Exercise 12 Semiconductors EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the operation of a diode. You will learn how to use a diode to rectify ac voltage to produce dc voltage. You will also be introduced to the light-emitting diode. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Introduction to semiconductors The diode Operating principles of a diode. Characteristic voltage-current curve of a diode. Diode types. Procedure to test a diode using a multimeter. Single-phase half-wave rectifier The light-emitting diode (LED) DISCUSSION Introduction to semiconductors Diodes, transistors, integrated circuits, and other so-called "solid state" devices are made from crystals of a semiconductor material, usually silicon or germanium. At room temperature, the crystals of pure silicon and germanium are neither good insulators nor good conductors. This is why they are called semiconductors. The diode A diode is a semiconductor device that acts like a conductor while directly biased and like an insulator while inversly biased. Figure 198 shows a typical low-power diode. The diode has two terminals, called the anode and the cathode. A ring mark on the diode case identifies the terminal corresponding to the cathode. The other terminal corresponds to the anode. Ring mark Anode Cathode Figure 198. The diode. Festo Didactic 89688-00 249
Exercise 12 Semiconductors Discussion Figure 199 shows the construction and schematic symbol for a diode. P layer N layer The arrowhead points toward the cathode, i.e., in the direction of conventional current. A Anode Cathode K A K Construction Symbol Figure 199. Construction and schematic symbol for a diode. As the figure shows, the diode consists of two layers of semiconducting material (semiconductors): A P-type semiconductor layer containing positive charge carriers (holes). The P-type layer corresponds to the anode (A) terminal of the diode. An N-type semiconductor layer containing negative charge carriers (electrons). The N-type layer corresponds to the cathode (K) terminal of the diode. Operating principles of a diode The diode is an essential component of rectifier circuits. When used in a rectifier, the diode operates as a high-speed switch which has no movable parts. When no voltage is present across the diode terminals, the diode is in the off (blocked) state. No current flows through the diode, and the diode acts like an open switch, as Figure 200 shows. No voltage Switch open A K Figure 200. When no voltage is present across the diode terminals, the diode acts as an open switch. Therefore, no current flows through the diode. 250 Festo Didactic 89688-00
Exercise 12 Semiconductors Discussion The + and signs next to voltage in the figure indicate the convention of measurement of this voltage. These signs indicate that the voltage at point A () in the figure is higher than the voltage at point K () when voltage is positive (e.g., when ). Conversely, the value of is negative when the voltage at point A () is lower than the voltage at point K () (e.g., when ). When a voltage is present across the diode terminals, and the voltage at the anode is lower than the voltage at the cathode, the diode acts as an open switch. Therefore, no current flows through the diode. In this condition, the diode is said to be reverse biased (see Figure 201). A Reverse voltage K Switch open Figure 201. When the voltage at the anode is lower than the voltage at the cathode (i.e., when voltage is negative), the diode acts as an open switch: no current flows through the diode. When a voltage is applied across the diode terminals and the voltage at the anode is higher than the voltage at the cathode, the diode passes from the off (blocked) state to the on (conducting) state. In this case, the diode is said to be forward biased: it acts as a closed switch, allowing the current to flow from the anode to the cathode (see Figure 202). Forward voltage A K Switch closed Figure 202. When the voltage at the anode is higher than the voltage at the cathode (i.e., when voltage is positive), the diode acts as a closed switch and the current flows through the diode in the direction indicated. As long as current flows through the diode, the diode remains forward biased and acts like a closed switch. When the current stops flowing through the diode (even for a very brief lapse of time), the diode becomes like an open switch and the voltage across the diode terminals drops to 0 V, as Figure 203 shows. 0 V Switch opens A K 0 A Figure 203. When the current stops flowing through the diode, the diode becomes like an open switch and the voltage across the diode terminals drops to 0 V. Festo Didactic 89688-00 251
