LIGHTINGTOWN

The Best Solution for LIGHTING Your TOWN!

Lighting Source Expert

Energy Saving Pioneer

Knowledge

 1.What is Light Emitting Diodes(LEDs)?
 

Example:       Circuit symbol:   

Function

LEDs emit light when an electric current passes through them.  

Connecting and soldering

LED connections LEDs must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method).
LEDs can be damaged by heat when soldering, but the risk is small unless you are very slow. No special precautions are needed for soldering most LEDs.
Testing an LED

Testing an LED

Never connect an LED directly to a battery or power supply!
It will be destroyed almost instantly because too much current will pass through and burn it out.
LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1kohm resistor is suitable for most LEDs if your supply voltage is 12V or less. Remember to connect the LED the correct way round!

Colours of LEDs

LED colours LEDs are available in red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colours.
The colour of an LED is determined by the semiconductor material, not by the colouring of the 'package' (the plastic body). LEDs of all colours are available in uncoloured packages which may be diffused (milky) or clear (often described as 'water clear'). The coloured packages are also available as diffused (the standard type) or transparent.  

Tri-colour LEDs

Tri-colour LED The most popular type of tri-colour LED has a red and a green LED combined in one package with three leads. They are called tri-colour because mixed red and green light appears to be yellow and this is produced when both the red and green LEDs are on.
The diagram shows the construction of a tri-colour LED. Note the different lengths of the three leads. The centre lead (k) is the common cathode for both LEDs, the outer leads (a1 and a2) are the anodes to the LEDs allowing each one to be lit separately, or both together to give the third colour.

Bi-colour LEDs

A bi-colour LED has two LEDs wired in 'inverse parallel' (one forwards, one backwards) combined in one package with two leads. Only one of the LEDs can be lit at one time and they are less useful than the tri-colour LEDs described above.

Sizes, Shapes and Viewing angles of LEDs


LED Clip, photograph © Rapid Electronics
LED Clip
Photograph © Rapid Electronics
 
 
LEDs are available in a wide variety of sizes and shapes. The 'standard' LED has a round cross-section of 5mm diameter and this is probably the best type for general use, but 3mm round LEDs are also popular.
Round cross-section LEDs are frequently used and they are very easy to install on boxes by drilling a hole of the LED diameter, adding a spot of glue will help to hold the LED if necessary. LED clips are also available to secure LEDs in holes. Other cross-section shapes include square, rectangular and triangular.
As well as a variety of colours, sizes and shapes, LEDs also vary in their viewing angle. This tells you how much the beam of light spreads out. Standard LEDs have a viewing angle of 60° but others have a narrow beam of 30° or less.
Rapid Electronics stock a wide selection of LEDs and their catalogue is a good guide to the range available.

Calculating an LED resistor value

LED resistor circuit An LED must have a resistor connected in series to limit the current through the LED, otherwise it will burn out almost instantly.
The resistor value, R is given by:  
R = (VS - VL) / I
VS = supply voltage
VL = LED voltage (usually 2V, but 4V for blue and white LEDs)
I = LED current (e.g. 20mA), this must be less than the maximum permitted
If the calculated value is not available choose the nearest standard resistor value which is greater, so that the current will be a little less than you chose. In fact you may wish to choose a greater resistor value to reduce the current (to increase battery life for example) but this will make the LED less bright.  

For example

If the supply voltage VS = 9V, and you have a red LED (VL = 2V), requiring a current I = 20mA = 0.020A,
R = (9V - 2V) / 0.02A = 350ohm, so choose 390ohm (the nearest standard value which is greater).  

Working out the LED resistor formula using Ohm's law

Ohm's law says that the resistance of the resistor, R = V/I, where:
  V = voltage across the resistor (= VS - VL in this case)
  I = the current through the resistor
So   R = (VS - VL) / I
For more information on the calculations please see the Ohm's Law page.

Connecting LEDs in series

LEDs in series If you wish to have several LEDs on at the same time it may be possible to connect them in series. This prolongs battery life by lighting several LEDs with the same current as just one LED.
All the LEDs connected in series pass the same current so it is best if they are all the same type. The power supply must have sufficient voltage to provide about 2V for each LED (4V for blue and white) plus at least another 2V for the resistor. To work out a value for the resistor you must add up all the LED voltages and use this for VL.
Example calculations:
A red, a yellow and a green LED in series need a supply voltage of at least 3 × 2V + 2V = 8V, so a 9V battery would be ideal.
VL = 2V + 2V + 2V = 6V (the three LED voltages added up).
If the supply voltage VS is 9V and the current I must be 15mA = 0.015A,
Resistor R = (VS - VL) / I = (9 - 6) / 0.015 = 3 / 0.015 = 200ohm,
so choose R = 220ohm (the nearest standard value which is greater).

Avoid connecting LEDs in parallel!

Do not connect LEDs in parallel! Connecting several LEDs in parallel with just one resistor shared between them is generally not a good idea.
If the LEDs require slightly different voltages only the lowest voltage LED will light and it may be destroyed by the larger current flowing through it. Although identical LEDs can be successfully connected in parallel with one resistor this rarely offers any useful benefit because resistors are very cheap and the current used is the same as connecting the LEDs individually. If LEDs are in parallel each one should have its own resistor.

Reading a table of technical data for LEDs

Suppliers' catalogues usually include tables of technical data for components such as LEDs. These tables contain a good deal of useful information in a compact form but they can be difficult to understand if you are not familiar with the abbreviations used.
The table below shows typical technical data for some 5mm diameter round LEDs with diffused packages (plastic bodies). Only three columns are important and these are shown in bold. Please see below for explanations of the quantities.  

Type Colour IF
max.
VF
typ.
VF
max.
VR
max.
Luminous
intensity
Viewing
angle
Wavelength
Standard Red 30mA 1.7V 2.1V 5V 5mcd @ 10mA 60° 660nm
Standard Bright red 30mA 2.0V 2.5V 5V 80mcd @ 10mA 60° 625nm
Standard Yellow 30mA 2.1V 2.5V 5V 32mcd @ 10mA 60° 590nm
Standard Green 25mA 2.2V 2.5V 5V 32mcd @ 10mA 60° 565nm
High intensity Blue 30mA 4.5V 5.5V 5V 60mcd @ 20mA 50° 430nm
Super bright Red 30mA 1.85V 2.5V 5V 500mcd @ 20mA 60° 660nm
Low current Red 30mA 1.7V 2.0V 5V 5mcd @ 2mA 60° 625nm
 

IF max. Maximum forward current, forward just means with the LED connected correctly.
VF typ. Typical forward voltage, VL in the LED resistor calculation.
This is about 2V, except for blue and white LEDs for which it is about 4V.
VF max. Maximum forward voltage.
VR max. Maximum reverse voltage
You can ignore this for LEDs connected the correct way round.
Luminous intensity Brightness of the LED at the given current, mcd = millicandela.
Viewing angle Standard LEDs have a viewing angle of 60°, others emit a narrower beam of about 30°.
Wavelength The peak wavelength of the light emitted, this determines the colour of the LED.
nm = nanometre.

Flashing LEDs

Flashing LEDs look like ordinary LEDs but they contain an integrated circuit (IC) as well as the LED itself. The IC flashes the LED at a low frequency, typically 3Hz (3 flashes per second). They are designed to be connected directly to a supply, usually 9 - 12V, and no series resistor is required. Their flash frequency is fixed so their use is limited and you may prefer to build your own circuit to flash an ordinary LED, for example our Flashing LED project which uses a 555 astable circuit.
2.Color Quality of White LEDs

Color quality is one of the key challenges facing light-emitting diodes (LEDs) as a general light source. This paper reviews the basics regarding light and color and summarizes the most important color issues related to white light LEDs.
Unlike incandescent and fluorescent lamps, LEDs are not inherently white light sources. Instead, LEDs emit light in a very narrow range of wavelengths in the visible spectrum, resulting in nearly monochromatic light. This is why LEDs are so efficient for colored light applications such as traffic lights and exit signs. However, to be used as a general light source, white light is needed. The potential of LED technology to produce high-quality white light with unprecedented energy efficiency is the impetus for the intense level of research and development currently being supported by the U.S. Department of Energy.
White Light from LEDs
White light can be achieved with LEDs in two main ways:
1) phosphor conversion, in which a blue or ultraviolet (UV) chip is coated with phosphor(s) to emit white light;
2) RGB systems, in which light from multiple monochromatic LEDs (red, green, and blue) is mixed, resulting in white light.
The phosphor conversion approach is most commonly based on a blue LED. When combined with a yellow phosphor (usually cerium-doped yttrium aluminum garnet or YAG:Ce), the light will appear white to the human eye. A more recently developed approach uses an LED emitting in the near-UV region of the spectrum to excite multi-chromatic phosphors to generate white light.
The RGB approach produces white light by mixing the three primary colors red, green, and blue. Color quality of the resulting light can be enhanced by the addition of amber to “fill in” the yellow region of the spectrum. Status, benefits and trade-offs of each approach are explored here after.
 
What is White Light?
What appears to our eyes as “white” is actually a mix of different wavelengths in the visible portion of the electromagnetic spectrum. Electromagnetic radiation in wavelengths from about 380 to 770 nanometers is visible to the human eye.
 
Example of a Typical Incandescent
Spectral Power Distribution
Incandescent, fluorescent, and high-intensity discharge (HID) lamps radiate across the visible spectrum, but with varying intensity in the different wavelengths. The spectral power distribution (SPD) for a given light source shows the relative radiant power emitted by the light source at each wavelength. Incandescent sources have a continuous SPD, but relative power is low in the blue and green regions. The typically “warm” color appearance of incandescent lamps is due to the relatively high emissions in the orange and red regions of the spectrum.
 Comparison of White Light LED Technologies
Each approach to producing white light with LEDs (described above) has certain advantages and disadvantages. The key trade-offs are among color quality, light output, luminous efficacy, and cost. The technology is changing rapidly due to intensive private and publicly funded research and development efforts in the U.S., Europe, and Asia. The primary pros and cons of each approach at the current level of technology development are outlined below. 

  Advantages Disadvantages
Blue LED + phosphor • Most mature technology
• High-volume manufacturing processes
• Relatively high luminous flux
• Relatively high efficacy
• Comparatively lower cost
• High CCT (cool/blue appearance)
• Low CRI (typically in the 70s)
• Color variability in beam
Near-UV LED + phosphor • Higher color rendering
• Warmer color temperatures possible
• Color appearance less affected by chip variations
• Less mature technology
• Relatively low efficacy
• Relatively low light output
RGB • Color flexibility, both in multi-color displays and different shades of white
• Potentially very high color rendering
• Individual colored LEDs respond differently to drive current, operating temperature, dimming, and operating time
• Controls needed for color consistency add expense 
Approaches to producing white light with LEDs
Most currently available white LED products are based on the blue LED + phosphor approach. Phosphor-converted chips are produced in large volumes and in various packages (light engines, arrays, etc.) that are integrated into lighting fixtures. RGB systems are more often custom designed for use in architectural settings.
Typical Luminous Efficacy and Color Characteristics of Current White LEDs
How do currently available white LEDs compare to traditional light sources in terms of color characteristics and luminous efficacy? Standard incandescent A-lamps provide about 15 lumens per watt (lpw), with CCT of around 2700 K and CRI close to 100. ENERGY STAR qualified compact fluorescent lamps (CFLs) produce about 50 lpw at 2700-3000 K and CRI at least 80. Typical efficacies of currently available LEDs from the leading chip manufacturers are shown below. Improvements are announced by the industry regularly.

CCT             CRI   70-79 80-89 90+
2600-3500 K 23-43 lpw   16 lpw
3500-5000 K 33-47 lpw 27 lpw  
> 5000 K 33-56 lpw 38 lpw  
Sources: Manufacturer datasheets for Cree XLamp 7090 XR, Lumileds Luxeon K2 Emitter, Luxeon Warm White Emitter, and Osram Opto OSTAR-Lighting. April 2006.
Source: DOE (US Department of Energy)