Diodes, LEDs and Current-Voltage Characteristics

This lesson explains Diodes, LEDs and Current-Voltage Characteristics for Cambridge IGCSE Physics 0625. It covers Supplement content for Extended candidates. Focus on the cause-and-effect explanation and the exact quantities being compared. The goal is to explain the idea accurately and apply it in an unfamiliar context.

This is Supplement content for Extended candidates.

How does a diode behave in a circuit?

When a diode is connected in the forward direction, it can conduct after the forward potential difference becomes sufficient. Its resistance then becomes low and the current can rise rapidly.

When the diode is connected in the reverse direction, it has a very high resistance and the current is approximately zero under the conditions considered at this level.

The direction matters. Reversing the diode in a simple d.c. circuit can change the current from a significant value to approximately zero.

What does the current-voltage graph look like?

A diode’s current-voltage characteristic is not a straight line through the origin.

• In the forward direction, little current flows at first. The current then increases rapidly as the forward potential difference increases.
• In the reverse direction, the current remains approximately zero over the range considered in IGCSE questions.

The component is therefore non-ohmic: its resistance is not constant.

Remember that resistance at a particular operating point can still be calculated using:

$R = \frac{V}{I}$

but the value changes as the current and potential difference change.

How does an LED differ from an ordinary diode?

An LED emits light when it is forward biased and current passes through it.

A series resistor is commonly included because the diode current can rise rapidly. The resistor limits the current and protects the LED.

LEDs are used as indicators, displays and light sources. In a circuit diagram, the LED symbol includes arrows pointing away from the diode to represent emitted light.

Original circuit question

A cell, resistor and LED are connected in series. The LED does not light. The student reverses the LED and it lights.

(a) Explain why the LED did not light in the first arrangement. [2]

The LED was connected in the reverse direction, so its resistance was very high and approximately no current flowed.

(b) State the purpose of the series resistor. [1]

It limits the current and protects the LED.

Original marking guidance

• one mark for identifying reverse connection or reverse bias
• one mark for very high resistance or approximately zero current
• one mark for limiting current or protecting the LED

Original graph question

At one operating point, a forward-biased diode has a potential difference of 0.80 V and a current of 25 mA.

Calculate its resistance at this operating point. [3]

Convert the current:

$25\text{ mA} = 0.025\text{ A}$

Use:

$R = \frac{V}{I} = \frac{0.80}{0.025} = 32\,\Omega$

Original marking guidance

• one mark for converting 25 mA to 0.025 A
• one mark for using $R = V/I$
• one mark for 32 ohms

How can a diode be used for control?

Because a diode permits current mainly in one direction, it can be used to:

• protect a circuit from a reversed supply
• select one direction of current
• produce a one-direction output from an alternating input in simple rectifier contexts
• indicate whether current flows in the correct direction when an LED is used

Keep the explanation at the level asked by the question. For a circuit-direction item, the key points are the diode orientation, resistance and resulting current.

Common mistakes

• Treating a diode like a fixed resistor. Its current-voltage graph is curved and its resistance changes.
• Saying a reverse-biased diode has literally infinite resistance. State that the resistance is very high and current is approximately zero in the IGCSE model.
• Forgetting to convert milliamps to amps. Divide by 1000 before using $R = V/I$.
• Saying the LED resistor makes the LED brighter. Its primary role in the circuit is to limit current and protect the component.
• Ignoring conventional current direction. Use the diode orientation shown in the circuit.
• Forgetting the graph axes. Current is normally on the vertical axis and potential difference on the horizontal axis.

Exam technique

For a circuit question, trace conventional current from the positive terminal and check the diode orientation before calculating anything. For a graph question, describe both regions: the rapid forward-current rise and the approximately zero reverse current.

How this is examined

Extended Papers 2 and 4 can test diode and LED symbols, circuit behaviour, current-voltage characteristics and calculations at a stated operating point. Practical papers may use a diode or LED in a circuit-planning, measurement or graph task, so connect this lesson to the standard ammeter-in-series and voltmeter-in-parallel method.