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Identify the types of diodes. Zener DiodeA Zener Diode is a special type of diode that is considered to function in the reverse breakdown region. An ordinary diode operated in this region will usually be ruined due to excessive current. This is not the circumstance for the Zener Diode.A Zener Diode is heavily doped to diminish the reverse breakdown voltage. This origin a very shrill depletion layer. As a result, a Zener Diode has a forceful reverse breakdown voltage Vz. This is clear from the reverse characteristic of Zener diode as shown in Figure 1. Figure 1. Zener Breakdown RegionFigure 2 shows the schematic symbol of Zener Diode. Figure 2. Schematic Symbol of Zener DiodeNote that the reverse characteristic falls in an almost vertical manner at reverse voltage Vz. As the curve exposes, two things happen when Vz is reached: The diode current increases rapidly. The reverse voltage Vz across the diode remains almost constant.In other words, the Zener Diode functioned in this region will have a relatively constant voltage across it, unrelatedly of the value of current through the device. This permits the Zener Diode to be used as a voltage regulator. Light – Emitting Diode (LED)A light – emitting diode (LED) is a diode that provides off visible light when forward biased. LED are not finished from Silicon and Germanium but are done by using substances like gallium, phosphorous and arsenic. By changing the quantities of these fundamentals, it is likely to yield light of different wavelengths with colors that include red, green, yellow and blue.  Figure 3. Light – Emitting DiodeWhen LED is forward biased, as shown in Figure 3, the electrons from the n – type material cross the pn junction and recombine with holes in the p – type material.  Figure 4. LED schematic symbolFigure 4 shows the schematic symbol of LED. LEDs have the following advantages: Low Voltage Longer life (more than 20 years) Fast on-off switching Photo – DiodeA photo – diode, as shown in Figure  5, is a reverse – biased silicon or germanium pn function in which reverse current increases when the junction is exposed to light. The reverse current in a photo – diode is directly proportional to the intensity of light dropping on its pn junction. This means that greater the intensity of light falling on the pn junction of photo – diode, the larger will be the reverse current. Figure 5. Actual Photo – DiodeIn Figure 6, the schematic symbol of photo – diode is shown. When a rectifier diode is reverse biased, it has a very insignificant reverse leakage current. The same is right for a photo – diode. The reverse current is produced by thermally generated electron – hole pairs which are cleared across the junction by the electric field formed by the reverse voltage. In a rectifier diode, the reverse current rises with temperature due to an growth in the number of electron – hole pairs. A photo – diode differs from a rectifier diode in that when its pn junction is exposed to light, the reverse current increases with the increase in light intensity and vice – versa. Figure 6. Schematic Symbol of Photo – Diode Tunnel DiodeA tunnel diode is a pn junction that exhibits negative resistance, as shown in Figure 7, between two values of forward voltage. Figure 7. Negative Resistance RegionFigure 8 shows the schematic diagram of Tunnel Diode.  Figure 8. Schematic Diagram of Tunnel DiodeThe tunnel diode is basically a pn junction with heavy doping of p – type and n – type semiconductor materials. In fact, a tunnel diode is doped approximately 1000 times as heavily as a conventional diode. This heavy doping results in a large number of majority carriers. Because of the large number of carriers, most are not used during the initial recombination that produces the depletion layer. As a result, the depletion layer is very narrow. In comparison with conventional diode, the depletion layer of a tunnel diode is 100 times narrower. The operation of a tunnel diode depends upon the tunneling effect and hence the name. Varactor DiodeA junction diode which performances as a variable capacitor  under varying reverse bias is known as a Varactor Diode.When a pn junction is formed, depletion layer is created in the junction area. Since there are no charge carriers within the depletion zone, the zone acts as an insulator. The p – type material with holes (considered as positive) as majority carriers and n – type material with electrons (-ve charge) as majority carriers act as charged plates. Thus the diode may be considered as a capacitor with n – region and p – region forming oppositely charged plates and with depletion zone between them acting as a dielectric. This was demonstrated in Figure 9. Figure 9. Varactor DiodeFor normal operation, a varactor diode is always reversed biased. When the reverse voltage through a varactor diode is enlarged, the thickness of the depletion layer increases. Therefore, the total junction capacitance of the junction decreases. On the other hand, if the reverse voltage across the diode is lowered, the measurement of the depletion layer decreased. Consequently, the total junction capacitance increases. Figure 10 shows the schematic diagram of the Varactor Diode. Figure 10. Schematic Diagram of Varactor Diode Shockley DiodeNamed after its inventor, a Shockley diode is a PNPN device having two terminals as shown in Figure 11. Figure 11. Different Symbols of Shockley DiodeWhen Shockley diode is forward – biased, diodes D3 would be forward – biased while the diode D2 would be reversed – biased. Since diode D2 offers very high resistance (being reverse biased) and the three diodes are in series, the Shockley diode presents a very high resistance. As the forward voltage increases, the reverse bias across D2 occurs. Since this