Reading Assignment Horowitz, Hill Chap. 1.25 1.31 (p35-44) Data sheets 1N4007 & 1N4735A diodes Laboratory Goals PHY 351/651 LABORATORY 5 The Diode Basic Properties and Circuits In today s lab activities, you will utilize LabVIEW to explore the basic properties of diodes and to interface with some simple diode-based circuits. As you should have learned from the assigned reading, the diode is an essential component in modern electronics: it is widely used in circuitry for things like signal conditioning, circuit protection and regulation, and signal generation. As well, because the physical properties of diodes are well understood and characterized, they are commonly used as sensitive temperature transducers. (For example, both low temperature labs in the SU Physics department employ silicon diodes for thermometry in cryogenic equipment over a wide temperature range, from ~300 K down to 4 K.) Moreover, from a pedagogical perspective, the diode is very important because it serves as a friendly introduction to the operation of non-ohmic devices, from which one can then move on to learn about transistors, operational amplifiers, and integrated circuits. The plan for the day is as follows. In the first part of the lab (Activities 1, 2 & 3), you will build two LabVIEW VIs, together which will enable you to measure, store and analyze the IV (current-voltage) characteristics of two common types of diodes: a rectifier diode and a Zener diode. In the second half of the lab (Activities 4 and 5), you will first build a new VI that essentially operates as a two channel digital oscilloscope. Then you will use this VI to observe and characterize the properties of several basic, but important, diode circuits, including the halfwave rectifier and the diode clipper. Today s learning objectives are as follows: o To gain additional experience in the use of LabVIEW to configure and operate your DAQ for analog input operations. This will include learning how to trigger your AI operation off of an externally applied TTL signal. o To learn how to use LabVIEW to analyze data with a least-squares fitting function. o To become familiar with the properties and operation of two common types of diodes. o To gain additional experience constructing circuits on the 503 proto-typing board. Equipment o PB-503 proto-typing board o BK Precision Function Generator or DG1022 Arbitrary Generator o 1N4007 Silicon Rectifier Diode Page 1
o 1N4735A Silicon Zener Diode o Various resistors and capacitors o BNC-2120, NI USB-6003 o Hand-held digital multi-meter, banana cables, coaxial cables, and BNC-to-minigrabber adapters Activity 1 Measuring the Current-Voltage (IV) Characteristics of the 1N4007 Diode As you learned from rout reading assignment, the diode is a non-ohmic, nonlinear electronic device (Fig. 1). What this means is that the current through a diode and the voltage across a diode are not simply proportional to one another; in fact, the current-voltage (or IV) relationship for a diode is exponential and largely unidirectional (Fig. 1b). Under many circumstances, it can be modeled with the following equation: I " = I $ e & '/)& * 1 (1) Equation 1 is known as the Shockley diode equation (named after William Shockley, one of the inventors of the diode). In Eq. 1, I " is the current through the diode; V " is the potential difference across the diode (defined, as shown in Fig. 1a, as the potential at the anode side minus the potential at the cathode side); η is the emission coefficient, which varies from material to material, and characterizes the nature of the charge transport properties in the diode; V / = k 1 T/e, where k 1 is Boltzmann s constant, T is the diode temperature, and e is the magnitude of the electron charge; and finally I $ is known as the reverse saturation current (the magnitude of the current when the diode is reversed biased (i.e. V " < 0). It is the goal of Activity 1 to verify this relationship for the 1N4007 diode that you have at your station. Page 2
Figure 1: Adapted from Diefenderfer Figure 5.8. (a) Diode symbol and layout; V D defined from anode to cathode. (b) Typical diode IV curves. Notice the difference in scale for the positive and negative portions of the I D axis. (c) Schematic of the 1N41007 silicon diode. The location of the cathode is indicated by an annular band. The simplest way to measure the IV characteristics of a diode like the 1N4007 is shown in Fig 2a. Such as circuit would involve using LabVIEW, via GPIB (general purpose interface bus), to both control a variable voltage source to supply V " and control an ammeter (placed in series with the diode) to measure the current. However, as you know, the computers in this lab are not equipped with a GPIB card, thus we have to use the slightly more complicated (but also more instructional) approach shown in Fig. 2b. To implement the circuit shown in Fig. 2b, you will need to follow these directions: 1. For the tunable voltage source, you have two options: (1) Use the PB-503 positive and negative power supplies; (2) Use LabVIEW analog output functions. 2. Wire up your circuit on the PB-503 breadboard. Be sure to use the resistors that are specified in the circuit diagram; these were chosen specifically to not overload the output of the 503 s power supplies or the USB_6003 (you should look at the specs for these to calculate what the power output is for the configuration in Fig. 2b to convince yourself). 3. Build a LabVIEW program that measures both the voltage across the diode (V " ) and the voltage across the resistor R 6 (from this, you can calculate I " ); and then plot this data in an xy graph. Note: you may want to incorporate the following features in the LabVIEW program: Use a sequence structure to implement first a step that samples the AI channels and saves the data to file; and then a step that retrieves the data from the file and graphs it. Depending on your program, you may also want Page 3
an initial step that fixes the output voltage on one of the AO channels. Use a DAQmx read function to take single readings of the two AI channels that you designate for this measurement. Place this in a for loop to make measurements I " at different diode voltages V ". Use the Write to Spreadsheet function to write the data to spreadsheet file. You do not need to follow this advice if you can think of another way to implement the IV. However, your program must contain the following components: A graph of the IV data on the front panel displaying all of the data points that you took in the IV measurement. An array indicator on the front panel displaying the same data. A control for choosing the name of the file that you save the data to. A control for choosing the AI channels for the measurement. Additional Notes for Activity 1 1. Be sure to measure (and save) the IV curve for both forward and reverse polarity. 2. To identify the terminals of your diode, look for the annular cathode band (Fig. 1c). As the name suggests, this indicates the location of the diode s cathode. 3. Your resistors are rated for 500 mw, meaning they will start smoking if that limit is exceeded. Thus be sure that each resistor in your circuit dissipates less than 500 mw for each measurement you do (you should calculate this before you start turning the power supply knobs to know what your upper limit is; you should also measure the voltage drop across each resistor for each power supply setting to confirm). Page 4
Figure 2: Circuit schematics for measuring IV characteristics of a diode. (a) The simplest way, using a PC to control a programmable power supply to output the diode voltage V D ; the PC then controls the digital ammeter to read I D. (b) The circuit you will use to measure the IV characteristics of 1N4007. V S is the hand-tunable PB-503 power supply. R 1 = 4700 Ω, R 2 = 470 Ω, R 3 = 100 Ω. For Your Lab Report 1. Include an image of the block diagram of your VI. Also include a front panel image for forward bias IV measurements and a front panel image for reverse bias IV measurements. 2. Calculate the maximum power supplied by your source V : during IV measurements. Also calculate the maximum power dissipated in each of the resistors in the measurement circuit. 3. Comment on whether the knee voltage of the diode in the forward bias measurements is consistent with what you have read for 1N4007 diode. 4. Discuss whether your measurements of the reverse bias diode current are consistent with what you expect for the measurement circuit you are using and typical values for the reverse saturation current I $ (you will need to look around to find typical values of I $ ). Page 5
Activity 2 Fitting Your IV Measurements to the Shockley Diode Equation Using a LabVIEW Least-Squares Fitting Function In Activity 2, you will take the forward biased measurements of the 1N4007 diode that you made (and saved) in Activity 1 and fit them to the Shockley diode equation (Eq. 1). To do this you will take advantage of a built-in LabVIEW least-squares fitting Curve Fitting express VI. If you are not familiar with least-squares fitting techniques, it s not a problem (I won t be holding you responsible for understanding the mathematics of it - but if you re interested please see Essick Chpt. 9). What you should know is that Curve Fitting Express takes a function that you specify (i.e. Eq. 1, I " (V " )), along with your best guess of the unknown parameters of that function (i.e. I $ and a = 1/ηV / here), and then outputs an estimate for the parameters using a least-squares fitting routine. In order to use Curve Fitting Express you will need to know how to (1) read (import) a spreadsheet file into LabVIEW; (2) Properly index the array that was imported; and (3) convert the indexed data into a format suitable for Curve Fitting VI. I will not give you explicit instructions on how to build this VI, but in the end, your front panel should look something like the front panel VI shown in Figure 3. As you can see, this includes an x-y graph of the data and the fit; there is an array indicator displaying the best fit parameters; and there is a control to choose which spreadsheet file the VI should read the data from. For Your Lab Report 1. Include an image of the front panel and block diagram for your fitting VI. 2. Discuss how your best fit parameters for I $ and 1/ηV / compare to typical values for the 1N4007 diode (you should find in the literature that η 2 and I $ ~ 1 5 na). Figure 3: Image of the front panel for the VI used to fit IV data for 1N4007 to Eq. 1 using LabVIEW s nonlinear least-squares fitting function. Note, with the Curve Fitting Express VI, you are not able to input the Initial Guess Coefficients this VI was made with a lower-level function. Page 6
Activity 3 Measuring the Characteristics of the 1N4735A Zener Diode Zener diodes are specially engineered to operate in the avalanche region of operation. Avalanche or reverse breakdown operation occurs when a reverse bias voltage applied to the diode is sufficient to strip bound valence electrons from their host nuclei; the freed electrons are accelerated and collide with other atoms, ejecting more bound electrons, which results in a cascade of current. The characteristic voltage at which this occurs is known as the reverse breakdown voltage. For diodes like the 1N4007, which you just measured, exceeding the reverse breakdown voltage could likely cause damage to the diode. However, Zener diodes are designed specifically to operate in this regime. And, as seen in Figure 4, when the reverse breakdown voltage (also known as the Zener voltage for Zener diodes) is exceeded, the reverse current through the diode is nearly independent of the applied voltage. In this activity, you will use the diode measurement VI from Activity 1 to observe the IV characteristics of the 1N4735A silicon Zener diode. You should perform measurements of both the forward biased mode of operation and the reverse biased mode. You can use the same measurement circuit (Fig. 2b) that you used in Activity 1. However, note that the 1N4735A has a Zener voltage of V Q = 6.2 V. Thus, when measuring the reversed biased mode, in order to observe the avalanche region, you will need to remove the voltage divider in order to supply a largest enough reverse bias. (a) (b) R = 100Ω V " 1N4735A R ( = 10kΩ AI 1 Figure 4: (a) Taken from Diefenderfer, Chpt. 5, Fig. 5.18. An illustration of the avalanche effect in a Zener diode. (b) Simple circuit to illustrate voltage regulation using a zener diode. Page 7
Finally, to get an idea of the usefulness of Zener diodes, construct the circuit shown in Figure 4b. Use your VI from Activity 1 to measure the voltage across resistor R T as a function of the voltage V :. (Be sure to record V : using one of the AI input channels in your VI.) What you should see as you increase V : is that above V : ~ 6V, the voltage across R T remains constant (Be careful not to increase V : above 10 V, or you will risk overloading the power supply). This is a very simple illustration of using a Zener diode as a voltage regulator. While it wouldn t be practical to use this specific circuit for an application that required tight regulation of a circuit voltage (for instance, the voltage is completely unregulated below V Q ), my research group uses a similar circuit to limit the maximum voltage that can be applied to a very sensitive amplifier in order to protect the amplifier from damage due to voltage spikes. For Your Lab Report 1. Include an image (or graph) of your data for the forward bias characteristics of the 1N4735A Zener diode. Also include an image (or graph) for the reverse bias characteristics illustrating the avalanche mode of operation. 2. Include an image or graph of your data for the voltage regulation circuit in Figure 4b. Optional: Activity 4 Building a Digital Oscilloscope VI with the Capability for Triggering off of an External TTL Signal In Activity 5, you will investigate the operation of several basic diode-based circuits. In order to do this, you will first need to construct a VI that enables you to continuously measure and display time-varying voltages on your analog input channels; essentially you need to construct a digital oscilloscope VI. Just like a physical digital oscilloscope, your virtual digital oscilloscope will need to have the capability to trigger off of an external signal in order to properly align each voltageversus-time trace from one reading of the buffer to