Draw and interpret circuit diagrams containing diodes and light-emitting diodes (LEDs) and know how these components behave in the circuit

Published by Patrick Mutisya · 14 days ago

IGCSE Physics 0625 – 4.3.1 Circuit Diagrams and Components (Diodes & LEDs)

4.3.1 Circuit Diagrams and Circuit Components

Learning Objective

Draw and interpret circuit diagrams that contain diodes and light‑emitting diodes (LEDs) and understand how these components behave in a circuit.

1. Symbols used in circuit diagrams

  • Diode – a triangle‑pointed arrow with a line at the cathode.

  • LED – the diode symbol with two small arrows pointing away, indicating light emission.

Suggested diagram: Standard diode symbol and LED symbol placed side by side.

2. How a diode works

A diode allows current to flow in only one direction – from the anode (+) to the cathode (–). This property is called unidirectional conduction.

  1. Forward bias: The anode is at a higher potential than the cathode. The diode conducts once the forward voltage \$V_F\$ is reached (typically ≈0.7 V for a silicon diode).

  2. Reverse bias: The cathode is at a higher potential. The diode blocks current (except a tiny leakage current) until the reverse‑breakdown voltage is exceeded.

3. Light‑Emitting Diodes (LEDs)

LEDs are diodes that emit light when forward‑biased. Their key characteristics differ from ordinary diodes:

PropertySilicon DiodeLED
Typical forward voltage \$V_F\$≈ 0.7 V0.8 V – 3.5 V (depends on colour)
Current ratingUp to several amperes (depends on type)Usually 10 mA – 30 mA for indicator LEDs
Light outputNoneVisible light (colour determined by semiconductor material)
Polarity markingFlat side or stripe on cathodeFlat side or stripe on cathode, plus often a longer lead for anode

4. Circuit behaviour

When a diode or LED is placed in a simple series circuit with a battery and a resistor, the voltage across the component is approximately its forward voltage \$V_F\$. The remaining voltage drops across the resistor, and the current \$I\$ is given by Ohm’s law:

\$I = \frac{V{\text{supply}} - VF}{R}\$

where \$V_{\text{supply}}\$ is the battery voltage and \$R\$ is the series resistor.

5. Example circuits

5.1 Simple forward‑biased diode

Battery \$V{\text{s}}\$, resistor \$R\$, diode D in series (anode toward the battery). The diode conducts when \$V{\text{s}} > V_F\$.

Suggested diagram: Battery → resistor → diode (arrow pointing right) → back to battery.

5.2 LED with current‑limiting resistor

To protect an LED, a resistor is used to set the desired current \$I_{\text{LED}}\$.

\$R = \frac{V{\text{supply}} - VF}{I_{\text{LED}}}\$

Example: \$V{\text{supply}} = 9\text{ V}\$, \$VF = 2.0\text{ V}\$ (red LED), \$I_{\text{LED}} = 20\text{ mA}\$.

\$R = \frac{9 - 2}{0.020} = 350\ \Omega\$

Suggested diagram: 9 V battery → 350 Ω resistor → LED (anode left, cathode right) → back to battery.

6. Common mistakes to avoid

  • Connecting the LED backwards – the cathode must be toward the negative side.

  • Omitting the current‑limiting resistor – can cause the LED to burn out.

  • Using a resistor that is too large – the LED will be dim or not light at all.

  • Assuming all diodes have the same forward voltage; colour‑specific LEDs vary.

7. Summary checklist

  1. Identify the correct symbol for a diode or LED.

  2. Determine polarity: anode (+) to cathode (–).

  3. Calculate the required series resistor using \$R = (V{\text{supply}} - VF)/I\$.

  4. Remember that LEDs need a resistor to limit current.

  5. Check the forward voltage rating for the specific LED colour.

8. Practice questions

  1. Draw a circuit diagram that lights a green LED (forward voltage 2.2 V) from a 5 V supply using a 150 Ω resistor. Indicate polarity on the LED.

  2. A silicon diode has a forward voltage of 0.7 V. If it is placed in series with a 1 kΩ resistor across a 12 V battery, what is the current through the diode?

  3. Explain why an LED will not light if it is connected in reverse bias, even though a small leakage current may flow.