Exercise 12 Semiconductors Discussion Characteristic voltage-current curve of a diode The characteristic curve of a diode represents the current flowing through the diode as a function of the voltage across its terminals. Figure 204 shows the characteristic curve of an ideal diode and that of a real diode. Ideal diode: when the diode is reverse biased, it acts like a perfect insulator. Therefore, no current flows through the diode. When the diode is forward biased, it acts like a perfect conductor. Therefore, current flows through the diode without a voltage drop across the diode. Real diode: when the diode is reverse biased, a small leakage current flows through it. When the diode is forward biased, the current flowing through it increases very rapidly as the voltage increases until the diode becomes fully conducting. Note that the diode conducts little when the forward voltage is below the minimum value, called the knee voltage. The knee voltage is the voltage drop across the diode (typically 0.7 V in the case of a silicon diode) when the current starts to increase very rapidly. Ideal diode Real diode 0.7 V Knee voltage across a silicon diode Figure 204. Characteristic voltage-current curves of an ideal diode and a real diode. Diodes have many applications. For example, diodes are used in alternators to change alternating current into direct current. Changing alternating current into direct current is called rectification. That is why diodes are also known as rectifiers. 252 Festo Didactic 89688-00
Exercise 12 Semiconductors Discussion Diode types There are many types of diodes. Some of the common types are small signal diodes, power rectifier diodes, zener diodes, light-emitting diodes, and photo diodes. Small signal diodes are used to change low alternating current into direct current and to absorb voltage spikes. They are a common component on printed circuit boards. Power rectifier diodes are used in alternators and other applications to rectify ac current into dc current. Zener diodes can operate in reverse bias within a specified range without being damaged. A zener diode will block reverse bias current up to a predetermined level. When that level is surpassed, a zener diode will allow current to flow in reverse direction. Light-emitting diodes, commonly known as LEDs, convert electrical current directly into light, or photons. LEDs can be manufactured to display different colors. They are commonly used for digital data displays. They are also used on heavy equipment for such things as clearance lights. LEDs are much more efficient than light bulbs, consuming a fraction of the electricity and lasting much longer. Photo diodes conduct current when subjected to light. The photosensitive material in photo diodes increases its resistance as light decreases, and it decreases its resistance as light increases. They are often used to turn outdoor lights on and off. Procedure to test a diode using a multimeter This procedure is a typical procedure; it may slightly differ from one manufacturer to another. Refer to the user guide of your multimeter for more information. Set the multimeter function to the diode function. Insert the black test lead banana plug into the negative COM jack and the red test lead banana plug into the positive jack. Use the MODE button to view the diode icon on the display. Touch the test probes to the diode under test. If one reading shows a value near 0 V and the other reading shows OL, the diode is good. Reverse voltage will indicate OL. Shorted devices will indicate near 0 V in both polarities and an open device will indicate OL in both polarities. The value indicated in the display is the forward voltage. Festo Didactic 89688-00 253
Exercise 12 Semiconductors Discussion Single-phase half-wave rectifier A single-phase half-wave rectifier converts an ac output to a pulsating dc output. The circuit simply consists of a diode connected between an ac source voltage and a load (resistor ) as shown in Figure 205a. Load (a) During the positive half of source voltage, the diode is forward biased. Source voltage Time Load current (rectifier output current) Time Load voltage (rectifier output voltage) Time (b) WavefoRMS of the circuit voltages and current Figure 205. Single-phase half-wave rectifier. 254 Festo Didactic 89688-00
