University of Alberta Department of Electrical and Computer Engineering. EE 250 Laboratory Experiment #5 Diodes

Transcription

1 University of Alberta Department of Electrical and Computer Engineering EE 250 Laboratory Experiment #5 Diodes Objective: To introduce basic diode concepts. Introduction: The diode is the most fundamental circuit element with non-linear characteristics. The very fact that it is non-linear makes it a useful tool in many applications, such as AC to DC conversions and logic implementation. The purpose of this laboratory is: a) To gain an understanding of diode operation. b) To recognize circumstances where modifications must be made to the ideal diode assumption. A new circuit board is introduced in this lab: Theory: The DCB (Diode Circuit Board) The DCB is a small circuit board containing four 1N4005 diodes. The specifications (peak inverse voltage, forward resistance, etc) for these diodes can be read from the data sheet provided. A diode is a two terminal device, represented as follows: Figure 5.1: A Diode There are two different cases for diode operation: a) Forward biased (turned on): If a current is applied in the direction shown in Figure 5.1 to the diode, the diode acts like a short circuit and the voltage drop across it is zero (in the ideal scenario). b) Reverse biased (turned off): If a voltage is applied across a diode, negative to the reference voltage shown in Figure 5.1, the diode acts like an open circuit and does not conduct any current. EE 250 Experiment #5 -July 2006 Page 1

2 The response of the diode to forward and reverse biasing gives it its unusual i-v characteristic, illustrated in Figure 5.2: Figure 5.2: i-v Characteristic of a Diode In this lab, some of the applications that exploit this i-v characteristic will be investigated: Half-Wave Rectifier: A simple circuit with one diode in it, like in Figure 5.3, can be used to reject one half of an alternating voltage source, introducing an average value (a DC component) into the output voltage. Figure 5.3: A Half Wave Rectifier and its Output Voltage The output waveform shown is for an ideal diode. In reality, there is a drop of about 0.6V across a forward biased diode, causing the waveform representing Vout to be slightly smaller than Vin. The Bridge Rectifier: This circuit is more complicated than a simple half wave rectifier; it requires four diodes hooked up in bridge formation, as illustrated in Figure 5.4. The advantage that is has is that for both the positive and negative halves of the input voltage cycle, there is a path through two forward biased diodes and a resistor, to give an output voltage. Study of Figure 5.4 reveals that the current will always pass through the positive terminal of the resistor, so the output voltage will be as shown. EE 250 Experiment #5 -July 2006 Page 2

3 Figure 5.4: A Bridge Rectifier and its Output Voltage The output waveform is shown for an ideal diode. In reality, there will be two voltage drops, one for each of the forward biased diodes the current will pass through, so the output waveform will once again be smaller than the input. Non-Ideal Characteristics of Diodes: As mentioned briefly above, an actual diode will not operate exactly as predicted in theory. Although a forward biased diode was previously described to act as a short circuit, it will actually have a small but significant voltage drop across it, around 0.6V. The consequences of this are: a) A diode does not conduct as soon as there is a positive voltage across it. It will instead conduct when the forward voltage exceeds 0.6V. b) Rectifier circuits can never completely recover the energy of the input source. Some power will always be dissipated in the diode. How much the non-ideal characteristic of a diode affects the performance of a rectifier can be measured in two ways. The conduction angle (the fraction of one entire cycle of the input voltage that the diode conducts) can be calculated, and the average value of the output (DC component) can be calculated. Comparing these values for different circuits can help a designer decide which circuit is best for their purposes. Conduction Angle: A diode does not start conducting until the forward voltage across it exceeds its barrier voltage (the voltage drop across the diode when it is forward biased). If the barrier voltage is large (a large fraction of the peak value of the input voltage), the fraction of the input cycle that the diode conducts will be small. A greater conduction angle in a rectifier circuit is more efficient. See the Appendix for information on calculating the conduction angle. Average Output: The average output of a diode cycle gives insight into how well the rectifier circuit is converting AC to DC. See the Appendix for information on calculating the average voltage output. In the previous applications, the output voltage was taken across the resistor in the circuit. Using a diode as the source for the output voltage results in another useful output signal. Limiter (Clipping) Circuits: It is sometimes desirable to keep the output voltage of a circuit to within upper or lower bounds. Variations of this can be achieved using any of the circuits shown below: EE 250 Experiment #5 -July 2006 Page 3

