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Here are some more information for Bulb Led:

PAR 20 BULBS

PAR20 LED bulb has the capability that it uses just 4.6 watts and generates up to 300 lumens of light. This is an improved output as compared to the previously used bulbs. With a certain beam angle and E26 base, this family of LED bulbs is available in different   white color temperatures, two of them are 2800k and 6000k cool white. Other colors for PAR 20 bulbs are also available in the market.  With special research and engineering the PAR 20 bulbs are designed to last 50,000 hours.  As a result of this extremely long life maintenance costs are cut down and almost eliminate and this helps you see a return on your investment as LED Bulbs typically offer a payback period of less than 2 years.  These LED light bulbs are also environmentally responsible as they consume less energy than standard bulbs. Talking about their disposal it is also not a concern because these bulbs are lead free and they don’t have any mercury in them. These lights generate very little heat and emit no UV rays which make them an ideal choice for the museum and retail industry. In applications where vibration is a factor in the life of a light bulb, LED lights are the perfect solid state choice. These lights are also perfect for store fronts and point of purchase displays because these places are  typically illuminated for long hours.

The most widely used form of the PAR 20 bulbs is the halogen bulbs. PAR20 halogen light bulbs have a halogen capsule light in a small reflector as result they  give a great amount of light output, along with that they deliver good beam control. There are wide verities of PAR20 lamps including many wattages and several beam spreads. Halogen bulbs are incandescent bulbs Halogen bulbs, however, are filled with a special gas that actually rebuilds the filament and provides longer average life spans than normal They come in different shapes and sizes for various different uses. PAR stands for parabolic aluminized reflector. These types of bulbs will provide you with a high light output, a more white-colored light and longer life than normal. That is the main reason why the halogen are many times used in energy saving lighting situations. The PAR20 is a good direct replacement for the incandescent R20 lamps that have been used in track lighting or small recessed cans over the past decades. Replacing these will give you more light while reducing energy costs. You can determine the kind of PAR 20 bulb by measuring the distance across the widest point of the bulb. The number after PAR wil represent the number of eighths of an inch the bulb diameter is. This means that PAR20 lamps are 2 1/2 inches across.

Here's a helpful tip about lighting a space or area with these lamps. Try to make the area you're attempting to light a true highlight by making the light there three to five times as bright as the surrounding ambient light. This will cause attention to be drawn to the highlighted area. In many cases, people will gravitate toward these areas, so it's good to have product or something interesting there for people to look at. The small size of these reflector lamps make it possible to put them in almost any small space or recessed lighting.  Many people also use these halogen PAR20 bulbs for showcasing trophies, elevator, restaurants, clothing stores, jewelry, art work, sculptures, etc. and when shown under this light will appear more brilliant.  In the case of retail lighting, the way you light something can make the difference in selling something or not.

PAR 20 BULBS

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Najeeb

What type of resistor should i use for an LED bulb, that is powered by a USB cable?

I am making a light using a LED bulb and a USB cable. i have tried a 47k ,100k and a 10k resistor and all resulted in low lighting. Is the Resistor the problem or is it something else? and if it is the resistor, then what resistance level should i use? i got the directions on how to make the Light from this site ( http://forums.pcworld.co.nz/archive/index.php/t-52676.html ) however i am not completely sure if all the directions are right.

Some pointers for LED's:
When an LED begins to conduct, the voltage increases gradually while the current increases rapidly. Too much current will overheat the LED and eventually destroy it. The light emitted by an LED is directly proportional to current through the LED. The light output from an overheated LED will soon decrease and the LED may be damaged.
The forward voltage and wavelenght of an LED are directly related, so, it is not always possible to substitute a different LED w/o changing the voltage and current.
Some LED's table for voltage and wavelenght:
Wavelenght(nm)------Voltage
565(green)-------------2.2-3.0
590(yellow)-------------2.2-3.0
615(orange)------------1.8-2.7
640(red)------------------1.6-2.0
690(red)------------------2.2-3.0
880(infrared)------------2.2-2.5
900(infrared)------------1.2-1.6
940(infrared)------------1.3-1.7
don't know blue which is also white, but not have much difference to green.
LED's specified operating current limit must be respected for longevity and that operating LED's at their current limit does not significantly produce more brillance than at 60% of their current rating and ensure long operating life for most LED's.
Use this formula to determine the series resistance:
Rs=( +V-Vled)/Iled
where +V= voltage supply
Rs= series resistance
Vled= 1.7 V average
I led= LED current
Max. current= 20 ma(most LED)
Note:
if you're lazy in determining the resistance, then try this values:
LED voltage= 3.2-3.4V
480 ohm at 12 volts DC supply
240 ohms at 6 volts DC supply
200 ohms at 5 volts(USB voltage) DC
current will be around 25 ma with 30 ma maximum

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