Thursday, January 28, 2010

Los Angeles is literally basking in a whole new glow based on LED Technology

Before: The orange glow of high pressure sodium lights on 6th Street Bridge over the Los Angeles River before they were replaced with LEDs.

The city has decided to replace its street lights and bus stop lighting with LEDs. The bus stop lighting will be solar-powered and off the grid.
LA's Bureau of Street Lighting has been actively testing out different types of energy efficient lighting to replace the public lighting that currently includes a combination of incandescent, mercury vapor, metal halide, and high pressure sodium lights.
In 2009, the agency began an LED street lighting energy efficiency program to actively replace its existing 209,000 streetlights. When complete, the city's energy consumption for public lighting should be cut by 40 percent and save 40,500 tons of carbon emissions per year, according to city statistics.
Now the city has decided on which specific lights to go with. Many of the street lamps will be lit by LED Lights. Because the bus lights are self-sufficient, they will not need to be tied into the city's electric grid and will allow the city to remain lit even in the event of a blackout.
These lights will also give the city more freedom to replace existing lights or introduce lights in new places without having to dig up sidewalks or tie into electricity poles, cutting down on installation costs.
But in addition to making the city more energy efficient, the switch from an abundance of high pressure sodium lights across the city's highways to LEDs is also drastically changing the city's look. Before and after photos provided by the city of the 6th Street Bridge over the Los Angeles River illustrate a clear change in tint from orange to whiter lighting.

After: The 6th Street Bridge after high pressure sodium streetlights were replaced with LEDs. The difference between before and after the replacement is spectacular.

Tuesday, January 26, 2010

Why do I need a resistor with an LED ?

The short answer: to limit the current in the LED to a safe value.
The long answer: LEDs are semiconductors, diodes in particular. The current flowing in an LED is an exponential function of voltage across the LED. The important part about that for you is that a small change in voltage can produce a huge change in current. That is the most important concept of this article. Resistors aren’t like that. The current and voltage in a resistor are linearly related. That means that a change in voltage will produce a proportional change in current. Current versus voltage is a straight line for a resistor, but not at all for an LED.

Because of this, you can’t say that LEDs have “resistance.” Resistance is defined as the constant ratio of voltage to current in a resistive circuit element. Even worse, there’s no real way to know exactly the relationship between current and voltage for any given LED across all possible voltages other than direct measurement. The exact relationship varies among different colors, different sizes, and even different batches from the same manufacturer. When you buy an LED, it should come with a rating that looks like this: 3.3V @ 20 mA typical. That gives you one point along the operating curve. Usually that’s a safe operating point. You may get a maximum rating in addition. It may be in the form of either a voltage or current. For example, a lot of people report buying “5V blue LEDs.” These are really not rated to operate continuously at 5V in most cases.

The other thing I’d like you to take away from this article is the idea that it’s more useful to talk about driving an LED with a current of a particular size, instead of a voltage. If you know the voltage across an LED, you can not determine the current flowing in it, unless you are operating it at the exact point along the curve that’s described in the specs. Worse, being “off by a little” in the forward voltage can have a drastic effect in the current. So the approach I prefer is to select a current-limiting resistor in order to achieve a target current in the LED.
Most 3mm and 5mm LEDs will operate close to their peak brightness at a drive current of 20 mA. This is a conservative current: it doesn’t exceed most ratings (your specs may vary, or you may not have any specs–in this case 20 mA is a good default guess). In most cases, driving the LED at a higher current will not produce substantial additional light. Instead, the junction (the working parts of the LED) has to dissipate the excess power as heat. Heating the junction will decrease its useful life, and can reduce the output of the LED substantially. Heating it enough will cause catastrophic failure (producing a dark emitting diode).

To illustrate these ideas, I conducted an experiment. I built a simple circuit consisting of a variable voltage supply driving an LED directly. I varied the voltage across the LED and measured the current that flowed. I had a 3000 mcd blue LED and a 5000 mcd white LED available to test, both 5mm. The results are in the graph below. It’s the most important thing in this article, and it’s worth repeating: a small change in voltage can produce a huge change in current. Note especially the portion of the curve between 3.2V and 3.4V. The current changes by a factor of 4 even though the voltage varies by 0.2V. While the specifics will be different for every LED, they all will have this sort of relationship. Overdriving an LED a little is going to degrade it substantially. Both the LEDs in the test were destroyed by the higher drive currents. They still lit up, but at a fraction of their original brightness.

Monday, January 25, 2010

What are the electrical characteristics of LEDs?

It’s useful to think about two main types of LEDs—the familiar indicator LEDs that come in 5mm and 3mm epoxy packages, and “illumination-grade” LEDs, which are high-output devices, designed for lighting.
A typical indicator LED has a forward voltage rating between 2 and 4 Volts of DC. You may see maximum ratings above that. A typical drive current for indicator LEDs, even high-brightness ones, is 20 milliamperes (mA). From this you can see an indicator LED dissipates a modest amount of power—a few tens of milliwatts compared to the few tens of Watts a familiar incandescent bulb uses. In other words, the power used by an indicator LED is one thousandth of that used by a familiar light bulb.

Arrays are constructed to take advantage of this low power consumption. A series string of ten blue LEDs will take a 33 VDC forward voltage to light, but still only draw 20 mA of current from the source. So the supporting wiring can be less expensive for an LED array compared to a light bulb (which may draw an Ampere of current—fifty times as much as the LED). For parallel arrays, the current combine. So you can drive 10 blue LEDs in parallel from a 3.3V source, but the current drawn will be 200 mA. This flexibility in array construction is part of what makes LEDs very popular in mobile, battery-powered devices. The designer can arrange LEDs to take best advantage of the power source that’s available.

