A Comprehensive Guide to LEDs

What are LEDs?

A light-emitting diode (LED) is a solid-state semiconductor that emits light when a forward bias voltage is applied. An LED is a P-N junction diode constructed from direct band-gap materials. In these materials, the energy is released in the form of photons when the electrons and holes combine. The type of semiconductor material used in the LED determines the frequency, wavelength, and color of the light emitted.

LEDs are used in seven-segment displays, optical switching, visual signals, aviation and automotive lighting, advertising, traffic signals, etc. Infrared LEDs can be used in optical fiber communications and remote control units of many commercial products, including televisions, home entertainment systems, and other domestic appliances.

How does an LED work?

A light-emitting diode works like a normal PN-junction diode. The application of a forward bias to the diode causes electrons to flow into the p region from the n region, and holes to flow into the n region from the p region. In the n region, the electrons are the dominant carriers, therefore called majority carriers, while the holes are the minority carriers. Likewise, in the p region, the holes are majority carriers, while the electrons are minority carriers. When the electrons and holes enter their respective new regions, the carriers constitute minority carriers amongst majority carrier types, rapidly leading to recombination. During recombination, some of the energy difference is given up in the form of heat and light (i.e., photons). The emitted light depends on the energy band gap (E_g) of the compound semiconductor.

E_g = hf

In this equation, h is known as a Planck constant, and f is the frequency of the emitted electromagnetic radiation.

Figure 1. Operation of an LED
Source: Nirmal Academy

The frequency of radiation is related to the velocity of light. The formula is:

f = c / λ

From the above equation, we can say that the wavelength of electromagnetic radiation is inversely proportional to the band gap (or forbidden gap). For LEDs, the wavelength of the emitted photon lies in the visible or infrared region.

Characteristics of an LED

There are many variables and characteristics that define LEDs. Some of the key characteristics are luminous intensity, wavelength, and forward voltage drop.

Luminous Intensity

Luminous intensity is the quantity of light emitted by an LED in a particular direction per unit solid angle. The quantity of light produced from a source in one second is called lumen and is evaluated based on visual sensation. One lumen per steradian (square radian) is the unit of luminous intensity, also known as a candela. Generally, the luminosity of an LED decreases with the increase in the junction temperature of the diode.

Dominant Wavelength

The dominant wavelength of an LED is generally defined as the wavelength of the specific color visible to the human eye that the LED produces. LEDs emit photons to emit light. The wavelength of the emitted photons determines the LED's color and brightness. For example, red LEDs have wavelengths of around 633 to 660 nanometers, depending on whether it's super red, high-efficiency red, or ultra-red. The dominant wavelength of an LED is also affected by temperature.

Forward Voltage

The forward voltage rating is the minimum voltage difference between the anode and cathode to allow current flow. The forward voltage of an LED is directly proportional to the forbidden (bandgap) energy gap (Eg) and typically ranges between 0.9V to 2.5V. The forward voltage of any diode (including LEDs) can change with current and temperature; therefore, it is an important parameter to consider when designing a circuit.

Difference between an LED and an Incandescent Lamp

The basic key differences between LED and an incandescent light bulb are mentioned as follows:

Types of LEDs

The following types of LEDs are available based on the applications and power delivered:

• Flashing LEDs: Flashing LEDs are similar to standard LEDs but contain an internal voltage regulator and a multivibrator circuit that causes the LED to flash. The average flashing rate is about once per second, but many designs incorporate controllers for adjusting the speed. Flashing LEDs are typically used in security, alarms, and highways/roads to warn drivers of anomalies or obstacles, such as construction, detours, or railroad tracks.
• Bi-Color LEDs: Bi-color LEDs contain two different emitters in one case. There are two types. The first type consists of two dies connected to the same two leads in an inverse parallel configuration. Current flow in one direction emits one color, and the current in the opposite direction emits another color. The second type consists of two dies with separate leads for both dies, and another lead for a common anode or cathode so that they can be controlled independently.
• Red Green Blue (RGB) LEDs: RGB LEDs are in general tri-color LEDs with red, green, and blue emitters using a four-wire connection with one common lead (anode or cathode). These LEDs can have either a common positive lead (in the case of a common anode LED) or a common negative lead (in the case of a common anode LED). RGB LEDs find use in applications, such as decoration lighting, stage lighting, or status indicators.
• Ultraviolet LEDs:Ultraviolet (UV) LEDs emit UV rays with a wavelength of approximately 400 nm or less. They are divided into near-ultraviolet (NUV) LEDs, whose emission wavelength is 300–400 nm, and deep-ultraviolet (DUV) LEDs, whose emission wavelength is 200–300 nm. UV LEDs are used in air purification, excitation light sources for spectroscopy (as used for banknotes), surface sterilization, water disinfection, and hospital sanitization.
• Alphanumeric LEDs: Alphanumeric LED Displays are display devices that show information in the form of characters (numbers or alphabets), with LEDs being used as the light source.
• Chip-on-Board (COB) LEDs: A COB LED package consists of multiple LED chips mounted on a common board or substrate. The number of LED chips can range from 5 to 50. The LED chips are mounted as closely as possible to achieve the highest light output in the smallest amount of space. More light is produced when compared to multiple individual lights. Due to their high power capability, COB devices are used in applications that require a huge luminous flux, such as high bay lighting, street lighting, and downlights.
• Surface mount diode (SMD) LEDs: An SMD (surface mounted device) LED is an LED package manufactured by soldering LED package structures onto a circuit board utilizing a surface mount technique. They are flatter and smaller in design than conventional LED bulbs. SMD-LEDs are used for backlighting, home illumination, and automobile interior lighting.

Configuration of LEDs

LEDs are available in configurations based on the various factors, including voltage, wattage, current, and manufacturer type:

Low-current LEDs: Low-current LEDs are high-efficiency LEDs that provide sufficient brightness at a very low forward current of about 2mA. Low-current LEDs find utility in portable battery-powered devices with limitations on power consumption, such as wearable electronics, mobile and handheld devices, and instrumentation systems.

Standard LEDs: Standard LEDs have a round cross-section of 3-5mm diameter. They have a forward current of about 20mA. The voltages required to drive them vary by color:

• 1.9 to 2.1 V for red, yellow, orange, and traditional green
• 3.0 to 3.4 V for pure green and blue
• 2.9 to 4.2 V for violet, pink, purple, and white

Ultra-high Output LEDs: Ultra-high output LEDs have a low forward voltage (V_f) to achieve a higher lumen/watt in an efficient lighting package. They typically come in packages of greater than 1W and are driven at relatively high currents of 350, 700, or 1000 mA, producing several hundred lumens per watt. 

LED Driver

What is an LED driver?

An LED driver is a circuit that regulates and supplies the ideal current to an LED or a string of LEDs. They convert the AC voltage from mains to a lower DC voltage (usually 12V or 24V).

Why do you need an LED driver?

LED drivers are required for the following purposes:

• Individual LEDs operate at voltages ranging from about 1.5 to 3.5 volts and at currents of up to a maximum of 30mA. An LED bulb or strip may consist of several LEDs, in series and parallel combinations, requiring a total voltage of between 12 to 24 V DC. However, the electricity supplied at most places is alternating current (AC) with higher voltage (120-240V). An LED driver rectifies high-voltage AC to low-voltage DC.
• The current being supplied to the LED may experience a change due to voltage fluctuations. The LED driver provides a constant current to LEDs, and can help to avoid thermal runaway.

How does an LED driver work?

Figure 2 represents the circuit diagram of a dual-stage LED driver. The circuit consists of a bridge rectifier with an LC filter on the input side, a boost converter stage in the middle for power factor improvement, and a buck converter stage on the load side. The first stage of the driver circuit (boost converter) is provided for input Power Factor Correction (PFC), and the second stage (buck converter) is designed for the current regulation of the LED lamp.

Figure 2. A dual stage LED driver
Source: Handbook of Smart Materials, Technologies, and Devices, Springer International Publishing

The operation of the LED driver circuit is divided into three different modes. In mode 1, both switches are ON. When switch S₁ is ON, the inductor L₁ inductors are energized through the diode rectifier. The capacitor C₁ charges the inductor L₂ when switch S₂ is ON and supplies power to the load. In mode 2, switch S₁ is in operation, and switch S₂ is OFF. The inductor L₁ is energized continuously through switch S₁, and the inductor L₂ supplies power to the load through diode D₆. In mode 3, both switches are turned OFF. The inductor L₁ provides energy to the capacitor C₁, and the inductor L₂ supplies energy to the load through diode D₆ in continuous conduction mode. Once the voltage across the capacitor is less than the supply voltage, diode D₅ becomes forward-biased. The capacitor charges to a maximum voltage, and mode 1 begins again.

This dual-stage LED driver is designed for 18W at a voltage of 60V and a current of 0.3A, respectively. The switching frequency of the converter is 50 kHz, and the supply voltage is 230V rms, 50Hz.

Types of LED drivers

Two different types of LED drivers are available based on the driving mode.

• Constant-Current Driver: A constant-current external driver powers an LED with a fixed output current and an array of varying output voltages. LEDs rated to operate on a constant-current driver require a designated current supply, usually specified in milliamps (mA) or amps (A). A constant current driver provides consistent brightness and reliable overall performance. They work well for LED downlights or high-powered LEDs.
• Constant-Voltage Driver: A constant-voltage external driver have a fixed output voltage and a maximum output current. The maximum current is already regulated within the LED with simple resistors or an internal constant-current driver. The voltage is typically fixed at either 12V DC or 24V DC. Constant-voltage drivers work well for driving brightly colored LED lighting.

LED-based Flashlight Project

Figure 3 depicts a circuit diagram of a simple LED flashlight, with circuitry to run as a chaser. In this circuit, the LEDs blink one by one, and the process is repeated, which gives the running light effect.  

Figure 3. Circuit diagram for LED based flash light

The blinking rate of the LEDs can be increased or decreased by changing the values of resistors R1, R2, and R3 resistors and capacitors C1, C2, and C3. Assuming that R1, R2, and R3 have identical values, and C1, C2, and C3 likewise have identical values, the frequency can be calculated as follows:

Using values given in the circuit diagram (R1=R2=R3=20kΩ and C1=C2=C3=100μF), the frequency is about 0.36 Hz, and the time period is 1.386 seconds.

Bill of Materials (BOM)

This project will require the following components:

Frequently Asked Questions (FAQs)

How do LEDs compare to LCDs and OLEDs?

Liquid Crystal Displays (LCDs) are translucent pigments that can change RGB intensities to give the desired color. They do not emit light of their own and require a backlight.

LED and OLED (Organic Light Emitting Diode) are similar, but have the following key differences:

• Material: LED diodes are made of inorganic materials such as gallium arsenide (GaAs), gallium phosphide (GaP), or indium gallium nitride (InGaN). OLED diodes, on the other hand, are made of organic materials such as small molecules or polymers that contain carbon, hydrogen, and other elements.
• Electroluminescence: LED diodes produce light through a process called electroluminescence, where electrons flow through the diode and release energy in the form of light. OLED diodes also produce light through electroluminescence, but the process is different. In OLEDs, organic materials are sandwiched between two electrodes, and when a voltage is applied, electrons flow from one electrode to the other and release energy as light.
• Power consumption: OLED diodes are generally more energy-efficient than LED diodes, as they require less power to produce the same amount of light. This is because OLEDs only emit light when a voltage is applied, whereas LEDs continuously emit light when powered on.
• Manufacturing process: The manufacturing process for LED diodes is more mature and well-established than for OLED diodes, which are still considered a relatively new technology. This means that LED diodes can be produced more efficiently and at a lower cost than OLED diodes.

In terms of displays, the differences between LED, LCD, and OLED displays are beyond the scope of this guide; however, in general:

• LED displays have a long lifespan, are energy-efficient, and have high brightness levels. However, they are not as flexible as other display types and can suffer from color distortion at certain angles.
• LCD displays are energy-efficient and provide sharp, clear images. However, they require backlighting, which can result in lower contrast and black levels. They also suffer from limited viewing angles, with the colors appearing washed out when viewed from certain angles.
• OLED displays offer high contrast, deep blacks, and vibrant colors, with wider viewing angles than LCD displays. They are also thinner and more flexible, making them ideal for curved screens. However, OLED displays are more expensive to produce than LCD and LED displays and can suffer from burn-in issues, where static images displayed on the screen can cause permanent damage.

What are the advantages of LEDs?

The advantages of LEDS are:

• Low energy consumption
• Low cost and available in different colors
• Smaller sizes and lighter in weight.
• Longer lifetime
• They operate very fast and can be turned on and off very quickly.
• Free from toxic materials like mercury 
• The brightness of light emitted by LED can be easily controlled by varying the current flow

How does an LED produce light?

An LED is an optoelectronic device that works on the principle of electroluminescence. Electroluminescence is the property of a material to convert electrical energy into light energy. An LED is made up of a semiconductor material, such as gallium arsenide (GaAs) or gallium nitride (GaN), which has been doped with impurities to create a p-n junction. When a voltage is applied to the p-n junction, electrons in the n-type region and holes in the p-type region combine and release energy in the form of light.

What voltage do LEDs use?

Most common LEDs require a forward operating voltage of approximately 1.2 to 3.6 volts. The exact operating voltage depends on the manufacturer because of the different dopant materials and wavelengths used.

What is the difference between LED and diode?

An LED is a type of diode that produces light when an electric current is passed through it, while a regular diode is a two-terminal electronic component that allows current to flow in one direction and blocks it in the opposite direction. A diode and is used primarily for rectification and switching applications.

How can you tell if a bulb is an LED?

An LED bulb will not contain a filament like an incandescent bulb. Unlike incandescent bulbs or fluorescent bulbs, which emit light from a filament or tube, LED bulbs typically have one or more small, distinct light sources that emit light in a specific direction. You may also notice that the light emitted by an LED bulb is brighter and more focused than that of an incandescent or fluorescent bulb.  A light bulb can also be identified by measuring the power consumption. The energy an LED bulb consumes is about 80% less than an incandescent or fluorescent bulb. Another way is to turn the bulbs ON for 10 minutes. The LED bulb will not generate much heat compared to incandescent or fluorescent bulbs (which are hot enough to hurt).

LEDs are also typically much smaller than other bulbs; however, their housing may be designed to mimic the size and design of traditional bulbs.

Can you test LED with a multimeter?

When DMM is used in its diode test mode, it will generally illuminate the LED when the red lead of the DMM is connected to the anode of the LED and the black lead to the cathode of the LED. The voltage displayed on the DMM will be the LED forward voltage at the meter test current (usually around 1mA). The illumination of the LED is a good test of its functioning.

Can you replace old bulbs with LED bulbs?

Yes, old bulbs can be replaced with LEDs. Most LED bulbs are designed to fit in the same sockets. LED bulbs have excellent performance, energy efficiency, and longer lifetimes, and they will save money over time.  However, consider compatibility with the fixture; some older lighting fixtures may not be compatible with LED bulbs due to their design or wiring.  Some dimmers, for example, will not work with LED bulbs. Make sure to check the manufacturer's recommendations or consult a licensed electrician if you are unsure.

How do I know if my LED is 12V or 24V?

A 12V LED will draw twice the amount of power as a 24V LED to achieve the same power level. The easiest way to determine the voltage of the LED is to check the label. The label on the LED driver, power supply, or on the bulb itself should indicate the input voltage (typically 100-240V AC) and the output voltage (either 12V DC or 24V DC). If the output voltage is 12V DC, then the LED is a 12V LED. If the output voltage is 24V DC, then the LED is a 24V LED.

How do LED lights affect humans?

LED lamps and displays appear to pose no direct adverse health effects among the healthy population. However, exposure to LEDs and other lights in the late evening affects our circadian rhythm (our natural sleep-wake cycle). In the case of UV LED bulbs designed for disinfecting, exposure over time can cause skin damage and other health problems.

Can an LED be run on any voltage?

No, an LED (Light Emitting Diode) cannot be run on any voltage. LEDs require a specific voltage and current to operate properly, and exceeding these limits can damage or destroy the LED.

Most LEDs have a recommended operating voltage and current listed in their datasheet or specifications. For example, a common 5mm LED may have a recommended operating voltage of around 2-3 volts and a current of around 20-30 milliamps (mA).

To operate an LED, you need to supply it with the correct voltage and current using a power source such as a battery or power supply. If the voltage is too low, the LED will not light up, while if the voltage is too high, the LED may be damaged or burnt out.

How do I know what power supply I need to run an LED?

To determine the power supply you need to run an LED, you need to know the voltage and current required to power the LED. The wattage required can be calculated by:

Wattage = Voltage x Current

For example, if your LED requires 12V and 0.5A to operate, then the wattage required would be:

Wattage = 12V x 0.5A = 6 watts

So, in this case, you would need a 12V power supply that can deliver at least 6 watts of power. It is important to note that the power supply you choose should be rated to deliver slightly more power than what the LED requires, to ensure reliable and stable operation. For example, if your LED requires 6 watts, you may want to choose a power supply that can deliver 7-8 watts or more.

When should you not use LED bulbs?

While LED bulbs are a popular and energy-efficient lighting option, there are certain situations where they may not be the best choice. Here are a few scenarios where you may want to avoid using LED bulbs:

In enclosed fixtures: LED bulbs produce heat as they operate, and if they are used in enclosed fixtures that don't allow for proper ventilation, this can cause the LED to overheat and shorten its lifespan. In some cases, this can even cause the LED to fail prematurely.

In dimmer switches: Some LED bulbs may not be compatible with dimmer switches, or may not work properly when used with a dimmer switch. This can cause flickering, buzzing, or reduced lifespan of the bulb. If you want to use LED bulbs with a dimmer switch, it is important to choose bulbs that are specifically designed for use with dimmer switches.

In sensitive environments: LED bulbs can produce high levels of blue light, which can be disruptive to our sleep-wake cycle and have other negative effects on our health. In environments where sleep and relaxation are important, such as bedrooms or living rooms, it may be better to use warm-white LED bulbs, which produce less blue light.

In outdoor fixtures: Not all LED bulbs are suitable for use in outdoor fixtures, particularly in cold or wet environments. Some LED bulbs may not be rated for outdoor use, and exposure to moisture or extreme temperatures can cause the bulb to fail prematurely.

Which resistor should I use with my LED?

LEDs typically require a current of 10 to 20mA. The datasheet for the LED will provide detailed information about the forward current and the forward voltage drop. The resistance can be calculated by using Ohms Law:

R = (V − V_LED) / I

Where V is the voltage of the source, V_LED is the forward voltage drop of LED, and I is the LED current.

Note that the resistor you choose should be rated to handle the power dissipated by the resistor. You can calculate the power dissipated by the resistor using the following formula:

P = I² x R

Where:

• P is the power dissipated by the resistor in watts
• I is the current flowing through the resistor in amps
• R is the resistance of the resistor in ohms

How long do LEDs last?

LED lights are known for their longevity and can last significantly longer than traditional incandescent or fluorescent bulbs. The lifespan of an LED bulb is typically measured in hours of use rather than years, as the number of hours a bulb is used can vary from person to person.

On average, LED lights can last anywhere from 25,000 to 50,000 hours of use, or even longer in some cases. This means that if you use an LED bulb for 3 hours per day, it could last for up to 20 years or more before needing to be replaced.

The lifespan of an LED bulb can be affected by a number of factors, including the quality of the bulb, the environment in which it is used (such as temperature and humidity), and the voltage and current it is supplied with. Using an LED bulb in a well-ventilated fixture and choosing a high-quality bulb can help to maximize its lifespan.

Do LEDs always require a driver circuit?

LEDs require a constant DC supply of 12v or 24v, which is much lower than the mains power supply voltage, so in general, LED lights need an LED driver to convert the power supply into a more suitable one. However, there are some low-power LEDs that can be powered directly from a voltage source, such as a battery or power supply, without the need for a driver circuit. These types of LEDs are typically used for simple applications such as indicator lights, and do not require the same level of control or protection as higher-power LEDs.

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