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SOLU T IONSOLU T ION

(f) For t h e zener to be at t h e edge of t h ebreakdown region, I Z = I ZK = 0.2 mA and V z ~V ZK = 6.7 V.

At t h is point t h e lowest (worst-case) currentsupplied t h roug h R is (9 - 6.7)/0.5 = 4.6 mA , and t h us t h e load current is 4.6 - 0.2 = 4.4 mA.T h e corresponding value of R L is

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6 2

Series Zener diodesSeries Zener diodes

As long as V i is greater th an t h e sum of V Z1 andVZ2, bot h diodes will be

in t h e "on" state and t h eth ree referencevoltages will beavailable

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Back-to back- Zener diodes

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IC Regulator IC Regulator

The 78 xx (also sometimes known as LM 78 xx) series of devices is afamily of self-contained fixed linear voltage regulator IC . The 78 xxfamily is a very popular choice for many electronic circuits whichrequire a regulated power supply, due to:

i.

ease of use andii. Low cost .

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78xx and 79xx IC

W hen specifying individual ICs within this family, the xxis replaced with a two-digit number, which indicatesthe output voltage the particular device is designed toprovide (for example, the 78 05 has a 5 volt output,

while the78 12

produces12

volts).

The78

xx line arepositive voltage regulators, meaning that they aredesigned to produce a voltage that is positive relativeto a common ground . There is a related line of 79 xxdevices which are complementary negative voltage

regulators . 78 xx and 79 xx ICs can be used incombination to provide both positive and negativesupply voltages in the same circuit, if necessary .