- 32 Rectifier Circuits Name Date Class Reading Floyd Sections 17 1 Through 17 3 Objectives After Performing This Exper 1 (200.71 KiB) Viewed 128 times
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32 Rectifier Circuits Name Date Class Reading: Floyd, Sections 17-1 through 17-3 Objectives: After performing this experiment, you will be able to: 1. Construct half-wave, full-wave, and bridge rectifier circuits, and compare the input and output voltage for each. 2. Connect a filter capacitor to each circuit in objective I and measure the ripple voltage and ripple frequency Summary of Theory: Rectifiers are diodes used to change ac to de. They work like a one-way valve, allowing current to flow in only one direction, as illustrated in Figure 32-1. The diode is forward-biased for one-half cycle of the applied voltage and reverse-biased for the other half- cycle. The output waveforms is a pulsating dc wave. This waveform can then be filtered to remove the unwanted variations. Input- Diode Output-pulsating de n r Figure 32-1 Rectifiers are widely used in power supplies that provide the de voltage necessary for almost all active devices to work. The three basic rectifier circuits are the half-wave, the center-tapped full-wave, and the full-wave bridge rectifier circuits. The most important parameters for choosing diodes for these circuits are the maximum forward current, Ip, and the peak inverse voltage rating (PIV) of the diode. The peak inverse voltage is the maximum voltage the diode can withstand when it is reverse-biased. The amount of reverse voltage that appears across a diode depends on the type of circuit in which it is connected. Some characteristics of the three rectifier circuits are investigated in this experiment. Rectifier circuits are generally connected through a transformer, as shown in Figure 32-2. Notice the ground on the primary side of the transformer is not the same as the ground on the secondary side of the transformer. This is because transformers isolate the ground connection of the 3-wire service connection. The oscilloscope chassis is normally connected to earth ground through the 3-prong service cord, causing the ground to be common; however, you cannot be certain of this. If there is no connection between the grounds, the reference ground is said to be a floating ground. You can determine if the ground is floating by testing the voltage difference between the grounds.
Materials Needed: One 12.6 Vrms center-tapped transformer with fused line cord Four diodes 1N4001 (or equivalent) Two 2.2 kN resistors One 100 uF capacitor For Further Investigation: One 0.01 uF capacitor One 7812 or 78L12 regulator Procedure: Do this experiment only under supervision. Caution! In this experiment, you are instructed to connect a low-voltage (12.6 V ac) transformer to the ac line. Be certain that you are using a properly fused and grounded transformer that has no exposed primary leads. Do not touch any connection in the circuit. At no time will you make a measurement on the primary side of the transformer. Have your connections checked by your instructor before applying power to the circuit. 1. Connect the half-wave rectifier circuit shown in Figure 32-2. Notice the polarity of the diode. Connect the oscilloscope so that channel 1 is across the transformer secondary and channel 2 is across the load resistor. The oscilloscope should be set for LINE triggering since all waveforms are synchronized with the ac line voltage. View the secondary voltage, Vsec and load voltage, VLOAD, for this circuit and sketch them on Plot 32-1. Label voltages on your sketch. VEC VLOAD Plot 32-1 Measure the rms input voltage to the diode, Vsec, and the output peak voltage, VLOAD. Remember to convert the oscilloscope reading of Vsec to rms. Record the data in Table 32-1. Fuse VLOAD TIT CHOCH HI 220 HU Figurs 2-2
2. The output isn't very useful as a dc source because of the variations in the output waveform. Connect a 100 uF capacitor (C) in parallel with the load resistor (RL). Note the polarity of the capacitor. Measure the dc load voltage, VLOAD, and the peak- to-peak ripple voltage, VRIPPLE, in the output. To measure the ripple voltage, switch the oscilloscope vertical input to AC COUPLING. This allows you to magnify the small ac ripple voltage without including the much larger de level. Measure the peak- to-peak ripple voltage and the ripple frequency. The ripple frequency is the frequency at which the waveform repeats. Record all data in Table 32-1. Table 32-1 Half-Wave Rectifier Circuit Without Filter Capacitor With Filter Capacitor Computed Measured Computed Measured Measured Ripple Frequency VIN (rms) Vsec (rm) VLOAD (peak) VLOAD (peak) VLOAD (de) VRIPPLE 12.6 V ac 3. Disconnect power and change the circuit to the full-wave rectifier circuit shown in Figure 32-3. Notice that the ground for the circuit has changed. The oscilloscope ground needs to be connected as shown. Check your circuit carefully before applying power. Compute the expected peak output voltage. Then apply power and view the Vsec and VLOAD waveforms. Sketch the observed waveforms on Plot 32-2. VSEC VLOAD Plot 32-2 Measure Vsec (rms) and the peak output voltage (VLOAD) without a filter capacitor. Record the data in Table 32-2. Fuse SEC VLOAD Comm00000 000000000000 CHI CHE HR 22 kበ Note ground position Figure 32-3 Connect the 100 uF capacitor in parallel with the load resistor. Measure VLOAD, the peak-to-peak ripple voltage, and the ripple frequency as before. Record the data in Table 32-2. 261
Table 32-2 Full-Wave Rectifier Circuit Without Filter Capacitor With Filter Capacitor Computed Measured Computed Measured Measured Ripple Frequency Vsec (rms) Vsec (rms) VLOAD (peak) VLOAD (peak) VLOAD (de) VRIPPLE 6.3 V ac 5. Investigate the effect of the load resistor on the ripple voltage by connecting a second 2.2 k load resistor in parallel with R, and in the full-wave circuit in Figure 32-3. Measure the ripple voltage. What can you conclude about the effect of additional load current on the ripple voltage? 6. Disconnect power and change the circuit to the bridge rectifier circuit shown in Figure 32-4. Notice that no terminal of the transformer secondary is at ground potential. The input voltage to the bridge, Vsee, is not referenced to ground. The oscilloscope cannot be used to view both the input voltage and the load voltage at the same time. Check your circuit carefully before applying power. Compute the expected peak output voltage. Then apply power and use a voltmeter to measure Vsec (rms). Use the oscilloscope to measure the peak output voltage (VLOAD) without a filter capacitor. Record the data in Table 32-3. Fuse wowotw00000 000000000000 VSE VLOAD CHI CHE 221 7. Figure 32-4 Connect the 100 uF capacitor in parallel with the load resistor. Measure VLOAD, the peak-to-peak ripple voltage, and the ripple frequency as before. Record the data in Table 32-3.
Table 32–3 Bridge Rectifier Circuit Without Filter Capacitor With Filter Capacitor Computed Measured Computed Measured Measured Ripple Vsec (rms) Vsec (rms) VLOAD (peak) VLOAD (peak) VLOAD (de) VRIPPLE Frequency 12.6 V ac 8. Simulate an open diode in the bridge by removing one diode from the circuit. What happens to the output voltage? The ripple voltage? The ripple frequency? Conclusion: