Objectives
The objective of this lab was to;
Explain the relationship between capacitance and ripple voltage for a capacitive filter circuit, and
Explain the relationship between load resistance, load current, and ripple voltage for a capacitive filter circuit.
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Equipment
The following equipment was used;
An auto-transformer;
A center-tapped transformer Oscilloscope;
A Proto-board;
A Diode Bridge;
A (0 – 100 kΩ) Decade Resistor: 0 – 100 kΩ;
A (0 - 10 μF) Decade Capacitor:
Procedure
Circuit Verification
The circuit was constructed, as shown in Figure 2. Using a (0 - 10 μF) decade capacitor for C and a (0 – 100 kΩ) decade resistor for R L .
Figure 2: Capacitive Filter Circuit
The auto-transformer was set so that eO = Vdc = 25.0 V, the measured value of C = 1 μF and value of RL an open circuit . The measured voltage across the capacitor C was also confirmed to be 25.0V hence V pk = 25.0 V.
The scope was set up Set with the vertical sensitivity set to 10 V/div, the AC-DC-GND switch set to DC, and the ground level set to the center of the display. Using 10X probes, the Line was used as the trigger source, and observations of the waveform at e O noted.
The capacitor was then disconnected, and the decade resistor reconnected. After setting the resistance to 10 kΩ the observe the waveform at e O was observed.
II. The Effect of Load and Capacitance on Ripple
Procedure
The decade capacitor was reconnected so that the circuit is as shown in Figure 2 with the capacitor was set to 1.0 μF.
The table 1 below was then completed by measuring and recording the voltage at eO. as described in the notes following Table 2. Calculate the percent ripple and load regulation from the measured values.
Table 1: Ripple Measurements with C = 1.0 µF
|
R L |
*Calc. Vr(p-p) |
*Calc. Vdc |
*Calc. Vr(rms) |
**Meas. Vr(p-p) (scope) |
***Meas. Vr(rms) (DMM) |
Meas. Vdc (DMM) |
Calc. Ripple Factor r% |
Calc. Load Reg. % |
|
10 kΩ |
32.64 |
25V |
3.72 |
10V |
3.05V |
20.4V |
14.9% |
86% |
|
20 kΩ |
31.21 |
25V |
2.08 |
6V |
1.82V |
21.85V |
8.33% |
91% |
|
50 kΩ |
28.41 |
25V |
0.91 |
3V |
0.84V |
23V |
3.65% |
96% |
|
100 kΩ |
29.65 |
25V |
0.49 |
2V |
0.46V |
23.5V |
1.96% |
98% |
The resistor R L was set at10 kΩ and the table 2 completed using the provided values of capacitor C. The table two included the measured voltage at e O , a calculation of the percent ripple and load regulation from the measured values.
Table 2: Ripple Measurements with RL = 10 kΩ
|
C |
*Calc. Vr(p-p) |
*Calc. Vdc |
*Calc. Vr(rms) |
**Meas. Vr(p-p) (scope) |
***Meas. Vr(rms) (DMM) |
Meas. Vdc (DMM) |
Meas. Ripple Factor r% |
Calc. Load Reg. % |
|
1.0 μF |
32.84V |
25V |
3.725 |
10V |
3.05V |
20.4V |
14.9% |
86% |
|
2.0 μF |
31.63V |
25V |
2.07 |
6V |
1.8V |
21.78V |
8.26% |
92% |
|
5.0 μF |
29.41V |
25V |
0.92 |
4V |
0.84V |
22.9V |
3.67% |
96% |
|
10.0 μF |
28.21V |
25V |
0.48 |
2V |
0.45V |
23.4V |
1.92% |
98% |
Observations
It was observed that after performing step 3 in the procedure above it was observed that the output waveform was a flat line, whereas when the resistor is used, it becomes a waveform.
It was also observed that after performing step 4 in the procedure of part 1 above the waveform at e O becomes a rectangular wave.
After performing step 2 on the effect of load and capacitance on ripple that seeks to investigate the effect of the load on the ripple the two waveforms in figure 1 and 2 were observed from the 10 kΩ and 100 kΩ resistors respectively.
Figure 1: A waveform showing the effect of a 10 kΩ load on the waveform
Figure 2: A waveform showing the effect of a 100 kΩ load on the waveform
After performing step 3 on the effect of load and capacitance on ripple that seeks to investigate the effect of the capacitor (C) on the ripple the two waveforms in figure 3 and 4 were observed from the 2.0 μF and 10.0 μF capacitors respectively.
Figure 3: A waveform showing the effect of a 2.0 μF capacitor on the waveform
Figure 4: A waveform showing the effect of a 10.0 μF capacitor on the waveform
Conclusion
From the observation, it is clear that the objectives of the experiments were met. The experiment shows the relationship of load and the ripple current and the relationships of the capacitor values to the ripple current. A lower load resistance proves to result to a higher load ripple current as summarized in table one and two. Also, the oscilloscope images in figure one and two indicate that a load resistance of 10-kilo ohms produces a higher ripple compared to a capacitor of 100-kilo ohms. The capacitance value, on the other hand, is also inversely proportional with the ripple factor. A lower capacitance value results in a higher ripple factor, whereas a higher capacitor value results in a lower ripple factor. Finally, the higher the capacitor and load resistor value, the lower the diode ripple current.
