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PHY 440 Lab 7: Diodes
V-I Characteristics – signal diode
Measure the voltage-current characteristic of a standard signal diode, the 1N914, using the circuit shown 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 on graph paper to show the rapid rise in forward current when the forward voltage approaches 0.7 volts. It should look something like the figure below. Notice that the current in milliamps is given by the voltage across a 1K resistor as measured in volts. Therefore, you can measure V vs. I by connecting a digital voltmeter alternately across the diode and the resistor. Repeat the measurement using the LabView program, I_vs_v using a 1K resistor for the standard. Make a printout of the curve and insert into your lab notebook.
Temperature dependence
Bias the diode to get a reverse current of about 0.2 microamps. Put a soldering iron 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.
Figure 1: Diode Circuit.
5V
Var.
1K
+ VD
-18V 0
ID (mA)
0.7V
VD
Figure 2: Diode V-I Plot.
The Half-Wave Rectifier
The basic half-wave rectifier with resistive load looks like the circuit above. The Input and Output waveforms are shown on the right. The half-wave rectifier with filter capacitor and resistive load is shown in Figure 6 with the input and output waveforms in Figure 5.
The capacitor is electrolytic. You must obey the polarity markings when you connect this capacitor in the circuit. Use an 8 VRMS 60-Hz transformer secondary as a source.
(a) Begin with the capacitor disconnected. Observe the half-wave rectified pattern on the oscilloscope. Verify that the frequency of the pattern is 60 Hz.
(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. We might imagine using such a source to power some electronics that need a stable source.
(c) Add the resistive load back into the circuit and see how the load causes the output of the power supply to “ripple”. You may need to AC couple the scope and increase the
Figure 6: Rectifier with Capacitor.^ Figure 5: Input and Output forms.
Figure 4: Input (a) and Output (b) waves.
V
V
Figure 3: Half-wave Rectifier.
Scope
10K
a)
b)
t
10 μF
Scope
1K
a)
b)
t
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 5.6 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.
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 PN323B 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.
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 10: LN516RK Pinout.
Com
Figure 9: PIN Diode Circuit.
5V 1K
Long Lead
Figure 11: Common Cathode LN516RK.
Pin 3 (^) Pin 8
The interrupter The interrupter consists of an LED and a phototransistor separated by a gap (see Figure 13). When the gap is empty the light (infrared) from the LED excites carriers in the phototransistor, which then conducts. When the gap is filled, the light is interrupted and the phototransistor does not conduct much.
Build the circuit in Figure 14 using an H22A1 from Harris Semiconductor. Can you interrupt the light path so fast that you test the speed of the response? Better be speedy. The turn-on time is 8μs. The turn-off time is 50μs. Interrupters are used with spinning disks with holes that allow the light to pass. A simple counter on the output of the phototransistor can count the number of revolutions, and with the addition of a timing circuit the speed of the disk can be determined.
Figure 14: The Interrupter Circuit.
Scope
Figure 13: The H22A1.
Figure 12: Measurement Circuit.
1 2
(^43)
3
2 1 4
1K
5V
1K 10K
10V
