Experiment 8: Diodes in Electronic Instrumentation - ENGR-4300, RPI, Lab Reports of Engineering

Download Experiment 8: Diodes in Electronic Instrumentation - ENGR-4300, RPI and more Lab Reports Engineering in PDF only on Docsity! Electronic Instrumentation ENGR-4300 Experiment 8 K.A. Connor, S. Bonner, P. Schoch -1- Revised: 4/13/2009 Rensselaer Polytechnic Institute Troy, New York, USA Experiment 8 Diodes Purpose: The objective of this experiment is to become familiar with the properties and uses of diodes. We will first consider the i-v characteristic curve of a standard diode that we can use in the classroom. We will also see how the diode can work as a rectifier, which is an essential part of most DC sources that are driven by AC voltages. A serious problem with simple rectifiers is that the DC voltage they produce is dependent on the load. A common way to make the rectifier less sensitive to the load is to add some regulation. This we can do by utilizing the avalanching effect that occurs if we reverse voltage the diode too much. We will also see what kind of voltage limitation can be achieved with a forward biased diode. Such limitation of voltages is usually applied to protect circuit components. Equipment Required: • Rensselaer IOBoard Rev D (with Mobile Studio Desktop) • Oscilloscope (Rensselaer IOBoard) • Function Generator (Rensselaer IOBoard) • Protoboard • Resistors, Capacitors, Diodes, Zener Diodes, 9V batteries, op-amp • OrCAD Capture and PSpice Helpful links for this experiment can be found on the links page for this course: http://hibp.ecse.rpi.edu/~connor/education/EILinks.html#Exp8 Part A – The I-V Characteristic Curve Background Diodes: An ideal diode is a device that allows current to flow in only one direction. The symbol of a diode, shown in Figure A-1, looks like an arrow that points in the direction of current flow. The current flows through the diode from the anode to the cathode. The cathode is marked on a real diode by a band. Figure A-1. A small positive voltage is required to turn a diode on. This voltage is used up turning the device on so the voltages at the two ends of the diode will differ. The voltage required to turn on a diode is typically around 0.6 - 0.8V for a standard silicon diode. I-V characteristic curve: In order to understand how a diode functions, it is useful to look at a plot of the voltage across the diode vs. the current through the diode. We call this type of curve and i-v characteristic curve. If we were to create an i-v curve of a resistor, where the current is directly proportional to the voltage (V=IR), we would see a straight line with a constant slope or R -1 . When we plot the characteristic curve of an ideal diode (that switches on when the voltage across it goes above zero), we see zero current when vD is negative and infinite current as soon as vD tries to go positive. This is shown in Figure A-2. Note that, when and ideal diode turns on, it is a short circuit and, therefore, the voltage across the ideal diode when it is on is always zero. ANODE D1 DIODE CATHODE Electronic Instrumentation ENGR-4300 Experiment 8 K.A. Connor, S. Bonner, P. Schoch -2- Revised: 4/13/2009 Rensselaer Polytechnic Institute Troy, New York, USA Figure A-2. I-V curve of a diode: Figure A-3 shows typical characteristics of a real diode. Figure A-3. Ideally, a diode is a device that allows current to flow in one direction only. In practice, diodes allow large amounts of forward current to flow when the positive voltage across them reaches a small threshold. They also have a small “saturation” current and a “breakdown” region in which a large amount of current will flow in the opposite direction when a large negative voltage is applied. In small signal diodes, the forward current will typically be up to a few tens of mA at a forward voltage of about 1V. The reverse-breakdown voltage might be about 100V, and the saturation current Is may be of the order of 1nA. Power diodes may allow forward currents up to many amps at forward voltage drops of 0.6 to 1.5V or so, depending on the type of diode. The reverse-breakdown voltage of power diodes may range from as low as 50V up to 1000V or even much more. The diode equation: The equation below gives a reasonably good representation of the i-v characteristics of a diode.       −= 1T D nV v SD eIi Is is the saturation current usually measured in microamps, or nanoamps. VT is the Thermal Voltage [K in your book] where VT = kT/q = 0.0259V at 300K and n is a somewhat arbitrary parameter which depends on construction and usually lies between 1 and 2. Note that this equation characterizes the basic features of the diode i-v curve, but leaves out some details like reverse breakdown, junction capacitance, etc. Electronic Instrumentation ENGR-4300 Experiment 8 K.A. Connor, S. Bonner, P. Schoch -5- Revised: 4/13/2009 Rensselaer Polytechnic Institute Troy, New York, USA • Wire the circuit shown in Figure A-7 on your protoboard. The IOBoard can’t create a large enough output voltage by itself, so you need to build a gain of 10 op-amp circuit. Use either the uA741 or the LF351 op-amp for this circuit. Don’t use the MAX473, it doesn’t run at 9V. Use two 9V batteries to power the op-amp. R2 1k D2 D1N4148 0 ADC2-ADC2+ ADC1- ADC1+ U1 uA741 + 3 - 2 V + 7 V - 4 OUT 6 OS1 1 OS2 5 R4 1k R5 10k Vf unction_generator V3 9Vbattery V4 9Vbattery 0 0 0 V5 FREQ = 1k VAMPL = .6 VOFF = 0 Figure A-7. o Diodes look like very small resistors with only one stripe. The stripe corresponds to the straight line of the diode symbol at the cathode. The D1N4148 diode should have a “48” written on it. o When you wire the circuit, make sure your diode is placed so that the cathode faces toward ground as shown in the figure above. o V5 isn’t real. Instead connect Func Gen 1 (or AWG1) Output and Ground to these points. o Use ADC2 to sense the current. Be careful to get this right. Remove the jumper between ADC2- and GND, if you have one. For RED2 you must select the DIF (differential) mode rather than SE (single ended) for the scope channels. Make sure that you are using ADC1+, ADC1-, ADC2+ and ADC2- of connector J10 of the RED1 IOBoard or A1+, A1-, A2+, and A2- on RED2 to connect to the labeled points on this circuit. See the figure of the IOBoard below. You should NOT be using the GND inputs. Figure A-8. IOBoards RED1 (left) and RED2 (right). • Set Function Generator 1(AWG1) to a 1kHz, triangle wave, 1.2Vp-p. • Observe the diode voltage on channel 1 of the Oscilloscope, (ADC1 or A1). • Observe the diode current on channel 2 of the Oscilloscope, (ADC2 or A2). Gain of 10 Op-Amp R2 is current sensing resistor D2 is diode to be measured Electronic Instrumentation ENGR-4300 Experiment 8 K.A. Connor, S. Bonner, P. Schoch -6- Revised: 4/13/2009 Rensselaer Polytechnic Institute Troy, New York, USA o Remember that channel 2 is the current with a scale factor or 1000A/V, (1000A/V = 1mA/V). • Set the oscilloscope up to display a couple of cycles of the signal, for example set the time base to 250us/div. o To smooth the data, first make sure that the trigger is working very well and the trace repeats itself.
Click on Display Options (located at the top of the scope). Click on Averaging – try 16. • Save the data to a file once you have a clean plot on the screen. • Start MATLAB and Import the data file. o In MATLAB, plot the diode current vs. the diode voltage. You only want to display the useful data, so for example there isn’t a need to plot negative voltages because the current is zero.
