Chapter 3 - Chapter 3 Chapter Special Purpose Diodes Fahmi...

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Chapter 3 Chapter 3 Special Purpose Diodes Special Purpose Diodes Fahmi Samsuri Fahmi Samsuri Analog Electronics Analog Electronics Faculty of Electrical & Electronic Faculty of Electrical & Electronic Engineering, KUKTEM Engineering, KUKTEM
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FIGURE 3-1 Zener diode symbol. 1. ZENER DIODE Zener Diode :- is a silicon pn junction device that differ from rectifier diodes because it is designed for operation in the reverse- breakdown region . - if zener doide is forward-biased, it operates the same as a rectifier diode. Function :- to provide a stable reference voltage for use in power supplies, voltmeter & other instruments, voltage regulators.
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FIGURE 3-2 General diode V-I characteristic. Zener breakdown :- occurs in a zener diode at low reverse voltages. - zener diode is heavily doped to reduce the breakdown voltage. This causes a very thin depletion region.
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FIGURE 3-3 Reverse characteristic of a zener diode. V Z is usually specified at the zener test current, I ZT , and is designated V ZT . -When the reverse voltage (V R ) is increased, the reverse current (I R ) remains extremely small up to the “knee” of the curve. - From the bottom of the knee, the zener breakdown voltage (Vz) remains essentially constant although it increases slightly as the zener current,I z , increases.
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FIGURE 3-4 Zener diode equivalent circuit models and the characteristic curve illustrating Z Z . (a) Ideal model of a zener diode in reverse breakdown. (b) Practical model of a zener diode in reverse breakdown.
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Example 1: A zener diode exibits a certain change in Vz on a portion of the linear characteristic curve between Izk and Izm as ilustrated below. What is the zener impedence? Answer: 10 ohm Zz= Δ Vz = 50mV = 10Ω ΔIz 5mA SOLUTION:
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Example 2: From the datasheet, a 1N4736A zener diode has a Zz of 3.5Ω. The datasheet gives Vz = 6.8V at a test current, Iz, of 37mA. What is the voltage across the zener terminals when the current is 50mA? When the current is 25mA? Figure below represent the zener diode.
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