Exercise 12 Semiconductors Discussion The diode operates as a high-speed switch, allowing the current to flow only during the positive half-wave of the source voltage. At instant, the source voltage is zero. Therefore, the voltage across the diode is zero and the diode acts as an open switch, preventing current from flowing through the circuit. The voltage across the load (the rectifier output voltage ) is null. During the positive half of the source voltage waveform (i.e., between instants and ), the diode is forward biased, allowing current to flow through the circuit. Therefore, the waveforms of the rectifier output current and voltage have the same shape as the source voltage waveform. The voltage drop across the diode is very low: it is equal to the knee voltage. At instant, the load current (diode current) becomes 0 and the diode stops conducting current (i.e., the diode turns off). During the negative half of the source voltage waveform (i.e., between instants and ), the diode is reverse biased, preventing current from flowing through the circuit. Therefore, the rectifier output current and voltage are null. Meanwhile, all the voltage applied by the source (negative half of the source voltage) is present across the diode. The maximum value of this voltage is called the peak reverse voltage (PRV), or sometimes the peak inverse voltage (PIV). It corresponds to the maximum voltage the diode must withstand when it is reverse biased. The load voltage (rectifier output voltage) is, therefore, a pulsating voltage that is positive during half of the source voltage cycle, and null during the other half of this cycle. The rectifier output voltage is unipolar because it keeps the same polarity (positive) during the whole cycle. This occurs because the current can flow in one direction only. Neglecting the voltage drop across the diode, the amplitude of the rectifier output voltage is equal to the amplitude of the source voltage. As shown in Figure 205, the average value of the dc voltage at the output of a single-phase half-wave rectifier is equal to, or. Since single-phase half-wave rectifiers provide power to the load during half of the ac power source cycle only, they lack the efficiency required by most applications. The variations in the pulsating dc output of a half-wave rectifier are referred to as dc output ripple. In order to reduce (smooth) this ripple, a capacitor can be added across the output of the rectifier. The ability of the capacitor to charge up quickly and discharge slowly improves the waveform of the voltage at the output of the single-phase half-wave rectifier and thus increases the dc voltage. See Figure 206. Festo Didactic 89688-00 255
Exercise 12 Semiconductors Discussion (a) Circuit showing the location of the filtering capacitor Capacitor charge Capacitor discharge Rectifier output voltage with filtering capacitor Rectifier output voltage without filtering capacitor Load voltage (rectifier output voltage) Time (b) WavefoRMS of the rectifier output voltage with and without filtering capacitor Figure 206. Using a filtering capacitor across the output of a half-wave rectifier reduces the dc output ripple. The light-emitting diode (LED) A light-emitting diode (LED), like the one shown in Table 24, is an electrical component with two terminals that conduct the electricity only in one direction like a standard diode but which emits light when current flows. LEDs require dc voltage to operate. Table 24. Typical light-emitting diode (LED) and associated symbol. Component Symbol The current flowing through the LED excites the electrons in the diode, releasing energy in the form of photons. This effect is called electroluminescence. The materials used in the construction of an LED determine the color and brightness of the light. Figure 207 shows the mains parts of a typical LED. 256 Festo Didactic 89688-00
Exercise 12 Semiconductors Discussion Epoxy case Wire bond Post Reflective cavity Semiconductor die Anvil Flat spot (cathode side) Anode (+) Cathode (-) Figure 207. Construction of a typical LED. To emit light, an LED must be forward biased and the current flowing through it must be limited to prevent damage. Connecting an LED without protection will destroy the LED almost instantly. Usually, the current is limited using a resistor connected in series with the diode. The current-limiter resistance is determined using Ohm's law and using the forward current and voltage given in the datasheet of the LED, and the supply voltage. See Equation (31). (31) where is the current-limiter resistance in ohms () is the supply voltage in volts (V) is the forward voltage in volts (V) is the forward current in amperes (A) LEDs have many advantages over incandescent light sources: they are smaller in size, they last longer, they consume less energy, they have improved physical robustness, and they can be mounted to a printed circuit board. LEDs are now used in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, and camera flashes. Festo Didactic 89688-00 257