4 Figure 5.5: Clipping Circuits While the value of Vin is less than the forward voltage required to turn the diode on, no current flows and the output voltage is the same as the input voltage. However, once the diode has been turned on, the voltage drop across it is constant and equal to the barrier voltage. Even if the input voltage increases, the output voltage (measured as illustrated) will not. The transfer characteristics for circuits A and B are given in Figure 5.6. The transfer characteristics for the other two circuits will be predicted in the pre-lab. Experimental Procedure: Figure 5.6: Transfer Characteristics for Circuits A and B Throughout this experiment, results should be recorded in the report section of this handout. EE 250 Experiment #5 -July 2006 Page 4

5 Part 1: Basic Diode Operation A simple diode circuit is built to show how a diode operates under forward and reverse bias conditions. Figure A Forward Biased Diode 1.) Set the circuit up as shown in Figure ) Use the DMM to measure the voltage source, current in the circuit, the voltage drop across the diode, and the voltage drop across the resistor. 1.2 A Reverse Biased Diode 1.) Turn the diode in the circuit of Figure 5.7 around. 2.) Use the DMM to measure the voltage source, current in the circuit, the voltage drop across the diode, and the voltage drop across the resistor. 1.3 Current Dependence of V DO 1.) Turn the diode back to the way it was in part ) Change the resistance R, according to the values listed on the data sheet. For each value of R, measure the voltage drop across the diode. Part 2: Rectifier Circuits AC signals will be altered in circuits with diodes, because the diode will only conduct in one direction. This section investigates the effect of half and full wave rectifiers on an AC signal. 2.1 A Half Wave Rectifier Figure ) Construct the circuit of Figure ) Hook up the oscilloscope so that V s and V out are displayed simultaneously. Make sure both signals are on DC Coupling, selected from the channel menu. EE 250 Experiment #5 -July 2006 Page 5

6 3.) Sketch the input and output signal on the grid provided on the data sheet. 4.) Measure the upper peak value of the input and output voltages, and use them to calculate the voltage drop across the diode. 5.) Measure the conduction angle by measuring the time the diode conducts and the period of the input signal. 2.2 A Bridge Rectifier Figure ) Set up the circuit of Figure 5.9. ** For a bridge rectifier, the input and output waveforms cannot be displayed at the same time, due to the internally connected grounds.** 2.) Display the input waveform on the oscilloscope, and use cursors to measure the lower and upper peak values of the input signal. Make sure the waveform is vertically centered on zero. 3.) Measure the period of the input signal. 4.) Display the output waveform on the oscilloscope. **Triggering the output waveform** a) Connect Channel 1 of the oscilloscope across the output resistor. b) Connect the negative lead of Channel 2 to the same point in the circuit as the negative lead of Channel 1. Connect the positive lead of Channel 2 so that Channel 2 measures the voltage across one diode. c) From the Source section of the TRIGGER MENU, select Channel 2. d) There should now be clear waveforms for both channels. Channel 2 does not have to be displayed any longer. 5.) Measure the two peak voltages of the output signal. 6.) From the observations made in and 2.2.3, sketch the input and output waveforms on the grid provided on the data sheet. 7.) Measure the conduction angle by measuring the times where the diode conducts and the period of the input signal. Part 3: Clipper Circuits The circuit shown below will be constructed, and a graph of V out vs. V in will be plotted to show how a clipper circuit limits the output voltage in a circuit. EE 250 Experiment #5 -July 2006 Page 6

