Lab3 - L303-3.R23 Drexel University Electrical and Computer...

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L303-3.R23 3-1 Drexel University Electrical and Computer Engr. Dept. Electrical Engineering Laboratory III, ECEL-303 E. L. Gerber DC VOLTAGE SUPPLIES and REGULATION Object The objects of this experiment are to introduce you to a voltage regulator system, voltage regulator concepts, and the voltage regulator integrated circuit chip (IC). You will learn how to calculate voltage regulation and how to design a voltage regulator IC system. Also you will learn how to convert AC line voltage to a regulated DC voltage. Introduction All electronic systems require a good quality (regulated) DC voltage source to operate. Radios, TVs, CD players, boom-boxes, and computers all require a very stable DC voltage. Batteries are a possible source of power for these systems, but they have limited power and are expensive. Most home and laboratory systems utilize the 115 V-AC power line for power. The problem is this voltage is the wrong type (AC) and is too large (115 V- RMS). Most electronic devices use low DC voltages ranging from 5 to 24 volts. The first step in the process of converting the line voltage to a lower level DC is to reduce the level of the AC voltage with a step-down transformer; a 10:1 transformer will output 11.5 V-RMS. The second step is to convert the AC sine wave to a full-wave rectified sine wave (no negative content). The third step is to convert the varying rectified DC wave to a constant (flat) DC function. The final step is regulation, which keeps the DC level from changing if either the power line changes or the load connected to this DC power supply changes.
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L303-3.R23 3-2 Theory 1 - Rectification: A PN junction diode has the property of allowing current to flow in only one direction, and almost no current in the other direction. Fig. 1 depicts the output of a full-wave bridge rectifier circuit fed from a 10:1 step-down transformer. Fig. 1. Bridge Rectifier and Full-Wave Sine Output Voltage The four diodes permit the sine wave current to flow in only one direction through R C . The output of the rectifier circuit shown as V L . The period of the output is T = 1/2f = 1/120 sec. The peak value is V P = 2 V R MS = 2 12 volts, and it has a DC (average) value V DC = 2V P /π, but it is not a constant DC, see Fig. 1. 2 - Filters: The addition of a single capacitor across the filter resistor will greatly improve the DC waveform as shown in Fig. 2. The time constant, RxC, of the circuit affects the output. If RC >> T considerable improvement in the output occurs. R C is the equivalent load resistance of the device that the DC voltage is connected to and C is a design element.
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L303-3.R23 3-3 Fig. 2. RC Filter; a) Large RC Value, b) Small RC. The average or DC level is now midway between the max and min shown in Fig. 2. V
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This note was uploaded on 04/05/2010 for the course ECEC 303 taught by Professor Gerber during the Spring '10 term at Drexel.

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Lab3 - L303-3.R23 Drexel University Electrical and Computer...

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