13.1.2 Zener diode

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A Zener diode is a specialised type of semiconductor diode that behaves differently in forward and reverse biases:

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.

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:

Constant Voltage Source:

Supplies a stable voltage even when the input voltage fluctuates.

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.

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Example Problem: Calculating Resistor Value in a Zener Diode Circuit

In a practical scenario, a Zener diode is used in a circuit that supplies a constant 4V to a speaker. Here's how to determine the required resistor value:

Given Values:

Input Voltage, $V_{\text{input}} = 10\ V$
Zener Voltage, $V_Z = 4 \ V$
Minimum Operating Current for Zener, $I_{Z(\text{min})} = 5$ mA
Speaker Load Current, $I_{\text{load}} = 100$ mA

Step-by-Step Solution:

Calculate Voltage Across Resistor:
The Zener voltage is 4 V, so the resistor must drop the remaining 6 V (i.e., $V_R = V_{\text{input}} - V_Z = 10 - 4 = 6$ V).
Determine Total Current:
Total current flowing through the resistor is the sum of the minimum current required by the Zener and the current for the speaker load, i.e., $I_{\text{total}}$ = $I_{Z(\text{min})} + I_{\text{load}} = 5 \text{ mA} + 100 \text{ mA} = 105 \text{ mA}$ .
Calculate Resistor Value:
Using Ohm's law, $R = \frac{V_R}{I_{\text{total}}} = \frac{6 \text{ V}}{105 \times 10^{-3} \text{ A}} = 57 \Omega$ . Thus, a 57 Ω resistor is required to maintain the correct operation of the Zener diode circuit in this configuration.

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Important Notes on Zener Diodes in Circuits

Current Limiting: A resistor is commonly placed in series with the Zener diode to ensure that the current stays within a safe range, preventing damage to the diode.
Voltage Stability: The Zener voltage will remain stable as long as the diode operates within its specified current range, making it suitable for stabilising the voltage in circuits where stability is crucial.

Zener diode Simplified Revision Notes for A-Level AQA Physics

13.1.2 Zener diode

Key Characteristics of Zener Diodes

Applications of Zener Diodes

Circuit Requirements for Zener Diodes

Example Problem: Calculating Resistor Value in a Zener Diode Circuit

Important Notes on Zener Diodes in Circuits

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