When we apply a voltage across an LED, it produces light. This all comes from a tiny piece of semiconductor material inside the LED, which emits energy as photons. Interestingly, if we shine a light onto the LED, we are firing photons back into it, reversing the process and producing a small voltage. LEDs come in different shapes, colors, and sizes for various applications. LED stands for Light Emitting Diode.
LED Symbol and Basics
In engineering drawings, we use a specific symbol for LEDs. Notice it looks very similar to a diode symbol but includes arrows indicating that light is being emitted. Both LEDs and diodes operate on the same principle: a semiconductor material is sandwiched between electrical connectors. While both emit photons, only LEDs emit photons in the range visible to humans, specifically wavelengths around 400 to 700 nanometers. Depending on the wavelength, we perceive different colors.
For example:
- FM radio signals are photon waves around 3 meters.
- Wi-Fi signals are smaller at around six centimeters.
- Medical X-rays are tiny at around 0.01 nanometers.
These are all outside our visible spectrum. You might have noticed an LED in your TV remote emitting infrared light, typically around 940 nanometers—imperceptible to humans but visible on a phone camera.
How LEDs Differ from Standard Diodes
Inside the LED, electrons combine with holes, releasing photons in the process. Standard diodes use different materials in their semiconductor layers, producing photons in the near-infrared range. These photons are absorbed by the casing and converted to heat, causing diodes to become hot. In contrast, LEDs produce very little heat, making them much more energy-efficient compared to traditional incandescent lights, which generate significant heat by heating a filament to produce visible light.
Most LEDs you’ll recognize are the five-millimeter through-hole type, which often have one side with a flat edge. This flat edge indicates the cathode side, making it easier to identify the correct polarity. Through-hole LEDs are perfect for learning electronics, available in bulk cheaply and suitable for insertion into test boards or soldering into printed circuit boards. There are also smaller 3-millimeter versions and larger 10-millimeter versions, typically dome-shaped, as well as square-shaped variants.
Surface Mount Devices (SMD) and High-Powered LEDs
SMD LEDs, or Surface Mount Devices, are soldered directly to circuit boards for compact designs. These versions are much smaller, sometimes requiring a microscope to solder. SMD LEDs are commonly used in light bulbs. For example, a blue LED with a layer of yellow phosphorus combines yellow and blue light to create white light. High-powered LEDs, which are essentially many LEDs packed tightly together, are often used for torches and floodlights due to their brightness and visibility from great distances.
LED Polarity and Protection
LEDs only illuminate when the anode lead is connected to the positive and the cathode to the negative. The longest lead of the LED is typically the anode, making polarity identification straightforward. If the leads are trimmed, the flat edge on the LED’s case indicates the cathode side. Additionally, LEDs have two metal plates inside—the larger plate is the cathode. Some LEDs feature a small dot to indicate polarity, but it’s essential to check the manufacturer’s datasheet or test it yourself.
Connecting an LED directly to a higher voltage source, like a nine-volt battery, can destroy it. To prevent this, a resistor is used to reduce the current of electrons, protecting the LED by removing excess electrical energy as heat. For instance, with a 9-volt battery, a resistor might remove around 7 volts, allowing the LED to operate safely at the remaining 2 volts. The resistor sets the current for the circuit, which can be varied to control the LED’s brightness.
LED Drivers and Circuits
LED drivers are used in light bulbs and dedicated units powering strip lighting to provide the correct voltage and current. For example, a lamp running off 230 volts uses a rectifier to change alternating current to direct current, a capacitor to smooth it out, and a chip to provide a constant current to the LEDs, preventing flicker.
USB light strips are simple circuits where the USB port provides a 5-volt rail and a ground rail, with a resistor and an LED connected in parallel. This design allows for flexibility in length, as cutting the strip affects the current based on the number of LEDs.
Inside the LED
Inside the LED case, both the anode and cathode leads have metal plates separated by a small gap. The larger plate indicates the cathode side. The LED’s semiconductor consists of an n-type layer with free electrons and a p-type layer with holes. When powered, electrons flow from the n-type to the p-type layer, releasing photons at the PN junction. The wavelength of these photons determines the color of the light, which depends on the semiconductor material used.
Semiconductor Materials and Light Color
Different semiconductor materials produce different light wavelengths. For example:
- Silicon diodes emit near-infrared light, which humans can’t see.
- Gallium arsenic combined with gallium phosphide allows for a range of colors by adjusting the band gap of the semiconductor material.
By mixing different materials, scientists can achieve any color between red and green, enabling the production of various colors and white light by combining red, green, and blue LEDs.
Conclusion
LEDs are versatile, energy-efficient light sources used in a wide range of applications, from simple indicators to complex lighting systems. Understanding how LEDs work involves knowledge of semiconductor physics, circuit design, and material science, all contributing to the vibrant and efficient lighting solutions we rely on today.
Frequently Asked Questions (FAQs)
- What does LED stand for?
LED stands for Light Emitting Diode. It is a semiconductor device that emits light when an electric current passes through it.
- Why do LEDs produce different colors?
The color of light produced by an LED depends on the wavelength of the photons emitted, which is determined by the semiconductor material used in the LED’s construction. Different materials have different band gaps, resulting in different wavelengths and colors of light.
- How can I protect an LED from being destroyed by high voltage?
To protect an LED from high voltage, use a resistor in the circuit to limit the current flowing through the LED. The resistor reduces the voltage to a safe level, preventing excessive current that can damage the LED.
