More practice questions

  1. Q4. Which breakdown mechanism mainly acts in a Zener diode rated 3.3 V, and which in one rated 24 V?

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    In the 3.3 V diode, the Zener effect (tunnelling through a very thin, heavily doped junction); in the 24 V diode, avalanche breakdown. The dividing line is about 5 V (NEETS Module 7, page 3-3).

  2. Q5. Assertion: a Zener diode can work continuously in reverse breakdown when all its ratings are respected. Reason: its structure is designed for this operation, with current limited and heat adequately removed. Are both true, and does the reason explain the assertion?

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    Both are true, and the reason explains the assertion (NEETS Module 7, page 3-5; NCERT page 336).

  3. Q6. In a regulator with Vin = 15 V, RS = 300 Ω and VZ = 6.0 V, the load is disconnected. Find the Zener current and its power. Is a 0.25 W diode enough?

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    All of IS = (15 − 6.0)/300 = 30 mA now flows through the diode, so it dissipates 6.0 × 0.030 = 0.18 W. That is below 0.25 W, so it meets the nominal power requirement at the rating’s specified temperature. Real selection must include thermal derating and tolerances. With no load the diode takes the most current, so this is the case to check.

  4. Q7. A 10 V Zener regulator has RS = 100 Ω and a load that draws a steady 20 mA. The input varies between 14 V and 18 V. Find the smallest and largest Zener currents.

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    IS = (14 − 10)/100 = 40 mA up to (18 − 10)/100 = 80 mA, so IZ = IS − 20 mA runs from 20 mA to 60 mA. The ideal output stays at 10 V throughout, provided the diode can regulate at 20 mA and safely carry 60 mA (0.60 W at 10 V).

  5. Q8. While drawing the characteristic of a Zener diode, why is the reverse voltage raised in small steps of 0.1 V?

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    Near breakdown the current rises sharply for a very small rise in voltage, so large steps would jump past the knee of the curve and miss the breakdown voltage (reasoned; the 0.1 V steps are from the NCERT Laboratory Manual, Experiment 17, page 127).

  6. Q9. What is the purpose of the protective resistance RP in the circuit for the characteristic curve?

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    After breakdown the diode itself hardly limits the current, so RP in series keeps the current below the largest safe value PZ/VZ and stops the diode overheating (NCERT Laboratory Manual, page 126).

  7. Q10. A 5.0 V Zener regulator runs from a 12 V input and must supply 4.0 mA to its load with 10 mA through the diode. Find RS.

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    IS = 10 + 4.0 = 14 mA, so RS = (12 − 5.0)/0.014 = 500 Ω.