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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Identify the given values: V in = 9 V, V Z = 5.1 V, I Z = 5 mA.
- S2
Use Ohm's Law to calculate the resistance: R = (V in - V Z ) / I Z .
- S3
Substitute the values into the formula: R = (9 V - 5.1 V) / 5 mA.
- S4
Calculate the result: R = 3.9 V / 5 mA = 780 Ω.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Identify the given values: V in = 9 V, V Z = 5.1 V, I Z = 5 mA.
- S2
Use Ohm's Law to calculate the resistance: R = (V in - V Z ) / I Z .
- S3
Substitute the values into the formula: R = (9 V - 5.1 V) / 5 mA.
- S4
Calculate the result: R = 3.9 V / 5 mA = 780 Ω.
Final answer: The value of the resistor is 780 Ω.
Common mistakes
- Confusing forward bias and reverse bias in zener diodes. — 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. — 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. — 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) ). — 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. — 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. — 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
- Full worked solution (all marking points)4 marks