The figure below shows how you might want to do this. This is a guide. Figure A-9. • Add a plot of the diode current equation to the figure. o Start with n=2, guess a value for Is, plug in several values for vD, and plot the points. o Use 25.9mV for VT. o Vary the value of Is until you get a reasonable fit. • The figure below is the result of the MATLAB example on the output from the code on the previous page. You should be able to get a better fit. Consider using the “grid” command in MATLAB. • If time permits, vary the value of n also. For real diodes, n is in the range of 1 to 2. Electronic Instrumentation ENGR-4300 Experiment 8 K.A. Connor, S. Bonner, P. Schoch -7- Revised: 4/13/2009 Rensselaer Polytechnic Institute Troy, New York, USA 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 1 2 3 4 5 6 x 10 -3 Figure A-10. Diode current vs. diode voltage. Thin lines are plots of experimental data, thick lines are two attempts to fit the diode equation. One curve is for n = 2 & Is = 10 -10 , the other curve is for n = 2 & Is = 10 -9 A. • Include a copy of this plot in your write-up. List the final n and Is. • Keep the gain of 10 op-amp circuit on your protoboard. You will need it again. Summary A diode is a device that allows current to flow in only one direction. An ideal diode looks like a short when the current is flowing and an open circuit when the current is not flowing. A real diode has a small voltage across it when it is on, a small negative current through it when it is off, a region in which it switches from on to off, and a breakdown region where it stops blocking the current entirely. As long as the voltages we use do not reach the voltage of the breakdown region, the diode will work close to ideal. Part B – Diode Circuits: Rectifiers and Limiters Background Rectifiers: A rectifier is a circuit that converts an AC current to a DC current. At the very simplest level, a rectifier circuit is also a circuit that takes an AC voltage and modifies it so that the output voltage has only one sign. It will eliminate either all positive voltage or all negative voltage. A true DC current requires a constant DC voltage. The rectifiers we can build with only regular diodes give a first approximation to this. We will continue to refine the rectifiers presented here in other parts of this experiment. Half-wave rectifier: You will build a half-wave rectifier circuit. Build the circuit in Figure B-1. Be sure to use a 10kΩ resistor for this circuit. The op-amp circuit isn’t changed. ADC1or A1 senses the input signal to the half- wave rectifier circuit. ADC2 or A2 is connected to the output of the rectifier circuit. DC couple both oscilloscope channels. Electronic Instrumentation ENGR-4300 Experiment 8 K.A. Connor, S. Bonner, P. Schoch -10- Revised: 4/13/2009 Rensselaer Polytechnic Institute Troy, New York, USA o Modify your PSpice circuit by adding capacitor C1 in parallel with R1. o Perform a transient analysis as you did in the previous part. o Obtain a graph of the input and output voltages vs. time. o Include this plot in your report. (You will also be adding 5 experimental points to this plot based on the circuit you build in the next section.) • Modify the frequency of the input source and observe the output. o Modify the frequency of the source to 1kHz and run the simulation for 2ms. o Generate a plot of the Vin and Vout. o Note the time at which the output voltage reaches around 4V. o Keeping the time and step size the same, modify the frequency of the source to 10kHz and rerun the simulation. Run the simulation for 500us with a maximum step size of 1us. o Note the time at which the output reaches around 4V. o What is happening as the frequency increases? o Print a plot of the output from the second case only and include it in your report. Hardware Implementation of a Rectifier Now wire the rectifier on your protoboard and observe the output with and without the capacitor. Note: you may find it useful to use averaging of your traces. It is in the Display options menu. • Return to the circuit in Figure B-1, redrawn below 0 ADC1- ADC1+ ADC2- ADC2+D1 D1N4148 R1 10k Gain of 10 op-amp ckt Function_Generator 0 Figure B-7. Use the function generator to supply an input sinusoidal signal of 100Hz, 1.0Vp-p. This will supply a 10Vp-p signal to the rectifier circuit. • Observe both the input and the output voltages with the scope. o Be sure that your scope is set to DC coupling. o Take a picture with Mobile Studio or plot the data using either Excel or MATLAB (your choice). o Include this plot with your report. This is very similar to one of the previous plots except now the signal is a 100Hz sine wave. • Repeat the procedure above with a 4.7uF capacitor in parallel with the resistor R, as shown in Figure B-8. If your kit doesn’t include a 4.7uF capacitor, ask the instructor or TA. 0 ADC1- ADC1+ ADC2- ADC2+D2 D1N4148 R2 10k Gain of 10 op-amp ckt Function_Generator 0 C2 4.7uF Figure B-8. o Note that the sine voltage from the generator will become distorted when you add the large capacitor. o Record the input and output signals (ADC1 or A1 and ADC2 or A2) with Mobile Studio and include it in your report. Again you may choose to use a picture or plot the data using Excel or MATLAB. o Measure the output voltage at approximately 5 points during one cycle. • The data points can be selected from the Excel or MATLAB data files or the cursors can be used in Mobile Studio Electronic Instrumentation ENGR-4300 Experiment 8 K.A. Connor, S. Bonner, P. Schoch -11- Revised: 4/13/2009 Rensselaer Polytechnic Institute Troy, New York, USA
Get at good trace of Vout, (ADC2 or A2) on the scope.
