Experiment_23_Rectifier - Experiment 23 Diode Bridge...

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23-1 Experiment 23 Diode Bridge Rectifier OBJECTIVE To study the operation of a single phase diode bridge rectifier. DISCUSSION Rectifiers are a class of power converters which convert alternating current (AC) to direct current (DC). Rectifiers are classified as either controlled or uncontrolled. Today we will be exploring the uncontrolled, single phase, full-wave, diode bridge rectifier. It should be noted that many of the concepts discussed can be extended to three phase power system. This bridge rectifier circuit is comprised of four diodes usually followed by a filter capacitor as illustrated in Figure 23-1. The single phase bridge type rectifier is widely used in low power appliances because of simplicity and low cost. A transformer also usually placed before the rectifier to first step the voltage down. Figure 23-1 Single phase bridge rectifier In many cases the four diodes are contained in a single package as shown in the examples below.
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23-2 Figure 23-2 Bridge rectifier packages The bridge rectifier without filter capacitor functions as follows. When the AC voltage is positive relative to earth ground, current will flow through D1, through the load, and back through D4. Diodes D2 and D3 will be off during the first half of the cycle. Figure 23-3 Positive half cycle When the input swings negative, current will flow through D2, through the load, and back through D3. Diodes D1 and D4 will be off in the second half of the cycle. Current will always only flow in one direction through the load. Figure 23-4 Negative half cycle Figure 23-5 shows a typical voltage waveform applied to a resistive load without the filter capacitor. The DC output voltage is pulsed at twice the line frequency (120 Hz).
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23-3 Figure 23-5 Voltage waveforms without filter capacitor The average DC output voltage is related to the peak of the input AC waveform with the following expression: ˆ 2 AC DC V V (1) With the filter capacitor included, the DC output waveform is smoother and has a greater average value as illustrated below. Figure 23-6 Voltage waveforms with filter capacitor 0.025 0.03 0.035 0.04 0.045 0.05 0.055 -50 -40 -30 -20 -10 0 10 20 30 40 50 Time (sec) Voltage (V) DC Output AC Input 0.6 0.605 0.61 0.615 0.62 0.625 0.63 -50 -40 -30 -20 -10 0 10 20 30 40 50 Time (sec) Voltage (V) DC Output AC Input Discharge Charge
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