Section 1 of 5How an LED Works
1
An LED conducts only when it is connected in forward bias, so the current flows in the direction of its arrow.
2
When current flows through it, the LED emits light, so its light output shows that a current is present.
3
In reverse bias an LED does not conduct, so it does not light up and the current is zero.
4
An LED behaves as a diode, with the added feature that it emits light when current flows through it.
Section 2 of 5Why LEDs Are Efficient
Low current
- An LED needs only a very small current to produce light, so it uses little electrical power.
Energy conversion
- An LED converts most of the electrical energy it receives into light energy, with little wasted as thermal energy.
Running cost
- Because of its low power, an LED is much cheaper to run than a filament bulb that gives the same light output.
Comparison
- Of the common types of bulb, LEDs convert the largest proportion of electrical energy into light.
Section 3 of 5Uses of LEDs
Uses of LEDs
Indicators
- An LED lights when current flows, so it shows that a device is switched on or that a circuit is working.
Displays
- Arrays of LEDs form the screens of televisions and large road signs.
Aviation lighting
- LEDs are used for aircraft lights because they are bright and efficient, and they need little current.
Symbols
- An LED symbol has two arrows pointing away from it, while an LDR symbol has two arrows pointing towards it.
Section 4 of 5Using LEDs Well
Using LEDs Well
Connection direction
- An LED must be connected the right way round, with its arrow pointing in the direction of the conventional current.
Brightness control
- The brightness of an LED increases with the current flowing through it, so the circuit is used to control its output.
Energy saving
- Because an LED wastes little energy as heat, less electrical energy is needed for a given amount of light.
General lighting
- LEDs are now widely used for general lighting in homes because they use far less electricity than filament bulbs.
Section 5 of 5Exam Checklist & Tips
5.1.10 – Key Points for the Exam
- LED A light-emitting diode, which emits light when current flows through it in the forward direction.
- Forward bias A connection in which the current flows in the direction the diode allows.
- Reverse bias A connection in which the diode blocks the current.
- Diode A component that allows current to flow in one direction only.
- Indicator A component that shows whether a current is present in a circuit.
Common Mistakes
- Saying that an LED lights in both directions; it conducts and lights only in forward bias.
- Confusing the LED symbol, which has arrows pointing away, with the LDR symbol, which has arrows pointing in.
- Stating that LEDs waste most of their energy as heat; they are efficient and convert most energy into light.
- Forgetting that an LED needs a current flowing in the correct direction to light up.