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Engineering
How LED Lights Work
LED Lights
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LED lights are engineered devices that turn electrical energy directly into visible light using a semiconductor junction. LED stands for light-emitting diode, and the diode part means current is intended to flow mainly in one direction. LEDs matter because they are efficient, compact, long lasting, and easy to control electronically. They are used in home lighting, displays, traffic signals, vehicles, sensors, and optical communication.
Key Facts
- An LED emits light when a forward bias pushes electrons and holes together at the p-n junction.
- Photon energy is related to frequency by E = hf.
- Photon energy is related to wavelength by E = hc/λ.
- The approximate LED color is set by the semiconductor band gap: Eg ≈ hc/λ.
- LED current must be limited because a small voltage increase can cause a large current increase.
- Series resistor design uses R = (Vsupply - VLED) / I.
Vocabulary
- LED
- A light-emitting diode is a semiconductor device that produces light when current flows through it in the forward direction.
- P-n junction
- A p-n junction is the boundary between p-type and n-type semiconductor materials where light-producing recombination can occur.
- Forward bias
- Forward bias is the voltage direction that allows significant current to flow through a diode.
- Band gap
- The band gap is the energy difference in a semiconductor that largely determines the energy and color of emitted photons.
- Heat sink
- A heat sink is a material or structure that spreads and removes heat to keep the LED from overheating.
Common Mistakes to Avoid
- Connecting an LED directly to a battery without current limiting is wrong because the LED can draw too much current and burn out.
- Treating an LED like a normal resistor is wrong because its current does not increase linearly with voltage.
- Assuming all LEDs need the same forward voltage is wrong because red, green, blue, white, and infrared LEDs use different materials and have different voltage drops.
- Ignoring heat in high-power LEDs is wrong because excess temperature lowers efficiency, shifts color, and shortens lifetime.
Practice Questions
- 1 A red LED has a forward voltage of 2.0 V and should run at 20 mA from a 9.0 V battery. What series resistor is needed?
- 2 A blue LED emits light with wavelength 470 nm. Using E = hc/λ, h = 6.63 × 10^-34 J s, and c = 3.00 × 10^8 m/s, find the photon energy in joules.
- 3 Explain why an LED needs correct polarity and current limiting, while an incandescent bulb can usually be connected in either direction to a low-voltage source.