Zener diode (AQA A-Level Physics): Revision Notes

13.1.2 Zener diode

1. Forward Bias:

• Acts similarly to a standard diode, allowing current to flow easily in this direction once the applied voltage surpasses a certain threshold voltage. This is the minimum voltage required for current flow.

2. Reverse Bias:

• Unlike regular diodes, which are designed to withstand high breakdown voltages without damage, a Zener diode is manufactured with a lower, more specific breakdown voltage.
• The breakdown voltage $($also called Zener voltage, $V_Z)$ is the reverse voltage at which a Zener diode begins to conduct significantly without damage. This property allows it to be used reliably at its breakdown voltage.

Key Characteristics of Zener Diodes

• In reverse bias, once the breakdown voltage is reached, the voltage across the Zener diode remains approximately constant even if the current changes. This makes it suitable for applications requiring a steady reference voltage.
• For proper operation, the Zener diode requires a minimum current (often around 5 mA) to maintain its breakdown voltage.

Applications of Zener Diodes

The Zener diode's ability to maintain a stable voltage makes it ideal for:

3. Constant Voltage Source:

• Supplies a stable voltage even when the input voltage fluctuates.

4. Voltage Reference:

• In circuits where a measured voltage needs to be compared to a reference (e.g., sensing circuits), the Zener diode can provide a reliable reference voltage.

Circuit Requirements for Zener Diodes

For a Zener diode to function correctly:

• Input voltage must be greater than the Zener breakdown voltage $(V_{\text{input}} > V_Z)$.
• Current through the diode should remain above a certain minimum level (e.g., 5 mA) but within safe limits to prevent overheating.
• No current should be drawn from the output of the circuit, as this could affect the voltage stability provided by the Zener diode.