A diode is a two‑terminal device that lets current flow in only one direction, like a one‑way street 🚗.
An LED (Light‑Emitting Diode) is a special diode that emits light when forward biased, like a tiny streetlamp 💡.
Key Properties:
When a diode is forward biased (anode positive relative to cathode), it behaves like a very low resistance path.
When reverse biased (cathode positive), it blocks current (ideal diode: infinite resistance).
An LED behaves the same, but when forward biased it also emits light.
Diagram (text representation):
9V Battery ➜ 220Ω Resistor ➜ LED ➜ Ground
Use Ohm’s law and the LED forward voltage:
\$R = \frac{V{\text{supply}} - V{\text{forward}}}{I_{\text{desired}}}\$
For a 9 V battery, 2 V LED, and 20 mA desired current:
\$R = \frac{9\,\text{V} - 2\,\text{V}}{0.02\,\text{A}} = 350\,\Omega\$
Choose the nearest standard value, e.g., 330 Ω or 360 Ω.
Exam Tip: Show all steps in your calculation and round to the nearest standard resistor value. Remember to check the power rating: \$P = I^2 R\$.
- Diode symbol: a triangle pointing to a line (cathode). Current flows from the triangle tip to the line.
- LED symbol: same as a diode but with two arrows pointing outward from the triangle, indicating light emission.
- Current arrows in diagrams show the direction of conventional current (positive to negative).
- Polarity of batteries and diodes must be respected; reversing a diode will block current.
| Component | Symbol | Typical Value | Function |
|---|---|---|---|
| Battery | Two parallel lines (longer = positive) | 9 V, 12 V, etc. | Provides electromotive force (EMF) |
| Resistor | Zig‑zag line | 220 Ω, 330 Ω, etc. | Limits current |
| Diode | Triangle → line | 0.7 V drop | One‑way current flow |
| LED | Triangle → line + two outward arrows | 1.8–3.3 V drop | Light emission when forward biased |
Exam Tip: When given a circuit diagram, first identify all components and their orientation. Then determine the current path and check if any diode is reverse biased. Finally, calculate any required resistor values using the formula above.
1. If a 5 V battery powers an LED with a forward voltage of 2 V and you want 15 mA, what resistor should you use?
2. In a diagram, the diode symbol points away from the battery’s positive terminal. Is the diode forward or reverse biased?
3. Why do we need a resistor in series with an LED?