SKEU 3741 BASIC ELECTRONICS LAB

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1 Faculty: Subject Subject Code : SKEU 3741 FACULTY OF ELECTRICAL ENGINEERING : 2 ND YEAR ELECTRONIC DESIGN LABORATORY Review Release Date Last Amendment Procedure Number : 1 : 2013 : 2013 : PK-UTM-FKE-(0)-10 SKEU 3741 BASIC ELECTRONICS LAB EXPERIMENT 1 DIODE

2 BASIC ELECTRONIC LABORATORY FACULTY OF ELECTRICAL ENGINEERING UNIVERSITY OF TECHNOLOGY MALAYSIA EXPERIMENT 1 DIODE A The Half-wave and Full-wave Rectifiers Objective To study the elementary power supply circuits, which transforms energy from the, 240-V power line to steady dc that can be used by electronic circuits and equipment. Equipment Required Transformer 2 diodes (IN4001) 1 Capacitor (100 F, electrolytic) Resistor (100k, 0.25W and 100, 6W) Brief Theory In almost all dc power supply, a rectifier circuit with capacitor filter is used to convert an ac supply voltage to a dc voltage and to smooth out the dc level over time. Most electronic circuits require dc supply. Rectifier circuits are almost always found in electronic equipment that operates from the ac power lines. Although a sophisticated voltage regulating circuits are usually employed, the rectifier circuits in this experiments form the basis from which commercial dc power supplies are made. In a simple rectifier circuit, although the output is never negative, the output varies from zero to almost the peak value of the input. To be useful the power supply must maintain an output voltage that is constant over time, and does not change when the input voltage or the load resistance changes. The essential part of a dc power supply is shown in Figure 4-1 D Vi C VO Figure 4-1 The capacitor charges during the half cycle when the diode is forward biased, and retain its charge during the other half cycle. The output voltage is constant, with peak value approximately 0.7 V below the peak value of Vin. A load is connected to this circuit, but as soon as the load is connected, the output voltage is no longer constant, because the capacitor begins through the resistor.

3 Procedure Half wave Rectifier Power Supply 1. Construct the circuit in Figure 4-2. Observe the output waveform on the oscilloscope. What is the peak-to-peak voltage? Sketch the waveform. 2. Connect the filter capacitor (100 F, electrolytic) across the output of the circuit, parallel to the 1 k resistor. Be sure to connect the capacitor with proper polarity It may explode if you don t! Observe the output waveform on the oscilloscope. Sketch the waveform. Now add the 100-, 6 watt resistor. Observe and sketch the waveform. (Be sure to use dc couple on the oscilloscope, and mark the zero level on your sketch). 3. Calculate the ripple factor as a percentage using the following formula: 1N4001 % ripple = V r ( p p) x100 % V DC V(t) 240 VRMS 15 Vrms Vr(p-p) 1 k VO VDC 0 Figure 4-2 Figure 4-3 t The regulation of a power supply is a measure of how much the output voltage changes as more and more current is drawn from the output terminals. Percent regulation is defined by: % regulation = no load voltage full load voltage x 100% full load voltage Calculate the percent regulation for the halfwave rectifier in Figure 4-2, assuming that the 100 resistor constitutes a full load. Full wave Rectifier Power Supply 4. Construct the circuit in Figure 4.-. Sketch the output voltage WITHOUT the capacitor and the 100 resistor. State the difference between this waveform and the waveform in Figure 4.2? Then connect the capacitor and the 100 load resistor. Sketch the output waveform and calculate the percent ripple. 240 Vrms 50Hz Diode IN Vrms 100k 100 F 100 W To oscilloscope V o Figure 4-4

4 B THE ZENER DIODE REGULATOR Objective To study the behavior of zener diode and to use it as a voltage regulator to improve the performance of a dc power supply. Equipment Required Transformer Oscilloscope 2 diodes (IN4001) DC power supply zener diode (IN 4733) Ammeter Capacitor (100 F, electrolytic) Voltmeter Resistors (510, 2x1k, 6.8k, 100k ) Multi-meter Brief Theory All real diodes experience avalanche breakdown when the reverse voltage becomes large enough. In the breakdown region, the current may rise rapidly with very little increase in reverse voltage. A zener diode is designed to experience breakdown at a fixed reverse voltage. Commercial zener diodes are available for the range from 3.3V to several hundred volts. When forward biased, a zener diode has the same characteristic as any other forward biased diode. Figure 4-5 shows the I-V characteristic of a typical zener diode. The reverse breakdown region can be approximated to a straight line with a slope of 1/rz where rz is the incremental resistance in this region. rz can be as small as 5. The intercept Vzk is known as the zener knee voltage. id Reversed breakdown region -Vzk region Reversed biased Vf Forwardbiased region VD Figure 4-5

5 Procedure Zener diode I-V characteristics 1. Connect the circuit in Figure 4-6. For Vs between 0 to 20V, obtain values for iz and Vz. (Iz and Vz have negative values). Complete Table 4-1. Plot a graph of iz versus Vz. From your graph obtain the value of vzk and estimate rz. 1k iz + V S _ v z The zener regulator 2. Connect the circuit in Figure 4-7. A Figure Measure the voltage across the 100 k resistor and across the load resistor RL. Sketch the waveform and record the dc value and the ac ripple component. By how much (as a percentage) does the zener diode reduce the capacitor ripple voltage? 4. Measure the output voltage both with and without the load resistor present. What is the percent regulation of this supply when the zener diode is in place? 5. With the load resistor connected, touch RL and the zener diode with your finger. Comment on their temperature. Disconnect RL and touch the zener diode. Explain your observation. 6. Replace the load resistor with the 510 resistor. Measure the current flowing through R1, the zener diode and the load resistor. 7. Repeat step 6 using a 6.8k resistor. What is your conclusion? Diode IN4001 R1=1k To oscilloscope 15 Vrms IZ IRL 240 Vrms IR1 50Hz RL C 100k Vz 1k Vo 100 F IN4733 Figure 4-7