breakdown results in reduced resistance, the Shockley diode presents a very low resistance. From now onwards, the Shockley diode behaves as a conventional forward – biased diode; the forward current being determined by the applied voltage and external load resistance. When Shockley diode is reverse biased, diodes D1 and D3 would be reversed – biased while diode D2 would be forward – biased. If reverse voltage is increased sufficiently, the reverse voltage breakdown of Shockley diode is reached. At this point, diodes D1 and D3 would go into reverse – voltage breakdown, the reverse current flowing through them would rise rapidly and the heat produced by this current flow could ruin the entire device. For this reason, Shockley diode should never be operated with a reverse voltage sufficient to reach the reverse – voltage breakdown point. This behavior of Shockley diode is indicated on its V – I characteristics in Figure 12. Figure 12. V – I Characteristics of Shockley DiodeAs a summary,   Discuss, illustrate and derive the related equations of various rectifier circuits. When such DC supply is required, the mains AC supply is rectified by using diodes. The following two rectifier circuits can be used: Half – wave rectifierIn half – wave rectification, the rectifier provide current only during the positive half – cycle of input AC supply. The negative half – cycle of AC supply are suppressed. Therefore, current always moves in one direction through the load though after every half – cycle. Figure 13. Rectified Signals by Half – wave rectifierFigure 13 shows the circuit where a single crystal diode acts as a half – wave rectifier. The AC supply to be rectified is applied in series with the diode and load resistance. Generally, AC supply is given through a transformer. The use of transformer permits two advantages. Firstly, it allows us to step up or step down the AC input voltage as the situation demands. Secondly, the transformer isolates the rectifier circuit from power line and thus reduces the risk of electric shock.The AC voltage across the secondary winding AB changes polarities after every half cycle. During the positive half – cycle of input AC voltage, end A becomes positive w,r,t, end B. This makes the diode forward biased and hence it conducts current. During the negative half – cycle, end A is negative w,r,t, end B. Under this condition, the diode is reverse biased and it conducts no current. Therefore, current flows through the diode during positive  half – cycles of input AC voltage only; it is blocked during the negative half – cycles. In this way, current flows through load always in the same direction. Hence, DC output is obtained across the load. It may be noted that output across the load is pulsating DC. These pulsations in the output are further smoothened with the help of the filter circuits.The main disadvantages of a half – wave rectifier are: The pulsating current in the load comprises alternating component whose basic frequency is equal to the supply frequency. Therefore, an intricate filtering is required to produce steady direct current. The AC supply carries power only half the time. Therefore, the output is little.The output frequency of a half – wave rectifier is equal to the input frequency,  as shown in Figure 14. A waveform has a complete cycle when it repeats the same wave pattern over a given time. The output waveform also repeats the same wave pattern over 0 degrees to 360 degrees. This means that when input AC completes one cycle, the output half – wave rectified wave also completes one cycle. In other words, the output frequency is equal to the input frequency. Figure 14. Output Frequency of Half – wave RectifierThe ratio of DC power output to the applied input AC power is known as rectifier efficiency, Consider a half – wave rectifier as shown in Figure 15. Let v = Vmsin? be the alternating voltage that appears across the secondary winding. Let Rf and RL be the diode resistance and load resistance, respectively. The diode conducts during positive half – cycles of AC supply while no current conduction takes place during negative half – cycles. Figure 15 Half – wave RectifierDC Power. The output current is pulsating direct current. Therefore, in order to find DC power, average current has to be found out. AC Power input: The AC Power input is given by: For a half – wave rectified wave,  The efficiency will be maximum if Rf is negligible as compared to RL. This shows that in half – wave rectification, a maximum of 40.6% of AC power is converted into DC power. Full – wave rectifier Center – tap full – wave rectifierThe circuit engages with two diodes D1 and D2 as shown in Figure 16. A center – tapped secondary winding AB is contains with two diodes connected so that each gets one half – cycle of input  AC voltage. In other words, diode D1 utilizes the AC voltage seeming across the upper half (OA) of secondary winding for rectification while diode D2 uses the lower half winding OB. Figure 16. Center – tap Full wave RectifierDuring the positive half – sequence of secondary voltage, the end A of the secondary winding turn into positive and end B negative. This makes the diode D1 forward biased and diode D2 reverse biased. Therefore, diode D1 functions while diode D2 does not. The conventional current flow is through diode D1 load resistor RL and the upper half of secondary winding as made known by the dotted arrows. Therefore, diode D2 functions while diode D1 does not. The conventional current flow is through diode D2 load RL and lower half winding as made known by solid arrows. Referring to Figure 16, it may see that current in the load RL is in the similar direction for both half – cycles of input AC voltage. Therefore, DC is gotten across the load RL. Also, the polarities of the DC output across