Exercise 12 Semiconductors Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup Testing a diode using a multimeter Single-phase half-wave rectifier Operation without rectification. Operation with rectification. Operation with rectification and filtering. Light-emitting diode PROCEDURE High voltages are present in this laboratory exercise. Do not make or modify any banana jack connections with the power on unless otherwise specified. Setup In this section, you will install the training system modules in the workstation. 1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform this exercise. Install the equipment required in the workstation. Make sure that all fault switches are set to the O (off) position. Testing a diode using a multimeter In this section, you will use a multimeter to test the status of the diode on the Printed Circuit Board module. 2. Locate the diode on the PCB of the Printed Circuit Board module. Does the extremity of the diode near the test point N correspond to the anode or the cathode? 258 Festo Didactic 89688-00
Exercise 12 Semiconductors Procedure 3. Select the Diode function on the multimeter. On the multimeter, insert the black test lead banana plug into the negative COM jack and the red test lead banana plug into the positive jack. On the PCB of the Printed Circuit Board module, connect the black probe to the test point near the anode of the diode, and the red probe to the test point near the cathode of the diode. Measure the voltage and record the voltage. Voltage = V 4. Reverse the connections to measure the voltage. Record the voltage. Voltage V 5. Do the voltages measured in the previous steps correspond to a good diode or a damaged diode? Briefly explain. Festo Didactic 89688-00 259
Exercise 12 Semiconductors Procedure Single-phase half-wave rectifier In this section, you will observe that a diode can be used to rectify an ac voltage to produce a dc voltage. You will apply an ac voltage to a circuit having a resistive load, and observe that without rectification, there is no dc voltage at the output of the circuit. Then, you will add a diode (rectifier) in the same circuit, and observe that a dc voltage is present at the output of the circuit. You will also observe that adding a capacitor across the load of the circuit increases the dc voltage produced by the rectifier. Operation without rectification 6. Make sure that the main power switch on the Power Source module is set to the O (off) position, then connect it to an ac power outlet. Set up the circuit shown in Figure 208. Use resistors and in the lower section of the PCB to implement the load of the circuit. Lower section of the PCB Network voltage L N Figure 208. Circuit without rectification. 7. Turn the power source on. The ac source voltage ( applied to the load ( connected in parallel with ) corresponds to the output voltage of the control transformer. Measure the ac source voltage applied to the load through terminals S and M. Record the value below. AC source voltage ( applied to the load V 260 Festo Didactic 89688-00
Exercise 12 Semiconductors Procedure 8. Measure the dc voltage ( at the output of the circuit (across terminals U and P). DC voltage ( at the output of the circuit = V 9. Does the voltage you measured in the previous step confirm that, without rectification, there is no dc voltage at the output of the circuit? Yes No 10. Turn the power source off. Operation with rectification 11. Insert diode between the source voltage ( ) and the resistive load ( connected in parallel with ), as shown in Figure 209. Lower section of the PCB Network voltage L N Figure 209. Circuit with rectification (without filtering capacitors). 12. Calculate the average value of the dc voltage at the output of the singlephase half-wave rectifier using the ac source voltage measured in step 7 and the following equation: = (neglect the voltage drop across the diode). Record the value below. Average value of the dc voltage at the output of the single-phase halfwave rectifier, = V 13. Turn the power source on. Festo Didactic 89688-00 261
Exercise 12 Semiconductors Procedure 14. Measure the average value of the dc voltage at the output of the singlephase half-wave rectifier (in this circuit, the output voltage of the single-phase half-wave rectifier corresponds to the output voltage of the circuit as measured across terminals V and P). Average value of the dc voltage at the output of the single-phase halfwave rectifier = V 15. Does your result in the previous step confirm the presence of dc voltage at the output of the single-phase half-wave rectifier? Yes No 16. Is the average value of the dc voltage measured at the output of the singlephase half-wave rectifier approximately equal to the value calculated using the equation =? Yes No 17. Turn the power source off. Operation with rectification and filtering 18. Insert the filtering capacitor in parallel with the load as shown in Figure 210. Lower section of the PCB Network voltage L N Figure 210. Circuit with rectification and filter capacitor. 19. Turn the power source on. 262 Festo Didactic 89688-00
