NDJ 30003 - ELECTRONIC INSTRUMENTATION
LAB 1 - D'ARSONVAL GALVANOMETER
Name: ONG JUN JIE
Matrix No.: 192020315
Program: Computer Engineering
Date: 9/9/2021
OBJECTIVES
- To find the internal resistance and the current sensitivity of the galvanometer.
EQUIMENTS/COMPONENTS
- d'Arsonval Galvanometer (35-0-35mA)
- DC power supply
- Digital Multimeter
- 1 potentiometer, 1kΩ
- Composite resistor 510Ω, 220Ω, 10Ω, 5Ω, 2Ω
INTRODUCTION
The galvanometer contains a coil of wire in a magnetic field, which will experience a torque when a current passes through the wire of the coil. The coil is attached to a pointer and a spring so that the amount of deflection of the pointer is proportional to the current in the wire of the coil.
The value of the load resistor (R1) will be set to a specified value and the potential difference provided by the power supply will be varied to obtain a full-scale deflection of the pointer of the galvanometer. The voltage (VFS) required to obtain full-scale deflection will be recorded, without changing the applied voltage (VFS), Add a shunt resistor (RS) in parallel with the galvanometer. Vary the load resistance to get the full-scale deflection in the galvanometer. The new load resistance, R2 will be recorded. In both circuits, the potential difference supplied by the power supply is the same as is the current passing through the galvanometer (full-scale deflection in both circuits).
Application of Kirchhoff’s rules to the two circuits results in the following expression for the value of the internal resistance of the galvanometer (Rg). Assume Rg is 1.2Ω for the given galvanometer.
where, N = number of major divisions of the galvanometer scale for a full-scale deflection of the pointer
Rg = [ RS ( R1-R2 ) ] / R2
The current sensitivity (K) can be obtained form the measurement by using this formula
K = VFS / [ N( R1+R2 ) ]
R1 = load resistor in circuit 1
R2 = load resistor in circuit 2
RS = value of shunt resistor parallel to galvanometer
VFS = voltage at full-scale deflection of the pointer in galvanometer
EXERCISE
1. Set the potentiometer to 510 Ω. Connect the circuit as in the Figure 1.1 and keep the voltage source in minimum position such that voltage output from the voltage terminals are 0V.
Figure 1.1: Set up of circuit connection
2. Increase the voltage supply and the galvanometer pointer will deflect towards right hand side or left hand side. You can replace the galvanometer with the DMM/Ammeter in the Multisim. Assume Rg is 1.2 Ω for the given galvanometer. Vary the voltage supply until the galvanometer shows its maximum deflection, which the pointer should comes to 30 positions (divisions).
3. Connect the multimeter across the voltage terminal to measure the voltage and record this value as VFS.
4. Turn OFF the supply and do not disturb the voltage source. Now connect the resistance, RS and construct the circuit as in Figure 1.2. Select the 10 Ω for RS. Switch ON the instrument, then the pointer of the galvanometer will return back by a few divisions.
Figure 1.2: The circuit diagram with shunt resistor, Rs
5. Without disturbing the voltage source, adjust the potentiometer, R2 until the pointer scale comes to full scale deflection which is 30 positions (divisions).
6. Turn OFF the supply and disconnect R2. Measure the absolute values of R2 using multimeter and record the value in the Table 2.1.
7. The values of VFS, R1, R2, RS, are known, determine the galvanometer resistance, Rg by calculation. Then calculate its current sensitivity, K. Repeat from step 5 for RS = 5 Ω and 2 Ω and
R1 = 220 Ω.
8. Finally disconnect the galvanometer from the circuit and connect multimeter across the terminal of galvanometer to measure its internal resistance. Record the value as Rg (measured).
Note: Record all value for R2 which is measured from multimeter only.
RESULTS
CIRCUIT RESULT USING MULTISIM SOFTWARE
VFS when R1= 510Ω
R2 when VFS=15.34V and Rs=10Ω
R2 when VFS=15.34V and Rs=5Ω
R2 when VFS=15.34V and Rs=2Ω
R2 when VFS=6.65V and Rs=10Ω
R2 when VFS=6.65V and Rs=10Ω
R2 when VFS=6.65V and Rs=2Ω
CALCULATION
Attach ALL calculation and the multisim diagram for each answers in your report!
1. Connect the circuit in Figure 1.3 and run the simulation for both load resistance values for 10kΩ and 100kΩ. Observe the measured voltage using a voltmeter/DMM and conclude the answer with the prove of the diagram.
Figure 1.3
Measured voltage using a voltmeter for Load Resistance Values 10kΩ is 2.577V.
Measured voltage using a voltmeter for Load Resistance Values 100kΩ is 3.295V.
Observation
By increasing the load resistance, I had observed that the output voltage increase too. This is because of the "Ohm's Law'. From the formula V=IR, we know that higher resistance will get higher voltage.
2. Connect the circuit in Figure 1.4 and run the simulation for. The actual voltage across R2 is 7.5 V. Configure the circuit with the voltmeter has an internal resistance of 1 MΩ, 10MΩ and 100MΩ. Observe the measured voltage using a voltmeter/DMM and conclude the answer with the prove of the diagram.
Figure 1.4
Voltmeter has internal resistance of 1Mohm and the voltage is 7.143V.
Voltmeter has internal resistance of 10Mohm and the voltage is 7.463V.
Voltmeter has internal resistance of 100Mohm and the voltage is 7.496V.
Observation
From this question, I had observed that when voltmeter internal resistance increase, the voltage increase too. This is because of the loading error and causes this.
CONCLUSION
In the conclusion, I had learnt from this lab when during the measurement, the voltmeter circuit itself is in parallel with the measurement circuit component. Parallel combination of two resistors is less compare to single resistor. The resistance seen by the source is less with the voltmeter connected than without. Therefore, the voltage across the component is less whenever the voltmeter is connected. The decrease in voltage may be negligible or it may be appreciable, depending on the sensitivity of the voltmeter called as voltmeter loading effect. The resulting error is called a loading error. Lastly, Multisim is a great electronic schematic capture and simulation program which helps me to understand more of the D'Arsonval Galvanometer.
LAB 1 END
LET'S MAKE A LITTLE BIT PROGRESS EVERY DAY AND YOU SEE
THE POWER OF SMALL CHANGES
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