BME 3512 Biomedical Electronics Laboratory Three - Diode (1N4001)

Transcription

1 BME 3512 Biomedical Electronics Laboratory Three Diode () Learning Objectives: Understand the concept of PN junction diodes, their application as rectifiers, the nature and application of halfwave and fullwave rectifiers, operation of light emitting diodes (LEDs). Laboratory Equipment: Agilent Oscilloscope Model 54622A Agilent Function Generator Model HP33120A Agilent Power Supply Model E3631A Multimeter Supplies and Components: Breadboard 12 Volt AC Transformer 270 Ω Resistor 27 Ω Resistor 1 KΩ Resistor 10 KΩ Resistor 1 μf Capacitor Diode (4 each) LED PreLab Questions 1. Discuss the basic operation of a solid state diode. List the forward barrier voltage for germanium and silicon PN junctions. 2. What are the differences between conductors, nonconductors (insulators) and semiconductors? What category is germanium? 3. For a sine wave input, sketch the output wave form for both a halfwave rectifier and a fullwave rectifier. PostLab Questions 1. List several practical uses of diode rectifiers. 2. Explain the operation of a LED. 3. What is the purpose of the series resistor? What circuit parameters determine its value? 4. What are photodiodes? What are the main differences between LEDs and photodiodes? 1

2 Laboratory Three Diode () Laboratory Procedures 1) Measurement of VI curve of the pn junction of a silicon diode (). Multimeter to measure current 5V R 1 1K R 2 27 Ω A I V Figure 1. Circuit for measuring VI curve a) Build the circuit as shown in Figure 1, the power source is a 5V DC voltage. R 1 is a potentiometer to provide variable voltage supply. R 2 is used to limit the current passing through the diode (I < 15 ma). The current I is measured using a Multimeter. The voltage across the diode is measured using another multimeter (or an oscilloscope). b) Adjust R 1 to obtain the value of V as shown in the table below, and record the I at each value of V. V I 2

3 2) Halfwave rectifier Sine Wave 100 Hz 2 V pp V in R 10 K V out Figure 2. Circuit of halfwave rectifier In this experiment, a 100 Hz sine wave (V in ) is obtained from the Function Generator. In addition to be connected to the diode as shown in Figure 2, V in is also connected to CH1 of the oscilloscope for displaying. V out obtained from R is connected to CH2 of the oscilloscope for displaying. Adjust the setting of the oscilloscope so that the top half of the screen displays 2 3 cycles of V in with suitable size and the bottom half of the screen displays V out. Sketch the waveforms displayed on the screen on your lab notebook. 3) Fullwave bridge rectifier In this experiment, use 4 diodes to build a circuit as shown in Figure 3. Pay special attention to the connections of the 4 diodes that form a ring. A problem with the circuit of fullwave rectifier is that there is no common ground between V in and V out. As a result, we cannot use a signal from the function generator as V in and use CH1 of the oscilloscope to measure V out because the function generator and the oscilloscope CH1 has a common ground. In order to perform this part of experiment, V in is obtained form an isolated transformer, as shown in Figure V power outlet V in R 10 K V out Figure 3. Circuit of fullwave rectifier V out is connected to CH1 of the oscilloscope. Do not connect the oscilloscope to V in. When you obtained a satisfactory display, copy the waveforms to your lab notebook. 3

4 4) Fullwave rectifier with a capacitor for smoothing output. 110 V power outlet V in R 10 K C 1μF V out Figure 4. Circuit of fullwave rectifier with a smooth output The circuit for this experiment is almost the same as in Figure 3. The only difference is to add a capacitor of 1 μf in parallel with R. Observe the waveform of V out and compare with that in Figure 3. Try using a larger C, how does the output change? What is the role of C? Save the waveforms. 4

5 5) LED (Light Emitting Diode) Just like a diode, an LED also only allows current to pass in one direction. However, an LED is different from an ordinary diode in two aspects. First of all, an LED emits either visible or infrared light when a current passes through it. Second, its VI relation is significantly different than that of an ordinary diode: the voltage drop across the two terminals is much larger than 0.7V. In digital logic circuits, LEDs are usually connected to certain points in the circuit to provide a visible indication of the logic state: 0V (LED if OFF) for logic 0, 5V (LED is ON) for logic 1. According to the manufacture s spec sheets, each LED should be operated at a certain current (20 30 ma) and a certain voltage (3 4 V). It practice, one only needs to make sure that the current passing though the LED does not exceed a limit (e.g. 25 ma). Do not directly connect an LED to a voltage source that is higher than 4.5 V (the LED will be burned!). a) Build the circuit shown in Figure 5 and measure the current I and voltage V LED using a multimeter. b) based on your measurement, determine the value of R so that I = 10 ma when a power supply is V CC (may be other than 5V). Hint: I = (V CC V LED )/R c) Test your design by performing another measurement using the circuit in Figure 5. In this case, the power supply is 10 V and a new R is used based on your calculation. Measure I and V LED for the new circuit. Multimeter to measure current R = 270 Ω A I 5V LED V LED Figure 5. A test circuit for LED 5

6 Grading Rubric: Diode (Lab 3) Name: Points Cover Page I) Introduction Discuss the following principles of a Diode: What are the typical doping agents for ntype and ptype silicon? How does each doping agent bond with silicon? What is a hole in ptype silicon? / 10 What are forwardbiasing and reversebiasing? What is a forwardbarrier voltage? Explain how this makes a diode, in effect, a voltagesensitive switch. II) Circuit Diagrams (Circuit Maker Only) 1) Experiment 1 Measure the VI Curve of Diode Resistor and potentiometer should be labeled with their measured values! For the ammeter, you can choose to either: a) include an ammeter in your circuit diagram, or b) simply note the direction of the current (I). Also be sure to label the part number of the diode (). 2) Experiment 2 HalfWave Rectifier 3) Experiment 3 FullWave Bridge Rectifier For the stepdown transformer, there s actually a symbol in Circuit Maker called Transformer. You can search for transformer to find it. There s also a fullwave rectifier bridge; the symbol is called FW Bridge. 4) Experiment 4 FullWave Bridge Rectifier w/ Smoothing Capacitor Here, you only need one circuit diagram; indicate that two different capacitors (1μF and 10 μf) were used. 5) Experiment 5 LED There is an LED in Circuit Maker, too, under Diodes. Again, either include the ammeter in your circuit diagram, or simply indicate the direction of the current. III) Data and Results 1) Experiment 1 Measure VI Curve of Diode a. Measured Values or R 1 and R 2 b. Data Table for Values of V and I c. Graph of VI Curve from Experimental Data 2) Experiment 2 HalfWave Rectifier a. Measured Value of R b. Graph of V OUT vs. Time Save V OUT (35 periods) and xaxis data 3) Experiment 3 FullWave Rectifier a. Graph of V OUT vs. Time 4) Experiment 4 FullWave Rectifier with Smoothing Capacitor a. Graph of V OUT vs. Time Using 1 μf Capacitor b. Graph of V OUT vs. Time Using 10 μf Capacitor 5) Experiment 5 LED a. Measured Value for V LED, I, and R 6

7 IV) Discussion 1) Experiment 1 Measure the VI Curve of a) From the VI curve, can you identify (approximately) the forward barrier (bias) voltage? How did you identify where it is on the graph? 2) Experiment 2 HalfWave Rectifier a) Why is the negative part of the sine wave clipped? What property of the diode causes this? 3) Experiments 3 and 4 Full Wave Rectifier a) Plot all three output voltages (using no capacitor, the 1 μf capacitor and the 10 μf) on one graph (don t use the timeaxis data in this graph, since the three experiments weren t performed simultaneously; label the xaxis as Samples [n] ). Since the experiments weren t performed simultaneously, the peaks of the waveforms won t be aligned. However, you should be able to see the effects of the capacitor; explain what the capacitor does to the output signal. Why is this useful? V) PostLab Questions a) PostLab Question #3 / 2 Refer to Experiment 5 to see how to determine R based on V CC, V LED, and I. b) PostLab Question #4 / 2 VI) References / 1 7