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A Zener diode with breakdown voltage Vz = 9 V and maximum power dissipation Pmax = 0.27 W is used to provide a stabilized 9 V supply from a 12 V battery. The Zener is connected in parallel with the load terminals A and B, and a series resistor Rs is placed between the battery and the Zener/load combination. What is the minimum value of Rs that keeps the Zener within its power rating when no load is connected?

1. 111 ohm
2. 103 ohm
3. 100 ohm
4. 97 ohm

Correct answer: 100 ohm

Solution

With no load, the entire current through Rs flows through the Zener. To keep power within Pmax = 0.27 W at Vz = 9 V, the maximum Zener current is 30 mA. The voltage across Rs is 12 - 9 = 3 V, giving Rs_min = 3 / 0.03 = 100 ohm.

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