Illumination-grade LEDs have comparable forward voltages to indicator LEDs. This is a reflection of the fact that the junction material is the main determinant of the forward voltage. But the junctions in illumination-grade LEDs are typically larger, and can draw more current, and dissipate more power (while producing more light). A Luxeon Star LED has a drive current of 350 mA, and dissipates about 1 Watt of power.
Another important LED spec is maximum reverse voltage. A diode conducts current when a forward voltage is applied, but will not conduct if a reverse voltage is applied, up to a point. Reverse voltages in excess of the maximum can cause the diode to fail.

Sunday, January 24, 2010

What makes an LED such a good colored light source ?

If you want a colored light, LEDs can’t be beat for efficiency. The process that gives off the light makes light of a certain wavelength, which is a function of the junction material. The efficiency of that process is much higher than with incandescent lamps—15% for LEDs compared with 5% for incandescent. Then when you filter an incandescent light down to a single color, you give up as much as 90% in the process. So when you need a colored light, such as a traffic signal, or an exit sign, LEDs have a significant advantage.

Conversely, LEDs aren’t yet as efficient at making white light. LEDs commonly make white light by the process of exciting a phosphor, which gives off other wavelengths of light, giving a combined effect of a white light. The conversion of the phosphor isn’t perfect, so some efficiency is lost.

Wednesday, January 20, 2010

HomeLights LED quiz: Fill the quiz and win fantastic HomeLights LED items

1.LED is the English acronym for Light Emitting Diode


2.LED bulbs and spotlights do not contain Mercury or any other gas


3.There are 4 types of LEDs


4.LED give off much less heat than incandescent and halogen bulbs.


5.LED do emit either UV nor infrared radiation


6.LED is insensitive to temperature changes


7.Lumen: This is the unit that measures the luminous power emitted by a source (or flux)


8.CFL has longer lifetime than LED


9.Consumption in Watts: This expresses the bulb’s real electricity consumption.


10.It is possible to buy HomeLights Led items in France


Pleas send your answers to The winner will receive fantastic HomeLights LED light items!!!!

Wednesday, January 13, 2010

LEDs last for 100,000 or 50,000 hours.

This statement is a heresy! Everyone knows the game about the “weakest link”... This applies perfectly to the luminous sources with LED technology. LED spotlights and bulbs are assemblies of components, but which component is the weakest link in this assembly?
The diode itself has a life span of 100,000 hours, but the transformer (or ballast) does not work for more than 30,000 hours. The transformer that regulates the LED current is, therefore, the assembly’s “weakest link”, which will limit the life span of a LED luminous light for the general public to 30,000 hours. So, for the moment, systematically avoid products announced as having life spans of more than 30,000 hours.

Thursday, January 7, 2010

How to properly choose your LED spotlight or bulb?

How to properly choose your LED spotlight or bulb?
In order to properly choose your LED luminous source, you must first of all ask yourself some essential questions:

For which uses? (this will help you to decide on the desired luminous power)

Be careful with dreamlike salesmen! At present, it is impossible to find LED lighting luminous sources for less than €20/25. Of course, if you only need signposting or decorative lighting, the products under €20 will do the job perfectly.

Wednesday, January 6, 2010

3 types of LED

There are several types of electroluminescent diodes (LEDs) with very different technical specifications.
The low power (LoP) LEDs are generally used in spotlights and decorative, signposting or atmospheric bulbs, for their luminous power is relatively low. Nevertheless, this technology offers the advantage of being cheap.

The SMD (Surface Mounted Device) LEDS offer sufficient lighting power to be incorporated into the bulbs or spotlights called “support lighting”. This technology offers an excellent lighting power/price ratio.

The high power (HiP) LEDS offer sufficient power to be incorporated into the so-called “direct lighting” bulbs and spotlights. So, there are spotlights on the market fitted with these LEDs that are capable of replacing 40-watt halogen spotlights, but they only consume 4 watts (i.e. 10 times less). However, this technology is relatively expensive to buy, but it proves to be very advantageous when used thanks to the three significant savings created.

So, 3 high power (HiP) LEDs will produce a luminous power that is well above 20 low power (LoP) LEDs. Consequently, the product will be more expensive.

Sunday, January 3, 2010

LED traffic lights don’t cause accidents, do save lives and energy

After an Illinois drive was killed by a car running a snow-covered red LED traffic light, a number of blogs and newspapers ran headlines such as LED traffic lights don’t melt snow, do cause accidents.

But in fact, LED traffic lights save lives. LEDs are up to four times brighter than incandescents and can be seen in bright sunlight as well as foggy conditions. Even more importantly, they last up to ten times longer than incandescents, so the danger of a burnout is greatly reduced.

So it’s a shame that these headlines based on a single unfortunate incident may discourage some holdouts from switching to LEDs that would actually save lives and energy. Switching to LEDs can cut energy costs by more than 80 percent, saving almost $600 per year in power per intersection. Portland, Oregon, for example, is saving $380,000 per year in energy and reduced maintenance.

Bill Kloos, Portland’s Signals and Streetlighting Manager, said, “The LEDs have reduced transportation maintenance costs by $45,000 a year in off-hour call out costs and replacement bulbs. LED modules have a life of six years or more while the current bulbs have only a two-year life. In addition, we’ve been able to save 1,400 hours of valuable staff time per year previously used for group relamping and apply that time to other maintenance needs.”