Experiment 2 Diode Applications: Rectifiers

Transcription

1 ECE Practicum Fall 2007 Experiment 2 Diode Applications: Rectifiers Objectives 1. To investigate the characteristics of half-wave and full-wave rectifier circuits. 2. To recognize the usefulness of modeling in predicting results. Preparation 1. Read sections 3.13 to 3.16 of Jaeger and Blalock. 2. In Figure 1, the voltage v 1 (t) equals V p sin 2π60t. Develop the equation for the average dc voltage across the load resistor if the diode is modeled by an ideal diode. Let V p = 5 volts. 3. Repeat if the diode is modeled by a constant voltage drop model. Use V on = 0.6 volts. 4. In Figure 2, the voltage v 1 (t) equals V p sin 2π60t. Develop the equation for the average dc voltage across the load resistor if the diode is modeled by an ideal diode. Let V p = 8 volts. 5. How do you determine the capacitance value of a rectifier circuit that uses a capacitor filter? Preliminary Lab Work Set your function generator to display the true peak-to-peak voltage at its terminals refer to Appendix 2 for details. Parts 1N4148 silicon diode 2.2 kω resistor 4.7 μf (polarized) capacitor Line operated low-voltage center-tap transformer Exercise 1: Silicon Diode Check 1) Select the diode function on the digital multimeter (DMM). This function is used to check the conduction/nonconduction state of a diode. In this mode, the DMM sends a current of 1 ma out of the HI terminal, through the device under test (DUT), and back through the LO terminal. The display is the diode forward voltage (if conducting) and the beeper threshold is in the range 0.3 < eas < 0.8 V. The range of the display is fixed at 1 V (refer to the manual). 2) Connect the leads of the DMM to a small-signal silicon diode, such as a 1N4148. What does the display mean? Reverse the leads. What voltage appears across the diode when it is conducting? Can you test an LED by this method? Exercise 2: Half-Wave Rectifier Circuit 1) Wire the circuit shown in Figure 1 using a 1N4148 diode and a 2.2 kω resistor. Turn the FG on and set it for a 200 Hz sine wave. Set the output voltage for 4 volts peak to peak (p-p). Note: Did you follow the steps in Appendix 2? Experiment 2 Diode Applications Rectifiers-sep 10 1

2 2) Connect your scope as follows: scope ground clip to the black terminal on the FG, CH 1 probe to the FG red terminal (input), and CH 2 probe to the load resistor (output). Turn the scope on and select Auto-Scale. Red D1 2 Function Generator V1 1 R1 2.2k Black Figure 1. Half-wave rectifier circuit. 3) Examine the waveforms. Adjust CH1 and CH2 position controls so that both signal grounds line up in the middle of the screen. Set the volts/div controls to the same value for both channels. Note: Observe that the diode on-time and diode off-time of the output signal are not the same. Also observe the difference of the peak amplitudes of the input and output signals. 4) Select Cursors on the scope and perform the following measurements: - Measure the peak input voltage, V p. It should be 2.0 volts. - Measure the peak load voltage,. - Measure the voltage difference, = V p. What is the significance of the value of? 5) Measure the average dc load voltage,, with the digital voltmeter (DVM). What is the theoretical value of the dc voltage across the load? 6) Remove the DVM. Convert it to read dc milliamperes. Show your instructor how you will connect the digital ammeter (DAM) to read the dc load current, I dc. If approved, go ahead and measure the current. 7) Remove the DAM. Measure the dc load voltage with the scope using QuickMeas. 8) Record both waveforms. Record all scope settings. 9) Connect a 4.7 μf capacitor across the load resistor. Before you do this, see the note on the next page. The capacitor acts like a filter blocking high frequencies. 2