7 Figure ) Set up the circuit of Figure 5.10, with V s set to 0.5V. Use the fixed 5V supply for the constant 5V in the circuit. 2.) For the values of V s given on the data sheet, measure the output voltage V out. Use the DMM to measure the input and the output voltage. EE 250 Experiment #5 -July 2006 Page 7

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9 Experiment #5 Diodes Pre-lab Assignment Read the lab and Appendix over and answer the following questions. All work must be handed in with the pre-lab hand in sheet, and your answers should be copied into the lab data sheet for use during the lab. 1.) Refer to Figure 5.7 in the lab. Assuming a forward drop of 0.6V across the diode, predict the current in the circuit and the voltage drop across the resistor. 2.) In part 1.2, the diode direction is reversed. What will the current in the circuit and the voltage drop across the resistor be now? 3.) Refer to Figure 5.8 in the lab. Assuming a forward drop of 0.6V across the diode, predict the peak value and conduction angle of the current. 4.) For the circuit of Figure 5.9, predict the peak value of the output voltage, assuming a forward drop of 0.6V across the diodes. 5.) Predict the outputs of the two remaining clipper circuits in Figure 5.5 of the lab. Do this by drawing graphs like those in Figure 5.6. (For circuit D, use KVL.) EE 250 Experiment #5 -July 2006 Page 9

10 Experiment #5 Diodes Post Lab Questions 1.) What causes the voltage drop across the diode to differ for each resistance in part 1.3? 2.) Calculate the voltage drop across the diode from the bridge rectifier circuit. Assume that the larger (magnitude) peak output value corresponds to the larger (magnitude) peak of the input source. (This means there will be two calculations for V DO.) 3.) Calculate the average value of the output given by the half wave rectifier. 4.) Calculate the average value of the output given by the full wave rectifier. 5.) What happens to the frequency of the output voltage of a full wave rectifier, as compared to the input frequency? 6.) If the input voltage were increased, would the efficiency of the two rectifier circuits increase or decrease? 7.) Plot V out vs. V in for the data collected in part 3. On the graph, indicate the region where V in is unchanged, and the clipped value of the output voltage. EE 250 Experiment #5 -July 2006 Page 10

11 EE 250 Laboratory Pre Lab Experiment #5 Diodes STATION NUMBER Lab Section: Lab Date: Group Members/ ID Numbers: Signatures: Pre-lab Questions: Question 1: I V R Question 2: I V R Question 3: I P (θ 2 θ 1 ) Question 4: V P EE 250 Experiment #5 -July 2006 Page 11

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13 EE 250 Laboratory Lab Report Experiment #5 Diodes STATION NUMBER Lab Section: Lab Date: Group Members/ ID Numbers: Signatures: Pre-lab Questions: Question 1: I V R Question 2: I V R Question 3: I P (θ 2 θ 1 ) Question 4: V P Measurements: Remember to include units with all measurements ) A forward biased diode V S V R V DO I 1.2.2) A reverse biased diode V S V R V DO I EE 250 Experiment #5 -July 2006 Page 13

14 1.3.2) Current dependence of V DO R(kΩ) V DO 2.1.3) Graph of the input and output signals of a half wave rectifier 2.1.4) The barrier voltage of a diode V P, in V P, out V DO 2.1.5) The conduction angle of a half wave rectifier Conduction time Period (θ 2 θ 1 ) 2.2.2) The positive and negative peaks of the voltage source V P (-) V P (+) 2.2.4) The peaks of the output voltage V P (lower) V P (upper) EE 250 Experiment #5 -July 2006 Page 14

15 2.2.5) Graph of the input and output waveforms of a bridge rectifier 2.2.6) Conduction angle of a bridge rectifier Conduction time Conduction time (lower peak) (higher peak) period (θ 2 θ 1 ) 3.2) V in (V) V out EE 250 Experiment #5 -July 2006 Page 15