the load should be distinguished.Its main disadvantages are: It is hard to place the center tap in the secondary winding. The DC output is small as each diode utilizes only one – half of the transformer secondary voltage. The diodes obtained must have high peak inverse voltage. Full Bridge rectifierThe requirement for a center tapped power transformer is removed in the bridge rectifier. It encompasses four diodes D1, D2, D3, and D4 connected to make bridge as shown in Figure 17. The AC supply to be corrected is useful to the diagonally opposite ends of the bridge through the transformer. Between two ends of the bridge, the load resistance RL is joined. Figure 17 Full – Wave Bridge RectifierDuring the positive half – cycle of secondary voltage, the end P of the secondary winding becomes positive and end Q negative. This obtained diodes D1 and D3 forward biased while diodes D2 and D4 are reversed biased. Therefore, only diodes D1 ad D3 conduct. These two diodes will be in series through the load RL as shown in Figure 18. The current flow is made known by the solid arrows. It may be seen that again current flows from A to B through the load. Therefore, DC output is attained across load RL. Figure 18. Operation of Full – wave Bridge Rectifier  Advantages: The requirement for the center – tapped transformer is eliminated. The output is two times that of the center – tap circuit for the same secondary voltage. The PIV is one – half that of the center – tap circuit.Disadvantages: It requires four diodes. As during each half – cycle of AC input diodes that conduct are in series, therefore, voltage drop in the internal resistance of the rectifying unit will be twice as great as in the centre tap circuit. This is objectionable when secondary voltage is small. The output frequency of a full – wave rectifier is double the input frequency, .Remember that a wave has a complete cycle when it repeats the same pattern as shown in  Figure 19. Figure 19. Full – wave Bridge RectifierFigure 20 shows the process of full – wave rectification. Let v= Vmsin? be the AC voltage to be rectified. Let Rf and RL be the diode resistance and load resistance respectively.  Figure 20. Full -wave rectifier waveformThe instantaneous current i is given by: DC Output Power AC Input Power. The AC input power is given by:  The efficiency will be maximum if Rf is negligible as compared to RL. This is double the efficiency due to half – wave rectifier. Therefore, a full – wave rectifier is twice as effective as a half – wave rectifier. Discuss the block diagram of a power supply. You may illustrate it and the corresponding waveforms. Figure 21. Block Diagram of Power SupplyA regulated power supply is created  to guarantee that the output remains constant even if the input varies. A regulated DC power supply is also called as a linear power supply, it is an fixed circuit and contains of several blocks. The regulated power supply will admit an AC input and provide a constant DC output. Figure 21 demonstrations the block diagram of a typical regulated dc power supply.The building blocks of a regulated dc power supply are as follows:1. A transformer (usually step down)2. A rectifier (usually a Full – wave Bridge Rectifier)3. A DC filter (usually capacitors)4. A regulator (usually IC)Operation of Regulated Power SupplyTransformerA step down transformer will used to go down the voltage from the AC mains to the needed voltage level. The turn’s ratio of the transformer is used as to get the needed voltage value. The output of the transformer is connected and will go to as an input voltage to the rectifier circuit which is the next block.RectificationAs for rectifier circuit, the usually type of rectifier is Full – wave Bridge rectifier since it is more efficient compared to the Half – wave rectifier. Figure 22. Full – wave Bridge RectifierDC FilterationEven if the rectifier circuit corrects the AC signal, the signal is still pulsating. However, through the use of filter, pulsating voltage will now be smoothening.  Figure 23 Rectification with Output WaveformAs shown in Figure 23, the instantaneous voltage begins to rise the capacitor charges, it charges until the waveform reaches its peak value. When the instantaneous value begins to decrease,  the capacitor begins to discharges exponentially and gradually through the load (input of the regulator in this step). Hence, an almost constant dc value having very low ripple content must be get.RegulationThis is the last step or block in a regulated DC power supply. The output voltage or electric current will modify or fluctuate when there is a modification in the input from AC mains or due to variations in load current at the output of the regulated power supply or due to other situations that might affect the circuit like temperature variations. This problem can be diminished by using a regulator. A regulator will make constant the output voltage even when variations at the input or any other changes happened. Transistor, IC and Zener can be used as regulators. State other applications of a diode. (rectifier circuit is an application of diode). As a Power Indicator. A LED can be used to indicate whether the power is on or not as shown in Figure 24.  Figure 24. LED as Power Indicator Seven – Segment Display. LEDs are often grouped to form seven – segment display. Figure 25 shows the schematic diagram of seven – segment display. Figure 25. LED as Seven – segment Display Alarm Circuit. Figure 26 shows the use of photodiode in an alarm system. Light from a light source is allowed to fall on photodiode. Figure 26. Photodiode used in Alarm Circuit Counter Circuit. A photodiode may be used to count items on a conveyor belt as shown in Figure 27. Figure 27. Photodiode used to count items 

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