A 1-watt, 10-volt Zener diode with the following characteristics: Imin = 5mA; Imax = 95 mA; and Z = 8 ohms, is to be used as part of a voltage regulator in a 20-volt power supply. Approximately what size current-limiting resistor would be used to set its bias to midpoint of its operating range?

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Multiple Choice

A 1-watt, 10-volt Zener diode with the following characteristics: Imin = 5mA; Imax = 95 mA; and Z = 8 ohms, is to be used as part of a voltage regulator in a 20-volt power supply. Approximately what size current-limiting resistor would be used to set its bias to midpoint of its operating range?

The important idea is biasing the Zener regulator at the middle of its allowable current range to get a stable and predictable output. The midpoint current is (Imin + Imax) / 2 = (5 mA + 95 mA) / 2 = 50 mA. With a 20 V supply and a 10 V Zener, the resistor must drop about 10 V when the Zener carries 50 mA. So the required resistance is roughly (20 − 10) / 0.05 A = 200 Ω. The Zener’s dynamic resistance (8 Ω) means the Zener voltage will vary a bit with current, giving a small adjustment (roughly 0.05 A × 8 Ω ≈ 0.4 V), but for an approximate design, about 200 Ω is the right choice.

The important idea is biasing the Zener regulator at the middle of its allowable current range to get a stable and predictable output. The midpoint current is (Imin + Imax) / 2 = (5 mA + 95 mA) / 2 = 50 mA.

With a 20 V supply and a 10 V Zener, the resistor must drop about 10 V when the Zener carries 50 mA. So the required resistance is roughly (20 − 10) / 0.05 A = 200 Ω.

The Zener’s dynamic resistance (8 Ω) means the Zener voltage will vary a bit with current, giving a small adjustment (roughly 0.05 A × 8 Ω ≈ 0.4 V), but for an approximate design, about 200 Ω is the right choice.

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