3 Important Note: Observe the polarity shown on the side of the capacitor the tiny arrows point to the negative ( ) lead. If the capacitor polarity is incorrect, the capacitor may explode causing serious injury to your eyes or skin. 10) Again line up both signal grounds on the scope. What is the significant effect on the output signal? Measure the following items using Cursors: - The peak input voltage, V p, - The peak load voltage,, - The voltage difference, = V p, - The average dc load voltage,, using the DVM. - The p-p ripple voltage, V r, across the load (see Footnote 1 to see how to do this). 11) Measure the dc load current as you did in step 6. 12) Reset CH2 coupling to DC. Record both waveforms. 13) Remove the capacitor. Exercise 3: Full-Wave Rectifier Circuits 1) The full-wave rectifier circuit shown in Fig. 2 is a bridge rectifier circuit. Wire it up. Adjust the FG for a 4-volt p-p, 200 Hz sine wave. 2) Unlike the case with the half wave rectifier, you cannot use both probes, simultaneously, to display the input voltage and the load voltage. Use CH1 to display the load voltage at node 1 (ground clip at node 2). Notice that the diode on-time is significantly greater than the diode off-time. Also, this waveform is not exactly what you expect, because there are two conducting diodes in series and they contribute a voltage drop of about 1.2 volts. 3) Most dc power supply circuits have a sinusoidal input voltage much greater than 4- volts p-p. Many are line operated (120 V rms, 60 Hz) and use a step-down transformer to reduce the 120 volts to some lower value. To investigate this, increase the input voltage of the FG to 20 volts p-p. Measure the following using Cursors: - The peak input voltage, V p (should be 10.0 V), - The peak load voltage,, - The voltage difference = V p, - The average dc load voltage,, using the DVM. How can you predict the dc load current without using the DAM? 1 Deselect CH1, and select CH2. From the Coupling menu for CH2, choose AC. Increase the volts/div setting to around 50 mv/div. 3

4 Red Function Generator V1 D3 1 R k D6 D5 D4 Black Figure 2. Full-wave bridge rectifier circuit. 4) Now connect the 4.7 μf capacitor across the load. Important Note: The capacitor polarity should be minus ( ) at node 2 and plus at node 1 and. Repeat the previous measurements as well as the p-p ripple voltage, V r, across the load. Exercise 4: Half-Wave Rectifier Circuit with Negative Output 1) What circuit should you wire up to produce a negative voltage at the output? Draw a schematic and show it to your instructor for approval. 2) Wire the circuit but use a line operated step down transformer instead of a function generator. 3) Make measurements as you did before and record the appropriate waveforms. Report The report starts with the following information on the first page: title of experiment, date experiment was performed, name of report writer, names of lab partners, and the goals and objectives of the experiment written in sentence form. The next page starts the second part of the report which consists of sections A through E plus a Conclusion, as follows. A) Half-Wave Rectifier First, describe, in sentences, the circuit and what data you are going to measure. Draw the schematic. List the measured data and the predicted (theoretical) data. A good way to present this data is by a table similar to this illustration. HW Rectifier State the model and the calculations that you used for the predicted data. 4

5 B) Half-Wave Rectifier with Filter Capacitor In this part, describe, in sentences, the circuit modification. List the measured data and the predicted (theoretical) data. A good way to present this data is by a table similar to this illustration. V r HW Rectifier with Filter Show the calculations used for the predicted data. C) Full-Wave Bridge Rectifier First, describe the circuit and what data you are going to measure. List the measured data and the predicted (theoretical) data in tabular form as follows: FW Bridge Rectifier Show the calculations used for the predicted data. D) Full-Wave Bridge Rectifier with Filter Capacitor Draw the schematic. List the measured data and the predicted in tabular form as follows: V r FW Bridge Rectifier with Filter Show the calculations used for the predicted data. E) Comparison of the Four Circuits Make a table comparing the measured values for the four rectifier circuits. The table might look like: HW Rectifier FW Rectifier No Filter Filter No Filter Filter 5

6 V r Conclusion Overall, what conclusions do you reach with respect to rectifier circuits. Consider circuit complexity and cost. Appendix 1: Diode Information Figure A.1 shows an outline drawing and the schematic symbol for a small signal diode. Note the band on the n-side end of the diode this end is the cathode. Note the band at one end Anode p-side Cathode n-side (a) Anode p-side (b) Cathode n-side Figure A.1 Outline and schematic symbol for diodes Appendix 2: How to Change the FG to Display True Peak-to-Peak Amplitude. The default setting for the Agilent 33120A function generator shows the peak-to-peak voltage across the OUTPUT connector provided that the FG output terminals (RED and BLACK terminals) are shunted (or terminated) by 50 ohms. In our lab we normally do not have that situation. In order to have the FG display the actual peak-to-peak voltage across the output terminals, do the following: Enter the following commands 2 on the front panel of the FG. Keys to Press What is in the Display Shift Menu A: MOD MENU On/Off D: SYS MENU 1: OUT TERM 50 OHM HIGH Z Enter ENTERED 2 Refer to the Agilent Function Generator manual, page 40. 6