In this tutorial, we will study about the Light Emitting Diode (LED), its different types, operation. This will help in selecting the right type of LED for a particular application.
Light emitting diode (LED) is a solid state device that converts electrical energy into single color light. It is basically a specialized type of PN junction diode that emits either visible light, infrared or laser light at different wavelengths, made from a thin layer of heavily doped semiconductor material.
LED produces ‘cold’ generation light resulting in high efficiency. Unlike LEDs, normal incandescent lamps and bulbs generate large amounts of heat radiating away energy within the visible spectrum. Being a solid state device, LEDs are more durable, small and provide much longer lamp life than other normal light sources.
Construction of LED
The construction of LED is not similar to a normal signal diode. LED consists of a PN junction surrounded by a hard, transparent plastic epoxy resin hemispherical shaped body. It protects the LED from shock and vibration. The LED junction does not emit much light, so the construction of epoxy resin body helps in reflecting away the emitted photons of light from the surrounding substrate base, thus focusing upwards through the top.
However, some LEDs have cylindrical or rectangular shape construction with a flat top or surface. The cathode and anode terminal of an LED is normally identified by a notch, or by one lead shorter than the other.
A light emitting diode (LED) is a type of semiconductor diode that emits light when a current flows from anode to cathode across the PN junction of the device. Hence, an LED requires a direct current supply to forward bias the junction with a positive voltage for normal operation. The voltage to current relationship of LED is non-linear as shown in Figure 2, so the LED turns on at a lower voltage and will rapidly draw much higher current as the voltage increases.
Figure 1 – Circuit symbol of an LED and the direction of the conventional current flow
Figure 2 – Current vs. voltage Characteristic of LED
Biasing Requirements of LED
The light output intensity of LED is directly proportional to the forward current flowing through it. However, doubling of current does not provide twice the light output. A better solution is to use multiple LEDs to achieve desired light output. Since LED is connected across a power supply in a forward bias condition, it should be protected from excessive current flow using a series resistor.
The simplest method to control the current for driving LEDs is by employing a series resistor as shown in Figure 3;
Figure 3 – LED biasing circuit with a Series resistor
Thus, using Ohm’s law we can express:
IF = forward current
VDC = supply voltage
VF = forward voltage
R = series resistor
Advantages of LEDs
Energy efficient source of light for small areas and short distances
- Small in size
- Durable and resistance to shock and vibration
- Very fast on-time
- Good color resolution
- Can integrate into a control system
- Can be powered from a portable battery
Disadvantages of LEDs
- May be unreliable for outdoor applications with great temperature variations
- Employ large heat sinks to protect semiconductors from heat damage
LEDs are used in a wide variety of applications. Some typical applications of LEDs include:
- Traffic lights
- Indication lights on devices, toys, clothing
- Medical applications
- Signs and indicators
- Optocouplers and opto-isolators
Types of LEDs
The types of LEDs are classified into three categories that are further subdivided as follows:
Figure 4: Classification of LEDs
Miniature LEDs are one of the commonly used LEDs that are considerably small and have a single color or shape. These LEDs are used as device indicators in remote controls, calculators and cell phones. They are further sub-divided into standard, low current and ultra high output LEDs that vary in specifications. With their simple design and small size, they can be used directly in a circuit without need of a heat controller.
Figure 5: Miniature LEDs in different sizes
High Power LEDs
High power LEDs are capable of providing much higher output in terms of luminosity compared to standard LEDs. These LEDs are usually used in high powered lamps, headlights and industrial settings. They are further sub-divided on the basis of voltage, luminous intensity and wavelength. Overheating is a considerable factor in high power LEDs; therefore they need to be mounted properly on heat absorbent material. Control over certain temperatures and maximum current can help ensure reliability and longevity of LED.
Figure 6: Surface Mount Power LED
Application Specific LEDs
The LEDs required for specific applications are termed as ‘application specific LEDs’. An example of application specific LEDs for a digital clock is shown below:
Figure 7: Digital clock with LED display
They are divided further into sub-categories as follows:
Flash – the flash LED is usually used as an attention indicator in signs, vehicles, etc. It flashes the light at a specific frequency. It contains an additional integrated circuit and do not require connecting a series resistor with power supply.
Bi-color and Tri-color – As the name explains, these LEDs can emit two or three colors respectively. They are able to eliminate one or all colors at the same time. Bi-color LEDs have two leads, while tri-color LEDs usually have three leads, including anode, cathode and common cathode.
RGB – These are red, green, blue LEDs that allow emitting a combination of primary colors with precision. They are used in various applications including video display, light shows, etc. They require electronic circuits to control the diffusion and blending.
Alphanumeric – Alphanumeric LEDs are used to display alphabets, numbers and symbols. They are usually used in display boards, digital clocks, etc. These LEDs are available in different number of segments increasing versatility with the increased segments.
Lighting – Lighting LEDs are available in a variety of sizes and shapes and they are also known as LED bars or lamps. These LEDs are more commonly used because they have increased total area to escape heat.
LED Driving Techniques
The main function of the LED drivers is to deliver constant current instead of constant voltage across the range of operating conditions regardless of input and output. The requirements of the driver are specified by the arrangement of LEDs and its operating parameters. The LED driving techniques is based on a number of methods that are as follows:
Resistor Limiting Method
Resistor acts as a current limiter, but it is not used as a current control circuit for LED. A typical resistor limiting LED driver circuit is shown in figure below. It is a simple and inexpensive method of driving LED. However, there are certain disadvantages of using this method including poor efficiency, current variation due to change in forward voltage, and resistor heat generation.
Figure 8: Resistor Limiting LED Driver
Linear Regulation Method
It is a simple method that utilizes a linear IC with constant current source to control the LED current. A linear current source provides a constant current to the LED over the entire range of supply voltage. It also provides benefits of diagnosis functions. External resistors can be added to adjust the LED current. A typical LED driver circuit using a linear IC with constant current source if shown in figure below:
Figure 9: Linear Regulation with Constant Current Source
Another method that is commonly used for driving LEDs is a DC/DC converter that provides constant current to the LED over the entire range of supply voltage. The different topologies used in this method include Buck, Boost, etc. This method reduces the power loss by optimizing the chain length of LED. It provides highest efficiency and high temperature protection. However, they have an increased count of components.
Figure 10: Block Diagram of DC/DC Converter for driving LED
Brightness Control – PWM
The brightness of LED is controlled by varying the current or by using a technique called PWM (Pulse Width Modulation). PWM is the most stable method to achieve dimming of LED. It varies the ratio of pulse ON time vs. the total time of a pulse (duty cycle) by keeping the current at rated level.
Figure 11: PWM Dimming for LED Brightness Control
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