Stop the scope.
Click the Zoom icon (a magnifying glass in Mobil Studio Desktop v2.2).
On the bottom plot, hold the shift key and click and hold the left mouse to select the region to expand.
Enable the Cursors (window – cursor, in cursor window click enable). Select the cursor in the upper window and move it into the expanded area. Use the cursor to read 5 voltages and times, including the minimum and maximum voltages, and 3 other points.
Right click in the bottom window to reset the zoom as necessary. o Add the experimentally measured points to the plot generated with PSpice. This can be done by hand. o Vary the frequency of the function generator and observe both the input and output voltage of the rectifier with the capacitor. Try connecting and disconnecting the capacitor as you vary the frequency. Try a larger capacitor, such as a 100uF capacitor. PSpice Simulation of Voltage Limitation Now we can use PSpice to build a limiter using a pair of diodes. • Draw the circuit shown in Figure B-9. 0 V1 R1 1k D1 D1N4148 D2 D1N4148 A B Figure B-9. o Use VSIN for the source (VAMPL = 5, FREQ = 100Hz, everything else = 0). o Perform a transient analysis in increments of 50us for 250ms. o Plot the input (point A) and output (point B) voltages. The input is the voltage source while the output is taken across the diode pair, as shown. o Print your plot and include it in your report. (You will also be adding 5 experimental points to this plot based on the circuit you build in the next section.) o Change the amplitude of the sinusoidal input, Vin, to 2V. Repeat the transient analysis. o Print this plot also and include it in your report. Hardware Implementation of a Limiter Now build the limiter on your protoboard. Note: you may find it useful to use averaging of your traces. It is in the Display options menu. 0 ADC1- ADC1+ ADC2- ADC2+ Gain of 10 op-amp ckt Function_Generator 0 R1 1k D1 D1N4148 D2 D1N4148 Figure B-10. • Wire the limiter circuit shown above o Use the function generator to supply a 100Hz, 1V, sinusoidal input. The gain of 10 op-amp will then provide a 10Vp-p signal. o Observe Vout (ADC2 or A2) and Vin (ADC1 or A1) with the oscilloscope. o Take a Mobile Studio picture of this circuit or save and plot the data in Excel or MATLAB. Include it in your report. Electronic Instrumentation ENGR-4300 Experiment 8 K.A. Connor, S. Bonner, P. Schoch -12- Revised: 4/13/2009 Rensselaer Polytechnic Institute Troy, New York, USA o Record at least 5 experimental voltages and add them to your PSpice output. The data point may be hand written on the PSpice plot. • What is the effect of altering the amplitude of your circuit? o Repeat the measurements with a 100Hz, 0.4V sinusoidal input. o Take a Mobile Studio picture of this circuit with 0.4V amplitude and include it in your report. • Alter the circuit to limit voltage in only one direction. o Return the amplitude of the function generator to 1.0V. o Remove one of the diodes from your limiter and observe the output. o What part of the signal is being limited? o Take a picture of this output or plot it and include it in your report. o Now reverse the polarity of the remaining diode in the circuit. (Turn it around). o What part of the signal is being limited now? Summary Diodes can be used as rectifiers, which convert an AC signal into a signal with an average non-zero DC voltage. The desired result of a rectifier is to create a DC signal from an AC signal. The signal from a simple rectifier circuit built with a diode can be improved by the addition of a smoothing capacitor. Diodes can also be used to limit voltage. This device is called a limiter. These can be used to limit voltage in one direction, or in both directions. Part C – LEDs, Photodiodes and Phototransistors Background LEDs: An LED is a device that emits light when it is subjected to a voltage. Just like a regular diode, an LED will not turn on (and emit light) until a certain threshold voltage is reached. This threshold depends upon the color of the LED and the diode manufacturer. Red LEDs turn on when the voltage across them exceeds about 2.2V. With green LEDs, the voltage is about 1.6V. Note that, although diodes often have a plastic coating that matches the color of the light emitted, the light that comes from a diode is not white. It is light of the wavelength of the desired color, i.e. a red diode (even with a clear plastic covering) will put out light in the red region of the electromagnetic spectrum. The following equation can be used to decide what resistance to use with an LED, given its threshold voltage and the desired current through the diode. 20mA is a reasonable value to use for the current through the diode. LED LEDin I VV R − = Photodiodes: A photodiode is a device that generates a current in the presence of light. As photons of light excite the PN junction inside the diode, a current is generated through the junction. The more light that shines on the photo-reactive surface, the more current flows through the device. Phototransistors: A phototransistor is similar to a photodiode except that it takes advantage of the ability of the transistor to amplify current in the active region. The current it generates is still proportional to the amount of incident light, but it is amplified by the properties of the transistor. The graph in Figure C-1 shows the linear relationship between incident light and current through a phototransistor similar to the one we will be using. Electronic Instrumentation ENGR-4300 Experiment 8 K.A. Connor, S. Bonner, P. Schoch -15- Revised: 4/13/2009 Rensselaer Polytechnic Institute Troy, New York, USA Ideal Zener Diode I -VZ V Figure D-2. Experiment I-V Characteristic Curve of a Zener Diode Now we will plot the voltage across a Zener diode vs. the current through the Zener diode. • Wire the circuit in Figure D-3 in PSpice. The Zener diode is D1N750. It is in the “EVAL” PSpice library. R1 1k B D1 D1N750 I 0 V V1 1v A Figure D-3. • Run a simulation and create the i-v characteristic curve. o Perform a DC sweep analysis of voltage source V1 from -20 to +20V in increments of 0.1V. o Select “Add Trace…” to plot current through diode D1, I(D1), or use the marker shown. o Change the x-axis of your plot as follows (same as with the regular diode): • In the “Plot” menu, choose “Axis Settings…”. • Then click on the X Axis tab. • Click on the “Axis Variable…” button at the bottom. • Enter the voltage at point B as the new X Axis Variable. (Example: V(R1:2). Note that the voltage at this point is the voltage across the diode.) • After you get the plot, remove all voltage markers, so that only I(D1) is displayed. o The plot produced will show the i-v characteristic curve for diode D1. It should look like the i-v curve for a Zener diode. o Print this plot and include it in your experiment. • Determine the knee current o Look up the typical Zener voltage in the spec sheet for the 1N750A diode. (You can find a link to this spec sheet on the links page.) o Draw a vertical line in the reverse bias region on your output plot corresponding to the negation of the rated Zener voltage. Note that this diode will keep its reverse bias voltage quite close to -VZ for a wide range of currents. The smallest current for which the bias voltage is about equal to -VZ is called the knee current. o What is the maximum current (or knee current) for which the reverse bias voltage is within 0.1V of -VZ? o Mark the knee current on the plot. Electronic Instrumentation ENGR-4300 Experiment 8 K.A. Connor, S. Bonner, P. Schoch -16- Revised: 4/13/2009 Rensselaer Polytechnic Institute Troy, New York, USA Hardware Implementation In the final part of this experiment, you will build a Zener diode voltage regulator on your protoboard. In this experiment, you will be measuring current. Current is always measured in series with a circuit. • Assemble the circuit shown in Figure D-4 on your protoboard. The Zener diode looks like the regular diode, except it should have “75” written on it. This is the same circuit as Figure A-7 with the 1N4148 replaced with the 1N750 (kit may have 1N751 instead). ADC2+ ADC2- 0 Function_Generator Gain of 10 op-amp ckt R1 1k D3 D1N750 0 ADC1+ ADC1- Figure D-4. • Measure the current vs. voltage characteristics of the 1N750 (or 1N751) Zener diode. o Set Function Generator 1 or AWG1 to a 1kHz, triangle wave, 1.2Vp-p. o Observe the diode voltage on channel 1 of the Oscilloscope, (ADC1 or A1). o Observe the diode current on channel 2 of the Oscilloscope, (ADC2 or A2).
Remember that channel 2 is the current with a scale factor or 1000A/V, (1000A/V = 1mA/V). o Set the oscilloscope up to display a couple of cycles of the signal, for example set the time base to 250us/div. o To smooth the data, first make sure that the trigger is working very well and the trace repeats itself.
Click on Display Options (located at the top of the scope). Click on Averaging – try 16. o Save the data to a file once you have a clean plot on the screen. o Start MATLAB and Import the data file. o See Figure A-9 for help in plotting the data in MATLAB. o Include the experimental plot of i vs. v in your report. Summary Zener diodes exploit the breakdown region of diodes to create a device that allows current to flow freely in one direction at a level above the Zener voltage. You can use a Zener diode to hold a voltage to the Zener voltage. An AC to DC converter of a desired voltage can be created by combining a Zener diode (rated at that voltage) with a rectifier and a smoothing capacitor. Report and Conclusions The following should be included in your written report. Everything should be clearly labeled and easy to find. Partial credit will be deducted for poor labeling and unclear presentation. Part A (16 points) Include the following plots: 1. I-V Characteristic curve of a diode, the PSpice Plot with 5 points marked. (3 pt) 2. MATLAB plot of diode current vs. voltage from data taken using Mobile Studio software. This should include the data points and a line found using the diode characteristic equation. (5 pt) Answer the following questions: 1. Use the data you took for the i-v characteristic curve of the 1N4148 diode to determine the mathematical representation of the i-v curve. What values did you find for IS and n (curve on the MATLAB plot)? (4 pt) 2. Why do you know that the current through the diode is Vout/R2? (2 pt) Electronic Instrumentation ENGR-4300 Experiment 8 K.A. Connor, S. Bonner, P. Schoch -17- Revised: 4/13/2009 Rensselaer Polytechnic Institute Troy, New York, USA 3. What differences, if any, did you notice between the i-v characteristic curve given by PSpice and the one you measured experimentally? (2 pt) Part B (32 points) Include the following plots: 1. Picture or plot of the Mobile Studio data for the circuit in Figure B-1, the half-wave rectifier. This figure has a triangle wave input signal. (2 pt) 2. PSpice plot of the half-wave rectifier circuit, Figure B-5. (2 pt) 3. PSpice plot of the rectifier with smoothing with 5 experimentally obtained points marked. (4 pt) 4. PSpice plot of the rectifier (output only) at 10kHz. (2 pt) 5. Mobile Studio plot of the rectifier (with no smoothing capacitor, sinusoidal waveform). (2 pt) 6. Mobile Studio plot of the rectifier (with a smoothing capacitor). (2 pt) 7. PSpice plot of the voltage limiter at 10V (with 5 experimental points marked). (2 pt) 8. PSpice plot of the voltage limiter at 4Vp-p (2V amplitude). (2 pt) 9. Mobile Studio plot of the limiter circuit with 10Vp-p input, (it is 10Vp-p after the gain of 10 op-amp circuit). (2 pt) 10. Mobile Studio plot of the limiter circuit with 4Vp-p input amplitude. (1 pt) 11. Mobile Studio plot of the limiter with only one diode. (1 pt) Answer the following questions: 1. Explain why Vout changes when you add the capacitor to the rectifier in parallel with R. Explain why this circuit would be better for use as a DC source than the circuit without the capacitor. (3 pt) 2. Did the circuit with the capacitor work better (more like a DC source) at high or low frequencies? (1 pt) 3. Comment on the similarities and differences between the PSpice and experimental results for the rectifier. (2 pt) 4. At what values (positive and negative) does the limiter cut off the voltage of the 10Vp-p input signal? (2 pt) 5. Why is this circuit called a limiter? (2 pt) Part C (10 points) Include the following plot: 1. Mobile Studio plot of the input and output for the optical link. (5 pt) Answer following questions/comments: 1. What are the minimum and maximum voltages of the output waveform under the best signal conditions? (2 pt) 2. Describe what happens to the output waveform at the higher frequencies. (1 pt) 3. Comment on two of the following: a) the effect of misalignment of the LED and transistor, b) the effect of the room lights, c) signal level through paper, skin, or other objects. (2 pt) Part D (14 points) Include the following plots: 1. PSpice Zener diode characteristic curve with vertical line and knee current marked. (5 pt) 2. Plot of Zener diode i vs. v from the experiment. (5 pt) Answer the following questions: 1. What are the Zener voltage and knee current for the diode you simulated? (2 pt) 2. Shown below is the i-v characteristic curve of the 1N4148 non-Zener diode we looked at in part A, but obtained over a much wider voltage range. Compare this plot with the one you obtained for the Zener diode. (2 pt)