What is Special Purpose Diodes – Electronics Tutorial

What is Special Purpose Diodes – Electronics Tutorial
Pages 16
Views 349
Downloads : 8 times
PDF Size : 256.3 KiB

Summary of Contents

What is Special Purpose Diodes – Electronics Tutorial

  • Page 1

    INTRINTRINTRINTRINTRODUCTIONODUCTIONODUCTIONODUCTIONODUCTIONThe most common application of diodes is rectification. The rectifier diodes are used in powersupplies to convert a.c. voltage into d.c. voltage. But rectification is not all that a diode cando. A number of specific types of diodes ar...

  • Page 2

    126 Principles of Electronics7.1 Zener DiodeA zener diode is a special type of diodethat is designed to operate in the reversebreakdown region. An ordinary diodeoperated in this region will usually bedestroyed due to excessive current.This is not the case for the zener diode.A zener diode ...

  • Page 3

    Special-Purpose Diodes 127almost the entire energy is given up in the form of heat and emitted light is insignificant. However, inmaterials like gallium arsenide, the number of photons of light energy is sufficient to produce quiteintense visible light.(i)(ii)Fig. 7.2Fig. 7.2 (ii) shows the...

  • Page 4

    128 Principles of Electronics∴RS=3101.620 10−−× = 420 ΩΩΩΩΩNote that resistor RS is also called current-limiting resistor.Example 7.2. What is current through the LED in the circuitshown in Fig. 7.5 ? Assume that voltage drop across the LED is 2 V.Solution. Current throu...

  • Page 5

    Special-Purpose Diodes 129actually contain two pn junctions that are connected in reverse-parallel i.e. they are in parallel withanode of one being connected to the cathode of the other. If positive potential is applied to the topterminal as shown in Fig. 7.7 (i), the pn junction on the left...

  • Page 6

    130 Principles of Electronicscommon positive voltage source of +5 V. This arrangement is known as *common-anode type. Inorder to light a particular LED, say A, we ground the point A in Fig. 7.9 (ii). It forward biases the LEDA which will be lit.7.7 Photo-diodeA photo-diode is a reverse-b...

  • Page 7

    Special-Purpose Diodes 131Fig. 7.11Calibrated Photo-Diode7.8 Photo-diode OperationFig. 7.11 shows the basic photo-diode circuit. The circuit has reverse-biased photo-diode, resistor R and d.c. supply. The operation of the photo-diode is as under :(i) When no light is incident on the pn jun...

  • Page 8

    132 Principles of Electronicsreverse-biased voltage VR, the reverse current IR increases as the illumination (E) on the pn junctionof photo-diode is increased.7.10 Applications of Photo-diodesThere are a large number of applications of photo-diodes. However, we shall give two applications...

  • Page 9

    Special-Purpose Diodes 133Fig. 7.16Solution.The current that flows when the incident light is removed from the photo-diode is called darkcurrent. The resistance of a photo-diode corresponding to dark current is called dark resistance.From the curve shown in Fig. 7.16, it is clear that for ze...

  • Page 10

    134 Principles of ElectronicsFig. 7.177.12 Tunnel DiodeA tunnel diode is a pn junction that exhibits negative resistance between two values of forwardvoltage (i.e., between peak-point voltage and valley-point voltage).A conventional diode exhibits *positive resistance when it is forward bi...

  • Page 11

    Special-Purpose Diodes 135V), the diode exhibits negative resistance i.e., as the forward bias is increased, the current decreases.This suggests that tunnel diode, when operated in the negative resistance region, can be used as anoscillator or a switch. (i)(ii)Fig...

  • Page 12

    136 Principles of ElectronicsFig. 7.207.14 Varactor DiodeA junction diode which acts as a variable capacitor under changing reverse bias is known as avaractor diode.When a pn junction is formed, depletion layer is created in the junction area. Since there are nocharge carriers within the...

  • Page 13

    Special-Purpose Diodes 137Fig. 7.22 shows the curve between reverse bias voltageVR across varactor diode and total junction capacitance CT.Note that CT can be changed simply by changing the voltageVR. For this reason, a varactor diode is sometimes calledvoltage-controlled capacitor.7.15 Appl...

  • Page 14

    138 Principles of Electronics (i) (ii)(iii)Fig. 7.24Working(i) When Shockley diode is forward biased (i.e., anode is positive w.r.t. cathode), diodes D1and D3 would be forward-biased while diode D2 would be reverse-biased. Since diode D2 offers veryhigh resistance (being reverse bia...

  • Page 15

    Special-Purpose Diodes 139through them would rise rapidly and the heat produced by this current flow could ruin the entiredevice. For this reason, Shockley diode should never be operated with a reverse voltage sufficient toreach the reverse-voltage breakdown point.Conclusion. The above disc...

  • Page 16

    140 Principles of ElectronicsChapter Review Topics1. What is a LED ?2. Explain the working of a LED.3. Give two applications of LEDs.4. Why do LEDs need series current-limiting resistors ?5. How does LED differ from an ordinary diode ?6. What is a photo-diode ?7. How does photo-diode work...