A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Zener Diodes and Their Applications — mark scheme explained
The short answer
Zener diodes are a special type of diode that can operate in reverse bias, allowing them to be used as voltage regulators. This section covers the characteristic curve of a zener diode, its anode and cathode, and how it can be used with a resistor as a constant voltage source or reference voltage.
The question
A zener diode with a breakdown voltage of 5.1 V is connected in series with a resistor to a 9 V power supply. No load is connected (no load current is drawn) and the zener operating current (I Z ) is 5 mA. Calculate the value of the series resistor.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For calculation questions, marks are typically awarded for identifying correct formulas, substituting values correctly, and providing the final answer with appropriate units. For explanation and description questions, marks are awarded for clarity, completeness, and accuracy of the response.
What the command words demand
- Calculate
- Perform a numerical calculation using given data and appropriate formulas.
- Explain
- Provide a detailed account of how or why something happens, including relevant concepts and principles.
- Describe
- Give a detailed account of the characteristics or features of something without necessarily explaining the underlying mechanisms.
- Identify
- Recognize and name specific components, values, or properties in a given context.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 2-3 minutes per mark to ensure you have enough time to read the question carefully, perform calculations, and write a well-structured response.
- Identify the given values: V in = 9 V, V Z = 5.1 V, I Z = 5 mA.0 marks
- Use Ohm's Law to calculate the resistance: R = (V in - V Z ) / I Z .1 mark
- Substitute the values into the formula: R = (9 V - 5.1 V) / 5 mA.1 mark
- Calculate the result: R = 3.9 V / 5 mA = 780 Ω.2 marks
Final answer: The value of the resistor is 780 Ω.
Work through every step correctly and you earn all 4 marks.
Another worked example
A zener diode with a breakdown voltage of 12 V is used to provide a reference voltage with no load connected (no load current is drawn). The input voltage from the power supply is 15 V, and the operating current (I Z ) is 10 mA. Calculate the value of the series resistor needed.
- Identify the given values: V in = 15 V, V Z = 12 V, I Z = 10 mA.0 marks
- Use Ohm's Law to calculate the resistance: R = (V in - V Z ) / I Z .1 mark
- Substitute the values into the formula: R = (15 V - 12 V) / 10 mA.1 mark
- Calculate the result: R = 3 V / 10 mA = 300 Ω.2 marks
Final answer: The value of the resistor is 300 Ω.
Work through every step correctly and you earn all 4 marks.
Common mistakes
Confusing forward bias and reverse bias in zener diodes.
Why it happens: Students often mix up the behavior of zener diodes in forward and reverse bias, leading to incorrect circuit configurations.
Fix: Review the characteristic curve of a zener diode and understand that it conducts heavily in reverse bias when the breakdown voltage is reached.
Forgetting to include the current-limiting resistor in the circuit.
Why it happens: Students may overlook the importance of the current-limiting resistor, which can lead to damage or improper functioning of the zener diode.
Fix: Always ensure a current-limiting resistor is included in the circuit when using a zener diode as a voltage regulator.
Using the wrong formula for calculating the value of the resistor.
Why it happens: Students might use incorrect formulas or mix up the variables, leading to incorrect calculations.
Fix: Use Ohm's Law correctly: R = (V in - V Z ) / I Z . Double-check the values and units used in the calculation.
Not considering the typical minimum operating current (I Z(min) ).
Why it happens: Students may not realize that a zener diode requires a minimum current to function effectively, leading to incorrect circuit performance.
Fix: Always ensure the current through the zener diode is above I Z(min) for optimal voltage regulation.
Confusing anode and cathode connections in reverse bias.
Why it happens: Students may get confused about which terminal to connect to the higher potential and which to the lower potential in reverse bias.
Fix: Remember that in reverse bias, the anode is connected to the lower potential (negative side) and the cathode to the higher potential (positive side).
Not understanding the role of a zener diode as a reference voltage.
Why it happens: Students may not fully grasp how a zener diode can be used to provide a stable and precise reference voltage in circuits.
Fix: Review the applications of zener diodes, particularly their use as reference voltages, and ensure the current through the diode is within the specified range.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Zener Resistor Calculation | Calculate the series resistor value using the voltage across it and the zener current. | 4 |
| Calculate Series Resistor | Find the series resistor value for a zener diode reference circuit with no load. | 4 |
| Zener Series Resistor | Calculate the series resistor value using the voltage across it and the zener current. | 4 |
| Zener Series Resistor | Calculate the series resistor value from the voltage across it and the operating current. | 4 |
| Total across these question types | 16 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.