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Transcript

PHYSC 3322 Experiment 1.3 4 May, 2017 Diodes Purpose In this experiment, you will investigate the current-voltage characteristic of semiconductor diodes, and consider their applications in DC power supplies. Background Select a 1 k resistor, and measure its exact resistance using the Fluke multimeter. Select one of the diodes and construct the circuit shown in Figure 1, using the resistor you just measured, the Tektronix power supply, and two Fluke multimeters. One meter measures the voltage drop across the diode, while the other monitors the current through the resistor and diode. By adjusting the output voltage of the power supply, measure the voltage across and current through the diode. As you vary the power supply voltage, monitor the power dissipated in the resistor ( P V 2 / R ). Do not allow the power to exceed 250 mW . Next, reverse the polarity of the power supply connections and measure the diode current and voltage in the reverse direction. Again, do not allow the resistor power dissipation to exceed 250 mW . Plot the diode's complete (both negative and positive polarity) I V curve using Sigma Plot. Repeat the above measurements with each of the four diodes (1N914, 1N34, 1N4733 and the red light-emitting diode). Plot your results. Construct a half-wave rectifier using a 1N4007 diode as shown in Figure 2. Doublecheck your circuit before applying power—if the circuit is incorrect, you will likely blow a fuse! View the output wave form with the oscilloscope, and sketch the wave form shape. Do this with (1) no capacitor in the circuit, (2) a 22F capacitor, and (3) a 220 F capacitor. Now convert the circuit into a full-wave rectifier by adding a second 1N4007 diode, as shown in Figure 3. Again, sketch the output waveform for the three different output capacitors. Figure 1. Diode current vs. voltage measurement. 1 PHYSC 3322 Experiment 1.3 4 May, 2017 Figure 2. Half-wave rectifier. Figure 3. Full-wave rectifier. Questions Discuss and compare the I V curves for the different diodes. A simplified model for forward conduction in a semiconductor diode relates the current and voltage by I I 0 expV /V0 1 , (1) where I 0 and V0 are constants that depend of the type of diode. Do your diodes obey this relation? Estimate I 0 and V0 for each diode. To estimate I 0 , consider that when V is negative and significantly larger than V0 in magnitude, Equation 1 reduces to I I 0 . (2) Once you have a value for I 0 , you can rearrange the equation and take the natural logarithm of both sides to get a linear relationship with which you can determine V0 using a least-squares fitting procedure. Explain the shapes of the output waveforms in the half-wave and full-wave rectifier circuits. 2