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Diodes As Rectifiers + Diodes as Rectifiers As previously mentioned, diodes can be used to convert alternating current (AC) to direct current (DC). Shown below is a representative schematic of a simple DC power supply similar to a au- tomobile battery charger. The way in which the diode rectifier is used results in what is called a half-wave rectifier. 0V 35.6Vpp 167V 0V pp 17.1Vp 0V T1 D1 Wallplug 1N4001 negative half−wave is 110VAC R1 resistive load removed by diode D1 D1 120 to 12.6VAC − stepdown transformer + D2 D2 input from RL input from RL transformer transformer (positive half−cycle) Figure 1: Half-Wave Rectifier Schematic (negative half−cycle) − D3 + D3 The input transformer steps the input voltage down from 110VAC(rms) to 12.6VAC(rms). The D4 D4 diode converts the AC voltage to DC by removing theD1 and negative D3 goingD1 and D3 part of the input sine wave. The result is a pulsating DC output waveform which is not ideal except for17.1V simplep applications 0V such as battery chargers as the voltage goes to zero for oneD2 have and D4 of every cycle. What we would D1 D1 v_out like is a DC output that is more consistent;T1 a waveform more like a battery what we have here. T1 1 D2 D2 C1 Wallplug R1 Wallplug R1 We need a way to use the negative half-cycle of theresistive sine load wave to to fill in between the pulses 50uF 1K 110VAC 110VAC created by the positive half-waves. This would give us a more consistent output voltage. Below is a circuit that does just that. When the diodesD3 are connected in this manner this circuit would be D3 2 called a full-wave rectifier. 120 to 12.6VAC 120 to 12.6VAC stepdown transformer D4 stepdown transformer D4 TITLE FILE: REVISION: PAGE OF DRAWN BY: Figure 2: Full-Wave Rectifier Schematic 1 The full wave rectifier uses the one-way action of the diodes to create the output from both positive and negative half-waves of the AC input signal. The path through the load passes through two diodes in this scheme. The different paths through the rectifier are shown below. 0V 35.6Vpp 167V 0V pp 17.1Vp 0V T1 D1 Wallplug 1N4001 negative half−wave is 110VAC R1 Figure 3: Different Paths throughresistive the Rectifier load Diodes removed by diode D1 D1 When the the top terminal is more positive,120 to 12.6VAC only diodes D1 and D3 can conduct. Because of the − stepdown transformer + D2 D2 voltage drop across the load, diode D2 is reverse biased. Diode D4 is reverse biased from itsinput con-from RL input from RL transformer nection across the transformer. As the current passes through each diode, the voltage drops by 0.7 transformer (positive half−cycle) volts. Thus, even if the load was a short circuit, diode D2 could not pass current back to the input (negative half−cycle) terminal since its anode side is at least 0.7 volts below its cathode because of diode D1. When− D3 + D3 the bottom terminal is more positive, only diodes D4 and D2 can conduct. In either positive or D4 D4 negative half-cycles, the positive output terminal remainsD1 and D3 positive.D1 and D3 17.1Vp 0V Now we have an output that is uni-polar put still has ripple.D2 We and canD4 remove most of the ripple with D1 D1 v_out the addition of a capacitor shown below.T1 The capacitor acts as a temporary storage reservoir that T1 1 D2 D2 C1 supplies current to theWallplug load between the input pulses.R1 The capacitor is charged on the risingWallplug edge R1 50uF of the half-cycles and supplies current to the load whenresistive the load input voltage falls below the voltage 1K 110VAC 110VAC stored on the capacitor. Such a circuit is able to provide a fairly clean output that is suitable for most electronic devices. D3 D3 2 120 to 12.6VAC 120 to 12.6VAC stepdown transformer D4 stepdown transformer D4 TITLE FILE: REVISION: PAGE OF DRAWN BY: Figure 4: Full-Wave Rectifier with Filter Capacitor Schematic The output from a spice simulation of the full-wave rectifier is shown in fig 5. The red trace is the input voltage. The blue trace is the output voltage. After the input waveform begins to fall below 2 the capacitor voltage, we see the discharge of the capacitor begin. Once the inverted negative half- cycle voltage exceeds the capacitor voltage, we see the capacitor being recharged. The capacitor holds-up the voltage between the voltage peaks. With an big enough capacitor the voltage droop between input voltage peaks can be made arbitrarily small. Figure 5: Spice Simulation - Filtered Full-wave Rectifier Showing Output Ripple 3.
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