gained gained on how a multimeter works and how resistors resistors can be used to manipulate current to make other measurements.
