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SUMMARY OF THEORY Rectifiers are diodes used to change ac to de. They work like a one-way valve, allowing current 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 waveform is a pulsating de wave. This waveform can then be filtered to remove the unwanted variations. Diode ^^^^ n Figure 32-11 Inpul-ac Ourpes--puliaring de 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, I, 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. Exp Adobe Expe Convert PDF or Excel Onli Select PDF F Lab321900 Convert to Microsoft W Document Lar English (US) Convert, edis forms &
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 ground 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 is floating by testing the voltage difference between the grounds. 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. 81°F Sunny
1. MISTUCIOT veron Zoom out (Ctrl+Minus) 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. VSEC th₁ 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 VSEC 61 000000000000 000000000000 110 Plot 32-1 VLOAD m R₁ 22k0 VLOAD Figure 32-2 CHI CHI
Computed V (ms) 126 Vac 3. Without Filter Capacitor Computed VLOAD (peak) Measured VSEC (rms) Zoom out (Ctrl+Minus) Table 32-1 Half-wave rectifier circuit. Visc Measured VLOAD (peak) With Filter Capacitor Measured VLOAD (de) VRIPPLE The output isn't very useful as a de source because of the variations in the output waveform Connect a 100 μF capacitor (C₁) in parallel with the load resistor (R₂). Note the polarity of the capacitor. Measure the de load voltage, VLOAD- and the peak-to-peak ripple voltage, Vin the output. To measure the ripple voltage, switch the oscilloscope vertical input to AC COUPLING. This allows you to magnify the small ne 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. FLOW Ripple Frequency 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 Vane and VLOAD waveforms, Sketch the observed waveforms on Plot 32-2. 81°F Sunny
soopper HI 3/7 Fuse 000000000000 000000000000 Measure VSEC (rms) and the peak output voltage (VLOAD) without a filter capacitor. Record the data in Table 32-2. the Note ground position Plot 32-2 Y www 2.2. VLOAD 100% Figure 32-3 > E CHI CHO
Computed Vanc (rms) 6.3 Vac 4. 5. 6. Table 32-2 Full-wave rectifier circuit. Without Filter Capacitor Computed Measured VLOAD (peak) VLOAD (peak) Measured V₁ (ms) With Filter Capacitor Measured VLOAD (de) VRIPPLE Ripple Frequency Connect the 100 pF 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 Investigate the effect of the load resistor on the ripple voltage by connecting a second 2.2 kt load resistor in parallel with R, and C, 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 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 the bridge, Vsre 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 Vse (rms). Use the oscilloscope to measure the peak output voltage (Vos) without filter capacitor. Record the data in Table 32-3.
129 power. Compute the expected pom measure Vsac (rms). Use the oscilloscope to measure the peak output voltage (VLOAD) without a filter capacitor. Record the data in Table 32-3. Computed VSLC (rms) 12.6 V ac Hl Fuse 14 51 Measured Vsec (rms) Vsec Without Filter Capacitor Computed VLOAD (peak) R₂ 2.2k Figure 32-4 Table 32-3 Bridge rectifier circuit. Measured VLOAD (peak) VLOAD 266 With Filter Capacitor Measured VLOAD (de) CHI CH VAIPPLE Ripple Frequency &
Connect the 100 μF capacitor in parallel with the load resistor. Measure VuDAD. the peak-to-peak 1 ripple voltage, and the ripple frequency as before. Record the data in Table 32-3. 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 EVALUATION AND REVIEW QUESTIONS What advantage does a full-wave rectifier circuit have over a half-wave rectifier circuit? beides rectifier circuit with a full-wave rectifier circuit. Which has the higher output
EVALUATION AND REVIEW QUESTIONS What advantage does a full-wave rectifier circuit have over a half-wave rectifier circuit? 2 a ma sa bizes 3. Compare a bridge rectifier circuit with a full-wave rectifier circuit. Which has the higher output voltage? Which has the greater current in the diodes? Explain how you could measure the ripple frequency to determine if a diode were open in a bridge rectifier circuit.
# (a) VA (b) Vuc 5+ >> w 2203 eo Figure 32-5 In step 3, you moved the ground reference to the center-tap of the transformer. If you wanted to look at the voltage across the entire secondary, you would need to connect the oscilloscope is shown in Figure 32-5 and subtract channel 2 from channel 1. (Some oscilloscopes do not have this capability.) Why is it necessary to use no channels to view the entire secondary voltage? CHE CH₂ What is the maximum de voltage you could expect to obtain from a transformer with an 18 V secondary using a bridge circuit with a filter capacitor? What is the maximum de voltage you could expect to obtain from the same transformer connected in a full-wave rectifier circuit with a filter capacitor?