What is LED?

LEDs are Light-Emitting Diodes, semiconductor electronic devices that produce visible light. They were initially used as simple indicator lights. Today, they are used extensively for indoor and outdoor commercial and industrial lighting.

Below is the electronic symbol of the LED:

Symbol of the light-emitting diode showing the cathode, anode, and the direction of light emitted.

LED dates back to the early 1900’s when a British physicist discovered silicon carbide crystals could produce light when electric currents were touched. In 1962, GE scientist Nick Holonyak developed the first visible light LED diode. This LED produced red light. Later George Craford developed a yellow light LED. The humble LED then cost close to $200 for each diode. Within a few years, advances and production scaled up and the cost dropped to 5 cents.

How does an LED work?

The P-N junction is the basis of the LED’s functioning. The LED has an anode and a cathode separated by a crystal of semiconductor material. By adding impurities to the semiconductor material, the result produces P-N electronic junctions within the LED chip. The entire assembly is within a plastic cover that can also double up as a lens to guide the light emitted by the LED.

A schematic diagram of a Light Emitting Diode. (Picture credit https://commons.wikimedia.org/wiki/File:LED_Device.jpg)

Voltage is applied across the electrodes. The current flows from the anode (P side) to the cathode (N side). When an electron meets a hole at the P-N junction it falls into a lower energy state.

The difference in energy of the two states is called the ‘Band gap’ which is a characteristic of the material comprising the P-N junction.

The excess energy of the electron is emitted as a Photon. More the ‘Band Gap’ higher the energy difference and the shorter is the wavelength of the light emitted.

The picture above shows the spectrum of electromagnetic radiation. Of visible light, red has the longest wavelength of 700 nm (least energetic) while violet has the shortest wavelength 400 nm (most energetic).

LED produces light in a narrow band of wavelengths. phosphors are often used to improve the spectrum of white light produced by an LED. It’s currently possible to combine several LEDs on a board producing custom wavelengths of full spectrum light.

LED Devices

The following are important for increasing the lighting efficiency of LEDs

• Advances in material sciences to create materials with better band gaps
• Better fabrication techniques for reducing the cost and increasing efficiency
• Improvement in heat dissipation
• Light extraction from the material comprising the diode. New materials allow more light to be extracted. This improves the lumen per watt characteristics of LEDs.
• Improvements in phosphor technology to increase the efficiency of conversion of light from one wavelength to a wider band of wavelengths.

A single LED is very small and produces a defined amount of light according to its design type. There are low-, mid- and high-power LEDs. Several LEDs need to be combined to produce the desired amount of light.

The picture below shows 9 LED menorah resting on a fingertip indicating the relative size of an LED:

This small size makes it possible to combine LEDs in any combination and imparts versatility to LED-powered lighting devices. According to the design of the printed circuit board, each LED will fail or a group of LEDs will fail together. It means the chances of failure of all LEDs simultaneously are very remote.

Now that you know what an LED is, it is time to understand what are LED bulbs made of. LEDs need to be combined to increase the light output per bulb to suitable values. You could try to do it on your own by soldering several LEDs on a metal core printed circuit board but it is better to choose packaged bulbs. Packaged bulbs are better because the LEDs have the same voltage, and requirements, and the integration of the sub-parts of the bulb is better. Also, the aesthetic appeal of a manufactured LED bulb is difficult to imitate.

The image below is a graphic representation of a section of an LED light:

An LED product contains the following components

LED cluster: Produces the required amount of light.

Drivers: Changes the household Alternating Current to Direct Current and maintains the right current to power the LED. The LED driver converts AC input voltage to 12V or 24V. Other Voltages are also possible, even AC driven LEDs. The driver is the brain of the LED bulb.

The two main components of an LED driver are

• The driver-integrated circuit
• The driver circuit driver circuit is also called the electrical control gear

Instead of using ICs and driver circuits, the required voltage drop can be achieved by using a resistor. But resistors can lead to unacceptable voltage drops and far higher currents than the LED is designed to handle.

Heat sink: The heat generated by the electronic parts and the LED needs to be dissipated. Heat sinks that are not designed well or with excess drive currents can result in overheating. This can result in elevated junction temperatures that in turn will compromise both the life and the light produced by the LED.

Three Things are Important For Good Thermal Management

Substrate material: Often a metal core PCB mounts LEDs. Besides providing a substrate for the mechanical mounting of LEDs, the metal core spreads the heat over a larger area. This helps to transfer it to the heat sink.

Interface materials: Usually a film or grease is used as an interface material. These help in removing heat while separating the passive heat sink from the energized components.

Heat sinks: Heat sinks are of two types. Active heat sinks often use fans to circulate air. Passive heat sinks use metal fins to dissipate heat. Other structural features can improve airflow around the metal fins and ensure better heat dissipation. Active heat sinks dissipate more heat but given the advances in passive heat sink design, these are not needed in most applications. Only when several LEDs are being used in a confined space an active sink may be needed to control temperatures.

Optics: LED light is directional light. The standard light distribution angle of an LED is 180 degrees. The light is emitted into the upper half-space. For some LEDs the distribution angle is adjustable, there are narrow, wide-beam till batwing optics available. The viewing angle can be altered by lenses. Lenses can be built into the structure of the LED (first optic) or a secondary lens can be used to further control the viewing angle. Either there can be one secondary lens for several LEDs in a bulb or each LED may be given a separate secondary lens for tighter control of light output.

Polycarbonate lenses are better as they have very little light loss and are easy to manufacture. The surface quality and accuracy of shape are vital to ensure the even spread of light and for restricting losses in light output.

Advantages of LED Technology

Light output: In 2002, light output from LEDs was in the region of 20 lumens per watt. Today LED Commercial lighting devices can produce 130+ Lumens per watt. New LED designs can produce as much as 200+ Lumens per watt. This amount is greater than the light created by incandescent light bulbs and fluorescent tubes.

Lower power consumption means that a typical LED light at www.ledlightingsupply.com can pay for itself in 1 to 2 years.

Life span: LED lights last anywhere between 30,000 to 100,000 hours. Most commercially available LED lights are rated for a 50,000 to 100,000-hour life span. This means that once installed an LED will last anywhere between 10 to 30 years, depending on the running hours per day. LED’s long life reduces maintenance expenses over traditional light sources like HID and fluorescent lighting and makes them perfect for difficult-to-reach locations.

Operating characteristics: LEDs operate and are not sensitive to low temperatures. Additionally, they are not affected by on-off cycling. This makes them safer and more efficient in cold environments. They are also better for applications requiring frequent switching of and off lights. The fixtures are not affected by vibrations making them the best choice for places like bridges.

Shock resistant: The energized components of the LED are separated from the outer surface with quality insulation. The electrodes are embedded in the bulb matrix and the driver electronics are encased in its shell. A layer of interface material between the LED and the heat sink ensures that no current can leak into the heat sink.

Vibration resistant: Looking at the LED at the beginning of the post you will see that the electrodes are encased in transparent acrylic. There are no suspended filaments as LEDs are resistant to vibrations. Many avid off-road driving fans use LED lights in their SUVs because of this feature alone.