the next. To implement this triggering capability, you will need to use the same DAQmx VIs that you used in Lab 4: Create Virtual Channel, Timing, Read, and Clear. However, unlike Lab 4, you will not create a circular buffer, but instead should place AI Read inside a while loop and program it to acquire a finite number of scans for each iteration of the while loop. In addition, you will need to configure the triggering options for the data acquisition by using the DAQmx Trigger function; set it so that that the DAQ starts acquiring data on either the rising edge or falling edge of a TTL signal that you supply to digital pin PFI0 or PFI1 (see the USB-6003 specs for these pins locations). See Essick Chapter 11 for the correct placement of the Trigger VI in your block diagram. Note: You will use the sync output of your function generator to provide the signal to PFI0 when it comes time to test out your oscilloscope. In the end, your oscilloscope VI should look something like the VI in Fig. 5. In order to carry out Activity 5, it should definitely have the following: 1. Waveform chart or graph that updates continuously and displays two AI channels. 2. Controls to select the triggering channel and to select the trigger edge. 3. Controls for the analog channel scan rate and number of samples. 4. A Boolean control for stopping the acquisition process. Page 8
Figure 5: Front panel VI of a virtual digital oscilloscope. In the displayed state, the VI runs continuously; it digitizes signals on two analog input channels at a rate of 1000 Scans/s, stores the samples in a buffer, and then reads them all at once from the buffer into LabVIEW. Once the data is read from the buffer, the acquisition starts all over again after the digital I/O channel PFI0 sees a rising edge of an externally supplied TTL pulse. The signals shown in the waveform graph demonstrate the operation of a full-wave rectifier circuit (see Fig. 6c). The white trace is the 40 Hz AC signal input to the rectifier; the red trace is the rectified (unidirectional) output. Note in older versions of LabVIEW PFI0 is denoted digital A. Activity 5 Building and Characterizing Some Important Diode-Based Circuits In this activity, you will build and characterize the three circuits shown in Figure 6. These circuits are: the half-wave rectifier (Fig. 6a); the diode clipper (Fig. 6b); and the full-wave rectifier (Fig. 6c). In order to make these measurements, please follow these guidelines: 1. Use your function generator to provide the input signal v VW to your circuit. As well, use the sync output to provide the trigger signal. 2. Measure v VW using one of the AI input channels you have designated in your oscilloscope VI. And measure v XYZ using the second AI channel. Page 9
3. For the diode clipper circuit, use the PB-503 s positive and negative power supplies to provide the circuit biases V T and V [. 4. When you are ready to build the full-wave rectifier, contact the instructor; he will provide you with the additional diodes that you need to put it together (unfortunately, there may not be enough to go around). 5. ***Important: The capacitors that you will use are electrolytic capacitors. Such capacitors have a polarity (the negative terminal is marked with a minus sign). In order to insure that they do not explode, you must connect them so that the negative terminal is connected to a point in the circuit at lower potential than the point to which the positive terminal is connected. **** For Your Lab Report 1. Include images of the front panel of your oscilloscope VI for each one of the circuits. In those images, the waveform graph should display both v VW and v XYZ. For the case of the half-wave and full-wave rectifiers, please provide an image for when the capacitor C is present and an image for the capacitor is removed. (For example, Figure 5 shows the VI waveform graph for the full-wave rectifier when the capacitor is not present; and Figure 7 shows the waveform graph for the full-wave rectifier when the capacitor is present. You should do something similar.) 2. For the half-wave and full-wave rectifier circuits, measure the ripple and compare to what you estimate given your circuit parameters. Page 10
Figure 6: Diode circuits for Activity 5. (a) Half-wave rectifier. (b) Diode clipper. (c) Full-wave rectifier. Use the following parameters: R L = 1 10 kω; C = 100 200 μf. For R, v in, V 1, and V 2, you should determine what values to use based upon what you learned in your reading assignment. For the signal frequency, try something in the range of 10 100 Hz. Page 11
Figure 7: Oscilloscope VI front panel showing the response of the full-wave rectifier in Fig. 6c when a 100 μf capacitor is shunting the output. The red trace is the rectified signal v out. The white trace is the input signal v in. Page 12