Special Purpose Diodes

Transcription

1 Special Purpose Diodes 1

2 Contents Zener Diode Schottky Diode Light Emitting Diode(LED) Photodiode Types Standard Signal Diode Photo diode Zener Diode Tunnel Diode Varicap Schottky Diode LED-Light Emitting Diode TVS-Transient Voltage Suppression Example application Reverse polarity protection. Rectifier Light Sensing Voltage Regulation Oscillators (microwaves) Voltage-controlled oscillators Power Applications Light signalization, lighting Over voltage protection 2

3 Zener Diode A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage." 3

4 Basic Function The basic function of zener diode is to maintain a specific voltage across its terminals within given limits of line or load change. Typically it is used for providing a stable reference voltage for use in power supplies and other equipment. This particular zener circuit will work to maintain 10 V across the load. 4

5 Normal Diode vs Zener Diode 5

6 The zener diode s breakdown characteristics are determined by the doping process. Low voltage zeners less than 5V operate in the zener breakdown range. Those designed to operate more than 5 V operate mostly in avalanche breakdown range. Zeners are available with voltage breakdowns of 1.8 V to 200 V. This curve illustrates the minimum and maximum ranges of current operation that the zener can effectively maintain its voltage. 6

7 6 Reverse bias in Volts V I Reverse current in ma V The voltage across a conducting zener is relatively constant. 7

8 Equivalent Circuit 8

9 Zener Diode applications A zener diode is used on reverse polarization, as a 1. voltage limiter 2. basic (shunt) regulator 3. For protection against voltage peaks. 9

10 Zener regulated power supply preliminary schematic. 10

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12 Example (Basic Regulator) Let's suppose that we have an input voltage of 12 Volt, and we want to limit the output voltage to 7.5 Volt. Then we choose a 1N V zener diode. On the datasheet given, we can see that I ZT = 34mA, which is the minimum current needed for reaching V z, and I ZM =121 ma is the maximum admisible current. 12

13 A good working point is I Z =80 ma, near the middle of the range. Note. A zener diode is used as a reference or regulator only on low current simple applications. 13

14 Zener Limiting Zener diodes can used for limiting just as normal diodes. Recall in previous chapter studies about limiters. The difference to consider for a zener limiter is its zener breakdown characteristics. 14

15 Example(Zener Limiting) 15

16 Schottky Diodes Special Features Low forward voltage drop Fast switching action 16

17 Applications Switching Power Supply Voltage clamping Discharge protection 17

18 Comparing Diodes Reference: 18

19 Varicap a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode which has a variable capacitance that is a function of the voltage impressed on its terminals. 19

20 Light Emitting Diode The light-emitting diode (LED) emits photons as visible light. Its purpose is for indication and other intelligible displays. Various impurities are added during the doping process to vary the color output. 20

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22 LED driving circuit Power supply V S R S V D LED I S = V S - V D R S The typical voltage drop for most LEDs is from 1.5 to 3.2 V. 22

23 LED driving circuit Example: Source Voltage = 9 volts Voltage Drop = 3.1 volts typical for a blue or white LED Desired Current = 13 milliamps So the resistor we need is: (9 3.1) / ( 13 / 1000 ) = 452 ohms so we will use a 470 Ω resistor. The forward voltage in the table is value for a typical forward current of 20mA. 23

24 Example Design the circuit to drive standard RED color LED from 5 Vdc supply. This LED requires 20 ma to generate proper brightness. 24

25 7 Segment LED 25

26 Photodiodes 26

27 The photodiode is used to vary current by the amount of light that strikes it. It is placed in the circuit in reverse bias. As with most diodes when in reverse bias, no current flows, but when light strikes the exposed junction through a tiny window, reverse current increases proportional to light intensity. 27

28 I-V characteristic In the 1st quadrant, the device acts as a photovoltaic detector. It produces a voltage proportional to the incident light intensity. In the 3rd quadrant, when a reverse voltage is applied, it acts as a photoconductive detector. In the dark, the reverse current(dark current, I DARK ) is very small. When light strikes the diode, there is little increase in the forward current, but the reverse current can increase significantly. The current is proportional to the intensity of the incident light. 28

29 Using a photodiode in photoconductive mode i A Reverse Current 29

30 Operation 30

31 IR LED and receiver IR, or infrared, communication is a common, inexpensive, and easy to use wireless communication technology. IR light is very similar to visible light, except that it has a slightlty longer wavelength. This means IR is undetectable to the human eye - perfect for wireless communication. An IR transmitter contains an LED that emits infrared light. Thus the name. The receiver contains either a photodiode or a phototransistor (usually the latter). The carrier frequency for the vast majority of IR remote controls is 38 KHz or something near. 31

32 Basic schematics of the IR emitter and IR detector circuits. 32

33 Counting System 33

34 Symbols 34