# LectureNotes_8 - (Vz is usually specified at the zener test...

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(V z is usually specified at the zener test current I ZT , and is designated V ZT .) Zener Equivalent Circuit The figures show the ideal and practical models of a zener diode in reverse breakdown. V z (a) Ideal (b) Practical It has a constant voltage drop equal to the nominal zener voltage. This constant voltage drop is represented by a dc voltage even though the zener diode does not actually produce an emf voltage. The dc source simply indicates the effect of reverse breakdown is a constant voltage across the zener terminals. The zener impedance Z Z is included in the practical model of a zener diode. Since the actual voltage curve is not ideally vertical, a change in zener current ( I Z ) produces a small change in zener voltage ( V Z )

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By Ohm’s law the ratio of Z V to Z I is the impedance as expressed in the following equation Z Z z I V Z Normally Z Z is specified at I ZT and is designated Z ZT . In most cases the assumption is made that Z Z is constant over the full linear range of zener current values and is purely resistive. Circuit Diagram of a simple voltage regulator + V i - Load Resistor R L Current limiting Resistor R S Input voltage to be regulated Note that the zener diode is reverse biased Circuit Approximations
When V V z , the zener is ON and behaves like a constant dc voltage source of value V z When 0 < V < Vz, the zener is OFF and behaves like an open circuit. Temperature Coefficient The temperature coefficient specifies the percent change in zener voltage for each o C change in temperature. For example, a 12V zener diode with a positive temperature coefficient of

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LectureNotes_8 - (Vz is usually specified at the zener test...

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