Electrical Measurements Name: Hasibullah Fnu Date: 09/08/2017 Instructor: Abebe
I.
Objectives
The purpose of this experiment was to introduce the basic concepts of electric circuits through 3 different experiments. In the first experiment, we set up and studied the behaviors of Direct Current (DC) circuits by measuring the current and voltage in two different set-ups. In the first circuit, the resistance R came from a resistor. Then, we calculated the ratio V/I and compared it to the actual value R of the resistor. In the second set-up, we changed the resistance of the circuit into a light bulb to see what would happen with the circuit. In the second experiment, we measured the voltage and frequency of Alternating Current circuits using an oscilloscope. Then we compared the data recorded from the oscilloscope with the output generated by the function generator.
I I . Set Set-up and T heo heor y For the first experiment, we set up two circuits in order to study the behaviors of the current, voltage and resistance of DC circuits. The first circuit looked like this:
Figure 1: DC circuit using the resistor
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Low Voltage AC/DC Power Supply (LVPS) was used as the power source of the circuit. After connecting all the wires, we started to measure, read and record the data of the circuit by using the Digital Multimeter (DMM). (DMM). DMM was connected in two different ways: paralle para llell and an d in series seri es when w hen measurin mea surin g the th e voltag vo ltagee and an d curre c urrent nt respecti resp ectivel vely. y.
Figure 2: Voltmeter Connection
Figure 3: Ammeter Connection Connection
In DC Circuits, the voltage and current do not change with time and their values follow the formula: V=IR
R = VI where V is the voltage difference between the two ends of the component, I is the current through it, and R is its resistance. In this case, the only component we had was the resistor. Since the resistance of the resistor is always a constant, the fraction
will remain remain constant regardless regardless of the values of
V and I. In the second circuit, we performed the same procedures as above with the light bulb. However, the resistance in this situation varies. 3
In the second experiment, we observed the operation of AC circuits. A function generator was used to generate an AC voltage output. An oscilloscope showed the voltage signal on the screen. Next, we measured the voltage and frequency of the generator. In order to find the voltage, we recorded the divisions from the bottom of a trough to the top of the crest of the sine wave and the voltage per divisions on the oscilloscope. We were able to retrieve the voltage of the AC circuit by these steps:
Peakk − to− Pea to − Pea Peakk volt voltag agee = divi divisision onss x volta voltage gess per divi divisision onss voltage voltage= peak−to−peak 2 After that, we recorded the number of divisions from a certain number of cycles and the seconds per division on the horizontal axis. We were able to find out the frequency by these steps:
Time ofof n cycles cycles = divisions divisions x seconds seconds per divisi division on Period= Time ofnn cycles f = T1 I I I . D ata and Ana A nalysi lysiss 1. V olta ltage, Cur C urrr ent and R esist si sta ance i n DC D C C i r cu cuii ts a. Resistance of the resistance with the Digital Multimeter Red
Black
Red
Gold
2
0
102
±5%
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b. Circuit with the resistor
From the data table below, we can see that the resistance value was roughly constant when we changed the voltage of the power supply. The reason is that resistor has a constant resistance. Therefore, with the resistor being the only component of the circuit, the fraction V/I is equal to the value of resistor we measured before (3.07 percent difference). Besides that, the voltage reading is almost equal to the voltage output indicated on the power supply. This equality is not always guaranteed. For example, in the circuit with two resistors in series, the voltage measured at two ends of each resistor would be different from the voltage output by the power supply since two resistors have to share the voltag e from the power supply. Trial 1
Voltage Output (V) 2
I (mA) 0.927
V (V) 2.030
R=V/I (k ) 2.190
2
4
1.862
4.050
2.175
3
6
2.850
6.060
2.126
4
8
3.850
8.110
2.106
5
10
4.840
10.050
2.076
Average of R(k ) (1)
2.135
Measured resistance of the resistor(k ) (2)
2.070
1−2| 100% = |1−2| (1+2 2 )
3.072%
Data Table 1: Circuit with with Resistor
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c. Circuit with the light bulb
The resistance value of the light bulb in this case was not a constant. It varied when we changed the voltage of the power supply. We could easily see an increasing trend when we increased the voltage. Trial
I (mA) 0.270
V (V) 2.060
R=VLVPS/I (k )
1
LVPS voltage (V) 2.000
2
4.000
0.390
4.040
10.256
3
6.000
0.480
6.040
12.500
4
7.000
0.570
8.020
12.281
5
10.000
0.640
10.000
15.625
7.407
Average of I (mA)
0.470
Average of V (V)
6.032
Power released by the light bulb =IxV (mJ)
2.835
Data Table 2: Circuit with with light bulb
d. V oltag oltage and and F r equency quency Me M easure asur ements with wi th the Oscill Osci llosco oscope pe a. Determine the voltage of the generator by the oscilloscope
Trial
Voltage/division (V/div)
Division (div)
Peak Voltage (V)
Voltage from the generator(V)
5.2
Peak to peak V (V) 10.4
1
2.0
5.2
5
2
5.0
3.0
14.0
7.0
7
3
0.5
4.0
2.0
1.0
1
4
1.0
4.0
4.0
2.0
2
5
2.0
6.0
12.0
6.0
6
6
Square
5.0
4.0
20.0
10.0
10
Data Table 3: Voltage of the function generator
Frequency (Hz)
Sweeps Division (div)
500
3
6.1
Time per Division (ms, s) 1.0 ms
Time of n sweeps (ms, s) 6.10 ms
Period (ms, s)
Measured frequency (Hz)
Percent error (%)
2.033 ms
491.8
1.64
800
2
5.0
0.5 ms
2.50 ms
1.250 ms
800
0.00
3,000
3
5.1
0.2 ms
1.02 ms
0.340 ms
2,941.1
1.96
10,000
3
6.0
50 s
300 s
100.000 s
10,000
0.00
12,000
3
5.2
50 s
260 s
86.667 s
11,538
3.85
Data Table 4: frequency of the function generator
SAMPLE CALCULATION Experiment 1: Voltage, Current and Resistance in DC Circuits.
1. Measure Resistance of the resistance with the Digital Multimeter Multimeter Resistor: Red/ Black/Red/Gold Coded resistance of the resistor = 20x102 = 2000 Ω
±
Resistor Tolerance = 2000 x ( 5%) Ω Experiment 2: Voltage and Frequency Measurements with the Oscilloscope:
1. Sine Wave output: Measurement #1: Volt/Div Setting: 2Volt/Div Time/Div Setting: 0.2x10-3 s/Div Peak-to-peak voltage = 5.2 div x 2V/div = 10.4 (V) Peak voltage = ½ peak-to-peak voltage = 10.4/2 = 5.2 (V)
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2. Square Wave output: Volt/Div Setting: 2Volt/Div Peak-to-peak voltage = 4 div x 5V/div = 20 (V) Peak voltage = ½ peak-to-peak voltage = 20/2 = 10(V) Conclusion:
This experiment is to measure electrical circus: voltage, current, and resistance with a digital multimeter. In experiment 1, the measured resistance values are constant. The resistor voltage reading values with the DMM are almost the same with the voltage out put reading values on the LVPS. The resistance values are also proportional to voltage. The percent error for this experiment is pretty low, 3.072% for 5 measurements. Experiment 2 is to measure the voltage and frequency output of a function generator with the oscilloscope. The percent errors for this experiment was ok, 0-3.85% for 5 readings. These These errors maybe caused by the wrong reading so we didn’t get the precise measurements. I learned a lot from this lab.
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