ANADOLU UNIVERSITY DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING EEM 102 INTRODUCTION TO ELECTRICAL ENGINEERING EXPERIMENT 9: DIODES AND DC POWER SUPPLY
OBJECTIVE: To observe how a diode functions and to build a DC power supply. PRELIMINARY WORK: Explain the working principles of half-wave and full-wave rectifier circuits. Also explain the working principle of dc power supply. Prepare the work with your own handwriting using maximum two A4-size papers. INTRODUCTION: Diodes: Diodes are unilateral devices, that is, they conduct current in one direction but block it in the opposite direction. They will have a voltage drop of 0.2 0.3 V or 0.6 0.7 V when they are conducting depending on the materials they are made of, Germanium or Silicon, respectively. The direction of the current is from anode to cathode. Cathodes are usually indicated with a painted band. Figure 1. A diode.
Figure 2. Forward and reverse biased diodes. Transformers: These are devices that are generally used to convert an AC voltage to another AC voltage at the same frequency. Thus, the input to the transformer may be 220 V rms at f = 50 Hz, and the output may be 12 V rms at f = 50 Hz. This is basically accomplished by two windings around an iron core. The core is made up of laminated sheets of iron. The input winding has N 1 turns, the output winding has N 2 turns. The turns ratio is defined as and this is indicated as 1 : a. Figure 3. A transformer. The relationships between the input and output voltages and currents are as follows (Fig.3).
The Half Wave Rectifier: Figure 4. Half wave rectifier and input and output waveforms. When the input signal is positive (between t 1 and t 2, t 3 and t 4, etc.), the diode is forward biased and conducts current. When the input signal is negative (between t 2 and t 3, t 4 and t 5, etc.), the diode is reverse biased and does not conduct current. Hence the output waveform V out is obtained. This is called half rectified sine wave. The Full Wave Rectifiers: 1) The center tapped full wave rectifier. Figure 5. Center tapped full wave rectifier.
During the positive half cycle of the input signal, the diode D1 is forward biased and the diode D2 is reverse biased. Hence the current flows through D1 and R. During the negative half cycle of the input signal, the diode D1 is reverse biased and the diode D2 is forward biased. Hence the current flows through D2 and R. Thus, the voltage obtained across R is full wave rectified as seen is Figure 6. Figure 6. Output of full wave rectifier. 2) The full wave bridge rectifier. Figure 7. The full wave bridge rectifier. During the positive half cycle of the input signal, the diodes D1 and D2 are forward biased, and D3 and D4 are reverse biased. So, the current flows from A to B over diode D1, to C over R, to D over diode D2, and back to A over the secondary windings of transformer. During the negative half cycle, the diodes D3 and D4 are forward biased, and D1 and D2 are reverse biased. So, the current flows from F to D and B over D4, to C over R, to A over D3,
and back to F over the secondary winding. Thus, the waveform shown in Figure 6 is again obtained. Since in each case, the current has to go through two diodes, there is a voltage drop in the bridge equal to two diodes drops. DC Power Supply: Figure 8. DC Power Supply If a capacitor is added to the output of the rectifier circuit, the half rectified or the full rectified voltage will charge the capacitor. Hence, depending on the load resistor, a voltage which is almost dc will be obtained. Figure 9. Output voltage From a to b, the capacitor is charged, from b to c, the capacitor is discharged, because the load resistor R draws current.
PROCEDURE: A. HALF WAVE RECTIFER: 1. Set up the circuit in Figure 4 with R = 1.2 KΩ. Use the 12 V rms output of the transformer (the black jack and the red jack next to it). 2. Observe the signal v 2 (t) on the oscilloscope and measure its peak value V p and its period T. 3. Observe the signal v out (t) on the oscilloscope, measure its peak value V pout and its period T out. Draw the waveform of v out (t) in your report. 4. Add a C = 220 μf capacitor between the output terminals. Observe and draw the waveform of v out (t) in your report noting the critical values on the waveform. 5. Observe the ripple voltage by switching to AC coupling on the oscilloscope and decreasing VOLTS/DIV scale. Draw the waveform. B. FULL WAVE RECTIFIER: 1. Set up the circuit in Figure 7 with R = 1.2 KΩ. Use the 12 V rms output of the transformer. 2. Observe the signal v out (t) on the oscilloscope, measure its peak value V p and its period T. Draw the waveform of v out (t) in your report. C. DC POWER SUPPLY: 1. Add a C = 220 μf capacitor between the output terminals. Observe and draw the waveform v out (t) in your report noting the critical values on the waveform. 2. Observe the ripple voltage and draw the waveform. 3. Change the load resistor R to 470 Ω. Observe and draw the waveform v out (t) in your report noting the critical values on the waveform. 4. Observe the ripple voltage and draw the waveform.
EXPERIMENT 9 DIODES AND DC POWER SUPPLY Name:.. No :... Table No:. Group :.. REPORT A. 2. V p2 =., T =. 3. V pout =., T out =. 4.
5. V pp, ripple =. Compare the graphs in A3, A4 and A5. Explain what is happening in A4 (and A5). B. 1. V pout =., T out =. 2.
C. 1. 2. V pp, ripple =. 3.
4. V pp, ripple =. What is the major difference between the waveforms of C.2 and C.4? What causes this difference? Date: Table No: Grades Name: Pre-Lab Report Performance Total No: