LEDs have replaced incandescent lamps in many applications because of the LED’s lower energy consumption, smaller size, faster switching and longer lifetime. Just as in an ordinary diode, the LED has an anode and a cathode that must be properly biased. The outside of the plastic case typically has a flat spot on one side which indicates the cathode side of the LED.
The material used for the semiconductor die will determine the LED’s characteristics. Figure 1 shows a source connected to a resistor and an LED. The outward arrows symbolize the radiated light.
In a forward-biased LED, free electrons cross the pn junction and fall into holes. As these electrons fall from a higher to a lower energy level, they radiate energy in the form of photons.
In ordinary diodes, this energy is radiated in the form of heat. But in an LED, the energy is radiated as light. This effect is referred to as electro-luminescence.
The color of the light, which corresponds to the wavelength energy of the photons, is primarily determined by the energy band gap of the semiconductor materials that are used.
By using elements like gallium, arsenic, and phosphorus, a manufacturer can produce LEDs that radiate red, green, yellow, blue, orange, white or infrared (invisible) light.
LEDs that produce visible radiation are useful as indicators in applications such as instrumentation panels, internet routers, and so on. The infrared LED finds applications in security systems, remote controls, industrial control systems, and other areas requiring invisible radiation.
LED Voltage and Current
The resistor of Fig. 1 is the usual current-limiting resistor that prevents the current from exceeding the maximum current rating of the diode. Since the resistor has a node voltage of VS on the left and a node voltage of VD on the right, the voltage across the resistor is the difference between the two voltages. With Ohm’s law, the series current is:
For most commercially available low-power LEDs, the typical voltage drop is from 1.5 to 2.5 V for currents between 10 and 50 mA. The exact voltage drop depends on the LED current, color, tolerance, along with other factors.
The brightness of an LED depends on the current. The amount of light emitted is often specified as its luminous intensity IV and is rated in candelas (cd). Low-power LEDs generally have their ratings given in millicandelas (mcd).
For instance, a TLDR5400 is a red LED with a forward voltage drop of 1.8 V and an IV rating of 70 mcd at 20 mA. The luminous intensity drops to 3 mcd at a current of 1 mA.
When VS is much greater than VD in above Eq., the brightness of the LED is approximately constant.
The best way to control the brightness is by driving the LED with a current source. This way, the brightness is constant because the current is constant.
An increase in ambient temperature of LED has a substantial negative effect on the LED’s light output. This becomes important when LEDs are used in applications with large temperature variations.
Typical power dissipation levels of the LEDs discussed up to this point are in the low milliwatt range. As an example, the TLDR5400 LED has a maximum power rating of 100 mW and generally operates at approximately 20 mA with a typical forward voltage drop of 1.8 V. This results in a power dissipation of 36 mW.
High-power LEDs are now available with continuous power ratings of 1 W and above. These power LEDs can operate in the hundreds of mAs to over 1 A of current.
An increasing array of applications are being developed including automotive interior, exterior, and forward lighting, architectural indoor and outdoor area lighting, along with digital imaging and display backlighting.
It has the benefit of high luminance for directional applications such as downlights and indoor area lighting. LEDs, such as this, use much larger semiconductor die sizes to handle the large power inputs.
Because this device will need to dissipate over 1 W of power, it is critical to use proper mounting techniques to a heat sink. Otherwise, the LED will fail within a short period of time.
Efficiency of a light source is an essential factor in most applications. Because an LED produces both light and heat, it is important to understand how much electrical power is used to produce the light output.
A term used to describe this is called luminous efficacy. Luminous efficacy of a source is the ratio of output luminous flux (lm) to electrical power (W) given in lm/W.
As a comparison, the luminous efficacy of a typical incandescent bulb is 16 lm/W and a compact fluorescent bulb has a typical rating of 60 lm/W. When looking at the overall efficiency of these types of LEDs, it is important to note that electronic circuits, called drivers, are required to control the LED’s current and light output. Since these drivers also use electrical power, the overall system efficiency is reduced.
Figure 2(a) shows a seven-segment display. It contains seven rectangular LEDs (A through G). Each LED is called a segment because it forms part of the character being displayed. Figure 2(c) is a schematic diagram of the seven-segment display.
External series resistors are included to limit the currents to safe levels. By grounding one or more resistors, we can form any digit from 0 through 9.
For instance, by grounding A, B, and C, we get a 7. Grounding A, B, C, D, and G produces a 3.
A seven-segment display can also display capital letters A, C, E, and F, plus lowercase letters b and d. Microprocessor trainers often use seven-segment displays that show all digits from 0 through 9, plus A, b, C, d, E, and F.
The seven-segment indicator of Fig. 2(c) is referred to as the common-anode type because all anodes are connected together. Also available is the common-cathode type, in which all cathodes are connected together.
Figure 2(b) shows an actual seven-segment display with pins for fitting into a socket or for soldering to a printed-circuit board. Notice the extra dot segment used for a decimal point.
In an LED, free electrons radiate light when falling from higher energy levels to lower ones. The free electrons fall randomly and continuously, resulting in light waves that have every phase between 0 and 360°. Light that has many different phases is called non-coherent light. An LED produces non-coherent light.
A laser diode is different. It produces a coherent light. This means that all the light waves are in phase with each other. The basic idea of a laser diode is to use a mirrored resonant chamber that reinforces the emission of light waves at a single frequency of the same phase. Because of the resonance, a laser diode produces a narrow beam of light that is very intense, focused, and pure.
Laser diodes are also known as semiconductor lasers. These diodes can produce visible light (red, green, or blue) and invisible light (infrared).
Laser diodes are used in a large variety of applications. They are used in telecommunications, data communications, broadband access, industrial, aerospace, test and measurement, and medical and defense industries. They are also used in laser printers and consumer products requiring large-capacity optical disk systems, such as compact disk (CD) and digital video disk (DVD) players.
In broadband communication, they are used with fiber-optic cables to increase the speed of the Internet. A fiber-optic cable is analogous to a stranded wire cable, except that the strands are thin flexible fibers of glass or plastic that transmit light beams instead of free electrons. The advantage is that much more information can be sent through a fiber-optic cable than through a copper cable.
New applications are being found as the lasing wavelength is pushed lower into the visible spectrum with visible laser diodes (VLDs). Also, near-infrared diodes are being used in machine vision systems, sensors, and security systems.