ELEC 2020 EXPERIMENT 6 Zener Diodes and LED's

Transcription

1 ELEC 2020 EXPERIMENT 6 Zener Diodes and LED's Objectives: The experiments in this laboratory exercise will provide an introduction to diodes. You will use the Bit Bucket breadboarding system to build and test several DC and AC diode circuits. The objectives of this experiment include: Review basic principles of diodes from ELEC 2210 An understanding of diode rectifier circuits More experience with the Bit Bucket breadboarding system and the oscilloscope Continue to develop professional lab skills and written communication skills. Introduction A thorough treatment of diodes can be found in Chapter 3 of the ELEC 2210 textbook, Microelectronics Circuit Design by R.C. Jaeger. Diodes are circuit elements that allow current to pass in one direction, while blocking it in the other direction. They are used primarily in rectifier circuits, which convert AC to DC, and in voltage regulation and voltage limiting circuits. They are also sometimes used to produce a fixed DC voltage drop (approx. 0.7 volts for Si p-n junction diodes). In the latter case, several diodes may be connected in series to achieve the desired voltage drop. The mathematical model of the diode (not including reverse breakdown) is given in Equation 1: vd nv i = I ( 1 + e T ) D S [1] The diode current i D, and voltage, v D are defined in Figure 1. + v D anode cathode i D Figure 1. Circuit symbol and nomenclature for the diode. Table 1 explains the parameters found in Equation 1. 1

2 Table1. Diode equation parameters. Symbol Name Meaning I S n V T Saturation current Nonideality factor Thermal voltage Ideal maximum reverse current. Also the scale factor for forward current. Proportional to junction area. Typical value(s) to 10 9 A PSPICE Default A Corrects for manufacturing variations and temperature uncertainty. 1.0 to This parameter has units of voltage and is proportional to absolute temperature. 25 mv at room temp mv (27 o C) From equation 1, we can see that the diode current increases nearly exponentially as the diode voltage becomes more positive (forward bias), but the current is asymptotically limited to I S when v D is negative (reverse bias). If the diode is forward biased substantially, Equation 1 can be simplified to a pure exponential, and then near room temperature we can deduce the 60 mv per decade rule, which says that v D increases approximately 60 mv for every factor of 10 increase in i D. In addition to the behavior described by Equation 1, all diodes have a reverse breakdown voltage, V BR, at which significant reverse current will begin to flow. For many applications, it is desirable to have V BR be as large as possible. Values of V BR = 1000 V are not uncommon for small-signal rectifier diodes. For other applications, such as voltage regulation, we require relatively low reverse breakdown voltages, such as 5 V or 9 V. For these applications, we use Zener diodes. The reverse characteristics of a typical Zener diode are shown in Figure 2. i D Slope 2 V Z V ZK Slope 1 I ZK I Z v D Figure 2. Piecewise linear approximation of the reverse characteristics of a Zener diode. The Zener knee resistance is 1 R ZK = Slope 1 2

3 and the Zener resistance is 1 R Z = Slope 2 Typcial values of R Z are on the order of 10 ohms. Real diodes have series resistance and junction capacitance which affect their behavior in circuits at higher currents and higher frequencies. Furthermore, I S is a strong function of temperature. These effects will not be considered in this lab experiment. An important simple model for the diode is the piecewise linear (PWL) model, illustrated in Figure 3. i D V Z V ON v D Figure 3. Piecewise linear (PWL) diode model. Figure 4 shows a series DC circuit with a diode. +V D - I + V R - Figure 4. Series circuit including a diode. When the circuit is connected, the diode will operate at a Q-point (I, V D ) determined by the intersection of the diode characteristic curve and the load line. The load line is established by the values of V DC and R. The circuit must satisfy Kirchhoff s Voltage Law (KVL), such that V D + V R = V DC. If the source is changed to an AC voltage source, current will flow in only the forward direction through the diode, as long as the peak negative value of the voltage does not exceed the reverse breakdown voltage of the diode. In this case, v R (t) will never be negative, thus the AC voltage is said to be rectified. 3

4 Pre-Lab: (1) Obtain the data sheet for the 1N4733 Zener diode from the class web site, or from Use this to determine the values of V Z, I Z, I ZK, R Z, and R ZK. Note that the datasheet uses the letter Z (impedance) instead of R (resistance). (2) Determine an expression for the RMS value of (a) a sine wave with peak value V P, and (b) a half-wave rectified sine wave with peak value V P. You may obtain these either by direct calculation, or by looking them up in any resource you choose. (3) Look up the turn-on voltage for red LED s and green LED s. Lab Exercise: There are four parts. Have your GTA sign off on each part before proceeding to the next part. (1) Fuse check, current measurement with the DMM, and verifying Ohm's law. In this part, you will learn how to measure current with the digital multimeter (DMM). You will also check to make sure the DMM fuse is not blown. (a) Identify the variable +15 VDC supply on the Bit Bucket. The output terminal is labeled 0~+15 as shown in the photo. Set the knob to Min. (fully CCW). (b) Connect this output to the + input on the Bit Bucket's DVM. Connect the input on the DVM to ground. Turn the 15V supply knob and verify that the voltage changes as expected. Record the minimum and maximum values. (c) Turn off the Bit Bucket Power, and adjust the variable voltage knob back to Min. (d) Obtain and measure a 100 ohm, ½-Watt resistor. Record your measurement. Leaving the DVM connected as above, connect the circuit shown below in Fig. 5. Set the DMM knob to the DC current position, and connect the positive lead (from the resistor) into the jack labeled 300mA. Connect the negative lead (returning to ground) to the jack labeled COM. (the labels may be slightly different if you are using a different model DMM). (e) Re-check to make sure the variable supply knob is set to Min. Turn on the Bit Bucket Power. 4