Exercise 12 Semiconductors Procedure 20. Measure the average value of the dc voltage at the output of the singlephase half-wave rectifier (across terminals V and P). Average value of the dc voltage at the output of the single-phase halfwave rectifier = V 21. Does the average value of the dc voltage measured in the previous step confirm that adding a filtering capacitor in parallel with the load improves the waveform of the voltage at the output of the single-phase half-wave rectifier and thus increases the dc voltage? Yes No 22. Turn the power source off. 23. Add a second filter capacitor ( ) in parallel with the load as shown in Figure 211. The addition of capacitor (in parallel with capacitor ) increases the capacitance of the filter (this is equivalent to using a larger capacitor ). Lower section of the PCB Network voltage L N Figure 211. Circuit with rectification and filtering capacitor. 24. Turn the power source on. 25. Measure the average value of the dc voltage at the output of the singlephase half-wave rectifier (across terminals V and P). Average value of the dc voltage at the output of the single-phase halfwave rectifier = V Festo Didactic 89688-00 263
Exercise 12 Semiconductors Procedure 26. What is the effect of increasing the capacitance of the filter on the average value of the dc voltage at the output of the single-phase half-wave rectifier? Briefly explain. 27. Turn the power source off. Light-emitting diode In this section, you will use the rectified and filtered voltage produced by the previous circuit to operate the light-emitting diode on the PCB. You will calculate the current-limiter resistance required by the light-emitting diode and then connect the circuit. 28. Calculate the current-limiter resistance required by the LED on the PCB by considering a forward current rated at 0.02 A (20 ma) and a forward voltage rated at 2 V. Use the average value of the dc voltage at the output of the single-phase half-wave rectifier measured in step 25 for your calculation. a To prevent damage to the LED on the printed circuit board due to a bad connection, the current-limiter resistor is permanently connected on the printed circuit board, and the resistor is numbered. Also for safety purposes, the value of the current-limiter resistor on the printed circuit board has been increased to 1.6 k. 29. Connect the branch containing the light-emitting diode to the current circuit as shown in Figure 212. In this branch, resistor is used as a currentlimiter resistor. 264 Festo Didactic 89688-00
Exercise 12 Semiconductors Procedure Lower section of the PCB Network voltage L N Figure 212. Circuit used to learn the operation of a light-emitting diode. 30. Set the multimeter to measure dc current. Note that the ammeter symbol in Figure 212 includes a polarity sign to indicate the instrument polarity. 31. Turn the power source on. 32. Is the light-emitting diode turned on? Yes No 33. Measure the forward current flowing through the light-emitting diode indicated by ammeter A. Forward current = A 34. Does the measured forward current, flowing through the light-emitting diode, confirm that the LED operates safely? Briefly explain why. Festo Didactic 89688-00 265
Exercise 12 Semiconductors Conclusion 35. Turn the power source off. Disconnect your circuit. Return the leads and the multimeter(s) to their storage location. CONCLUSION In this exercise, you were introduced to the operation of a diode. You learned that when a diode is forward biased, it acts as a closed switch and when it is reverse biased, it acts as an open switch. You learned how to use a rectifying diode. You also learned how to use an LED, and how to calculate the currentlimiter resistance. REVIEW QUESTIONS 1. How are the anode and cathode of a diode physically differentiated? 2. Is a diode forward biased or reverse biased when a voltage is present across its terminals, and the voltage at the anode is lower than the voltage at the cathode? 3. Does a diode act as an open switch or a closed switch when voltage is present across its terminals, and the voltage at the anode is higher than the voltage at the cathode? 4. How will the light emitted by an LED be affected if it is reverse biased instead of forward biased? 266 Festo Didactic 89688-00
Exercise 12 Semiconductors Review Questions 5. What can be done to reduce the ripple at the output of a half-wave rectifier? Festo Didactic 89688-00 267