How does a Zener diode hold its voltage steady?

A Zener diode conducts in reverse once its breakdown voltage is reached. The voltage across a Zener stays almost constant over a wide range of currents.

State the equation for the series resistor in a Zener regulator.

R = (Vs − Vz) ÷ (IL + Iz). Vs is the supply voltage, Vz the Zener voltage, IL the load current and Iz the Zener current.

A 12 V supply feeds a 5.1 V Zener and a 20 mA load, with 5 mA through the Zener. Calculate the series resistor.

The calculated value is 276 Ω, so the lower preferred value of 270 Ω is used. R = (12 − 5.1) ÷ 0.025 = 276 Ω.

Why is the lower preferred value chosen for a Zener series resistor?

A lower resistance passes slightly more current, keeping the Zener in regulation at full load. A higher value might starve the Zener and let the output voltage drop.

CCEA A-Level Technology and Design 8900

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Microelectronic Control Systems

Zener diodes and voltage regulators

3 things to know
A2 1

Overview

Regulated supplies, Zener diode regulators and series resistor calculations, power ratings, 7805 and 7812 regulators and their dissipation, decoupling capacitors and low-dropout regulators.

What you need to know

Answer each one before you reveal it.

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Key term

How is a Zener diode connected to regulate a voltage, and how does it hold that voltage steady?

Key term

Why do microcontroller circuits need a regulated power supply?

Key term

How is a Zener diode connected to regulate a voltage?

Questions from this subtopic

Each one with the model answer underneath.

1
Why do microcontroller circuits need a regulated power supply?
Model answer
A PIC and its sensors need a steady voltage to work and to read inputs accurately. A battery's voltage falls as it discharges, and heavy loads make an unregulated supply dip.
2
How is a Zener diode connected to regulate a voltage?
Model answer
The Zener diode is reverse-biased in parallel with the load, with a series resistor from the supply. The Zener holds a near-constant voltage across the load.
3
How does a Zener diode hold its voltage steady?
Model answer
A Zener diode conducts in reverse once its breakdown voltage is reached. The voltage across a Zener stays almost constant over a wide range of currents.
4
State the equation for the series resistor in a Zener regulator.
Model answer
R = (Vs − Vz) ÷ (IL + Iz). Vs is the supply voltage, Vz the Zener voltage, IL the load current and Iz the Zener current.
5
A 12 V supply feeds a 5.1 V Zener and a 20 mA load, with 5 mA through the Zener. Calculate the series resistor.
Model answer
The calculated value is 276 Ω, so the lower preferred value of 270 Ω is used. R = (12 − 5.1) ÷ 0.025 = 276 Ω.
6
Why is the lower preferred value chosen for a Zener series resistor?
Model answer
A lower resistance passes slightly more current, keeping the Zener in regulation at full load. A higher value might starve the Zener and let the output voltage drop.

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The output voltage is V.

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AThe total resistance is 2 kΩ.

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