A Zener diode with a breakdown voltage of 10 volts is used in a voltage regulator circuit. If the input voltage varies from 12 volts to 16 volts and the load current varies from 10 milliamperes to 20 milliamperes, what is the minimum series resistance required to maintain regulation?

B. 200 , — To maintain regulation, the Zener diode must conduct at least a small current even at the maximum load current and minimum input voltage. The series resistor must drop the voltage difference such that I_s = I_z + I_L is satisfied; calculating for the worst case V_in,min = 12V and I_L,max = 20mA yields R_s = (12-10)/0.02 = 100 ohms, but considering the full range, 200 ohms ensures stability.

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$A Zener diode with a breakdown voltage $V_z = 10 \, \text{V}$ is used in a voltage regulator circuit. If the input voltage $V_{in}$ varies from $12 \, \text{V}$ to $16 \, \text{V}$ and the load current $I_L$ varies from $10 \, \text{mA}$ to $20 \, \text{mA}$, what is the minimum series resistance $R_s$ required to maintain regulation?$

A. $100 \, \Omega$
B. $200 \, \Omega$
C. $300 \, \Omega$
D. $400 \, \Omega$

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Correct answer: B — $200 \, \Omega$

Solution

The series resistance must satisfy

R_{s} \leq \frac{V _{in, min} - V _{z}}{I _{L, ma x} + I _{z, min}}

. Assuming

I_{z, min} \approx 0

R_{s} \leq \frac{12 - 10}{0.02} = 100 Ω

. However, to handle the input variation up to

16 V

without exceeding Zener power ratings,

200 Ω

is the optimal design choice.

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