Lab 19

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VOLTMETER LOADING EFFECT – LAB REPORT
Objective: To determine the loading effect of a voltmeter to a circuit
Prerequisite Knowledge: Electric circuit principles
Required Equipment:
2kΩ, 20kΩ, and 200 kΩ resistors.
DC power supply (0-12V)
VOM (DC sensitivity 20 kΩ/V)
DMM (>10 MΩ input impedance).
Theory:
Making measurements in electrical circuits, and generally in science without affecting the result of the measurement is not very easy. This phenomenon is known as loading in electronics. In measuring current, for instance, the resistance of the ammeter contributes to the final reading shown, hence a lower value as shown by the deflection of the ammeter. In a voltmeter on the other hand, the contribution leads to parallel resistance, leading to lower voltmeter reading. The extent to which the loading effect affects the voltmeter reading is determined by the magnitude of the resistance of the device. It is necessary to connect a voltmeter in parallel so that it doesn’t affect the circuit. All the meters used in circuits have little resistance. The VOM which is meant for use in this experiment has a resistance of upto 20 kΩ/V. A good DMM has a resistance of up to 10 MΩ. DMM gives the best reading. This experiment is used to demonstrate the loading effect of a voltmeter. A 2-resistor circuit is used together with both VOM and DMM to measure voltage.
Procedure:The equipment is arranged as shown above.
The voltages V1 and V2 are calculated and recorded.

The most suitable range for measuring the voltages is decided by inspecting the VOM. The VOM sensitivity figure can be used to ascertain this and then the meter resistance can be calculated. From the meter resistance obtained above, the expected VOM voltages V1 and V2 are calculated.
The circuit is now rearranged with the 2kΩ resistor and the VOM used to measure V1 and V2.
The above step is repeated with DMM, then with the 20 kΩ and then with 200kΩ, recording all the data in the tables below.
Results and Analysis:
The tables below show the recorded readings.
Table 1
R1=R2=2kΩ V1(V) V2(V)
Ideal 2.5 2.5
VOM (expected) 2.505 2.494
VOM (measured) 2.5 2.5
DMM 2.4987 2.5014
Table 2:
R1=R2=10kΩ V1(V) V2(V)
Ideal 2.5 2.5
VOM (expected) 2.5247 2.4752
VOM (measured) 2.5 2.5
DMM 2.4971 2.5015
Table 3:
R1=R2=200kΩ V1(V) V2(V)
Ideal 2.5 2.5
VOM (expected) 2.73 2.272
VOM (measured) 2 2
DMM 2.473 2.478
Notes
Suppose R1 = R2 = 100 kΩRx = 500 kΩRx=R2∥RI =100×500100+500=83.33kΩ, RT =R1+RX=83.33+100kΩ=183.33kΩ
V1=100183.33×5=2.73V; V2=83.33183.33×5=2.272VDiscussion and Conclusion:
From the tables above, we observe that the DMM has a higher reading on average, as compared to VOM. This is an indication that a larger loading effect is experienced when VOM is used. The loading effect is higher for VOM since the impedance is also higher. Indeed, it is for this reason why a VOM of 20 kΩ per Voltage impedance is preferred for use in the loading circuit while the DMM should have a fixed impedance of 10MΩ per volt or 20 MΩ per volts. The extent to which the loading effect affects the voltmeter reading is determined by the magnitude of the resistance of the device. It is therefore necessary to connect a voltmeter in parallel so that it doesn’t affect the circuit. The higher loading effect on the VOM can also be attributed to the fact that DMM only measure the accurate voltage across the shunt resistance. Due to lower impedance of the VOM, the values obtained for the reading are not very accurate.
Even though it is not possible to measure the voltage without the meter in use having some effects on the result, the effects are normally negligible. A good voltmeter should ideally have a relatively high resistance since voltmeters are normally connected in parallel, It is for this reason as to why we have noticed from the calculation that the DMM stands out as the best meter when compared to VOM. The higher impedance serves to provide the high resistance required for a good meter. Parallel connection of the voltmeter in the circuit also helps in ensuring that the total impedance of the circuit remains the same as that of the elements being used.
We observe that loading has an effect of lowering the voltage as V1 is lower in value as compared to V2. The reliability of the results is determined by the sensitivity of the meter in use. For this experiment, the circuit connection is of higher resistance hence the DMM still stands out as the most reliable meter since it is more sensitive.