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Diodes
V-I Characteristics – signal diode
Measure the voltage-current characteristic of a standard signal diode, the 1N914, using the circuit shown in Figure 1 below. The purpose of the back-to-back power supplies is to make it easy to make measurements near zero supply voltage. Plot the V-I
characteristic to show the rapid rise in forward current when the forward voltage approaches 0.7 volts. It should look something like the curve in Figure 2. Notice that the current in milliamps is given by the voltage across a 1kΩ resistor as measured in volts. Therefore, you can measure I vs. V by connecting a digital voltmeter alternately across the resistor and the diode. In a moment you will use a LabView
program to repeat this measurement I vs. V.
Figure 1: Diode Measuring Circuit
Figure 2: Diode Characteristic Curve
Temperature dependence
Reverse bias the diode with ~-20 V and observe the reverse current (should be about 0.2 μA). Touch a hot soldering iron to the metal lead close to the diode and note the percentage change in reverse current. There is a big effect because the minority carriers that transport the reverse current are thermally generated. Now bias the diode in a forward direction to get a forward current of about 0.2 mA. Repeat the heat treatment and observe the change in current.
V-I characteristics of diodes using LabView
Repeat the measurement I vs. V for a standard signal diode using the LabView program, I_vs_v using a 1kΩ resistor for the standard. Make a printout of the curve and insert into your lab notebook.
Use LabView to measure the V-I characteristic for a Zener diode, the 1N4734, which has a Zener voltage of 5.6 V. (Note: In the forward condition, current flows out of the terminal labeled with the band.) Plot the V-I characteristic using the LabView program and note the dramatic increase in reverse current when the voltage reaches 5. volts. A Zener diode, biased at this breakdown condition, is a stable voltage reference because the voltage does not change much even though the current may change. Make a printout of the curve and insert it into your lab notebook.
The Half-Wave Rectifier
The basic half-wave rectifier with resistive load looks like the circuit in Figure 3. The Input and Output waveforms are shown Figure 4.
Figure 3: Half-wave Rectifier. Figure 4: Vin (solid) and Vout (dashed)
Build the circuit above. Use a 9-VRMS, 60-Hz supply.
(a) Begin with the capacitor disconnected. Observe the full-wave rectified pattern on the oscilloscope. Verify that the frequency of the pattern is 120 Hz. [Note: Because the two channels of your oscilloscope have a common ground, it is not possible to view both the input and the output of the rectifier at once.] (b) Add the 10-μF capacitor, getting the polarity right, but remove the load resistor. Observe how the capacitor integrates the rectified power to make a smooth DC source. (c) Add the resistive load back into the circuit and see how the load causes the output of the power supply to “ripple.” Verify that the frequency of the ripple is 120 Hz. (d) Reduce the resistive load by making the resistor 47kΩ instead of 10kΩ. Observe that the ripple decreases. Once again you may need to AC couple and increase the magnification of the DPO. Explain the advantage of the full-wave over the half-wave rectifier in making a stable power supply. In fact, neither supply is very good. Both are unregulated. Regulated power supplies will be studied in a later lab.
The PIN Diode
The PIN diode has intrinsic (not doped) material (I) in the junction between P and N type material. It makes a fast photo diode, wherein minority carriers are generated by photons. Because the photoeffect depends upon minority carriers, the diode is biased in the reversed condition for normal operation as a light-sensitive device. Use the PN323D pin diode in the circuit below to show a large increase in diode current when a bright lamp is brought close to the diode. What is the effect of elevated temperature on the reverse current? Reverse the diode to attempt to find the effect of light on the diode current in the forward-biased condition.
Figure 8: Vin (solid); Vout (dash)^ Figure 7: Full Wave Rectifier.
Figure 9: PIN Diode Circuit.
5V 1K
Long Lead
The Seven-Segment Display
The Seven-segment display uses 8 light-emitting diodes, including one for the decimal point. The LN516RK is a common cathode device, as shown in Fig11. The anodes are connected as shown in Fig. 10. Use the circuit in Fig. 12 to turn on segments and verify the diagram in Fig. 10.
Figure 12: Measurement Circuit
Figure 11: Common Cathode Figure 10: LN516RK Pinout. LN516RK.