5 (f) Slowly increase the variable DC. The voltage shown on the DVM should increase, and the current shown on the DMM should increase proportionally. If you get to 1 V and your DMM shows no current, the DMM most likely has a blown fuse. Check with your GTA for repair procedures. 0 to 15 V Variable DC Figure 5. Measuring current with the DMM (g) Record the the DVM voltage and the DMM current at three different voltages: roughly 1 V, 3 V, and 5 V. DO NOT EXCEED 60 ma OF CURRENT. Compute the resistance using Ohm's law, and compare with what you measured in part (d). (2) Diode I-V characteristics. Carefully measure both the forward and reverse characteristics of the 1N4733 diode, using the series circuits shown in Fig. 6. Use the variable 0 to +15 V DC supply on the bit bucket, and use a 100 ohm, ½ Watt resistor. Use a DMM to measure the current, and the DVM on the Bit Bucket to measure the diode voltage. In order to measure the forward characteristics, insert the diode so that the cathode is grounded, as shown in Fig. 6a. To measure the reverse characteristics, switch the diode end-for-end, so that the anode is grounded, as shown in Fig. 6b. For each part (forward and reverse) start with the variable DC voltage set to zero, and increase it carefully!! DO NOT EXCEED 60 ma OF CURRENT IN EITHER DIRECTION. 5

6 1/2 Watt DMM DC current 1/2 Watt DMM DC current 0 to 15 V Variable DC 1N4733 DVM DC voltage 1N4733 DVM DC voltage (a) (b) Figure 6. (a) Measuring the forward diode characteristics (cathode grounded). (b) Measuring the reverse diode characteristics (anode grounded). Black stripe on part indicates cathode. Measuring procedure. With the circuit connected as above, slowly increase the variable DC voltage until you have about 50 ma of current flowing through the diode. Record the diode voltage. Incrementally reduce the supply voltage, and continue recording I and V D pairs until the current is reduced to zero. You should let the meter readings stabilize for about 10 seconds at each step. You can use the example tables below as a rough guide for the number of values to record and the expected values. Table 2a. Example forward characteristics of the 1N4733. V D (volts) I D (ma)

7 Table 2b. Example reverse characteristics of the 1N4733. V D (volts) I D (ma) Diode Current (ma) _ 1N4733 Measured I-V Characteristics Diode Voltage (V) Example plot of the 1N4733 forward and reverse characteristics combined In your lab report, plot the I-V characteristics that you measured. See the example plot above. From the data you obtained, estimate the values of R ZK and R Z. Compare your values with those given in the data sheet. Are they within the stated limit(s)? In your lab report, calculate the value of I S from your data, at three different data points. Assume n = 1 and V T = 25 mv. Are the values consistent with each other? Do they fall within the expected range (see Table 1)? Compare your measured data with the 60 mv per decade rule at two different current values. How well does the rule match your data? Use approximate current values of (50 ma, 5 ma) and (30 ma, 3 ma) to test the rule. Compare the corresponding voltage differences with 60 mv, and express your comparison as a percent difference. 7

8 (3) Half-wave rectifier. Connect the Function Generator (FG) on the Bit Bucket to the oscilloscope and adjust the FG to sine, 2 V peak (4 V peak-to-peak), 1 khz. Use the MEASURE feature of the scope to determine the RMS value of the sine wave and compare with the value you expect from the prelab. Connect a series circuit as shown in Figure 7. Use the 1N4733 diode, a 1000 ohm, ¼ Watt resistor, and the Function Generator (FG) on the Bit Bucket. Oscilloscope Figure 7. Half-wave rectifier. If your circuit is connected properly, one end of the resistor should be grounded, and the other should be connected to the cathode of the diode. Use the oscilloscope to display and measure the voltage across the resistor. Be sure the oscilloscope ground is connected to the Bit Bucket ground. You should observe a half-wave rectified voltage across the resistor. Sketch your observation. Use the MEASURE feature of the scope to determine the RMS value of the voltage, and compare with the value you expect based on your expression in the prelab. Use the MEASURE feature of the scope to determine the MAXIMUM value of the voltage. Explain this value, considering KVL and the forward voltage drop you observed in Part (2). Increase the amplitude of the FG to its maximum value. Sketch and explain what you observe on the oscilloscope. It will help to take a moment to measure the FG output on the scope, and note that the peak negative output exceeds the Zener voltage of the diode. Now exchange the positions of the resistor and the diode, so that the anode is grounded, as shown in Figure 8. Reduce the FG voltage to 2 V peak. Use the scope to observe the diode voltage. Measure the most negative voltage. Increase the FG voltage to maximum. Explain your observation. Switch the FG to a square wave. What is the peak value of the output? Why? 8

9 Oscilloscope Figure 8. Measuring the forward turn-on and reverse breakdown. (4) Forward turn-on voltage of LED's. Replace the 1N4733 shown in Fig. 8 with a Red LED, and replace the 1kΩ resistor with a 330 Ω, ¼ W resistor. It does not matter which way the LED is inserted, since we are using an AC source. If your circuit is properly connected, one leg of the LED will be grounded. With the FG set to sine wave of maximum amplitude, confirm that the diode lights up, and observe the diode voltage with the scope. Sketch your display. What is your best estimate of the ON voltage? Compare with your expectations from the prelab. Replace the Red LED with a Green LED, and repeat. 9