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LED-Circuit
LED circuit
Simple LED circuit diagram
In electronics, the basic LED circuit is an electrical circuit used to power a light-emitting diode (LED). It consists of up to four components connected in series: a voltage source, a current limiting resistor, a LED, and optionally a switch to open and close the circuit. The switch may be replaced with another component or circuit to form a continuity tester Two diodes may be placed in parallel in the circuit, but connected anode to cathode; the second diode may be used to protect the LED against reverse bias, which can damage the LED, or it may be another LED which is illuminated when the polarity of the voltage source is reversed.
The LEDs used will have a forward voltage specified at the intended operating current. When the voltage source Ohm's law is used to calculate the resistor that is used to attain the correct current.[4][5] The resistor value is computed by subtracting the forward bias voltage from the supply voltage, and then dividing by the desired operating current.
This basic circuit is used in a wide range of applications, including many consumer appliances.
The formula to use to calculate the correct resistance for resistor to use is:
RESISTANCE(Ohms) =
(POWER SUPPLY VOLTAGE-LED VOLTAGE DROP)/LED CURRENT RATING
where:
Power supply voltage is the voltage of the power supply (such as a 9 volt battery)
LED voltage drop is the voltage drop across the LED (typically about 1.7 - 3.3 volts; this varies by the color of the LED)
LED current rating is the manufacturer rating of the LED (usually given in milliamperes such as 15 mA)
Simple LED circuit diagram
In electronics, the basic LED circuit is an electrical circuit used to power a light-emitting diode (LED). It consists of up to four components connected in series: a voltage source, a current limiting resistor, a LED, and optionally a switch to open and close the circuit. The switch may be replaced with another component or circuit to form a continuity tester Two diodes may be placed in parallel in the circuit, but connected anode to cathode; the second diode may be used to protect the LED against reverse bias, which can damage the LED, or it may be another LED which is illuminated when the polarity of the voltage source is reversed.
The LEDs used will have a forward voltage specified at the intended operating current. When the voltage source Ohm's law is used to calculate the resistor that is used to attain the correct current.[4][5] The resistor value is computed by subtracting the forward bias voltage from the supply voltage, and then dividing by the desired operating current.
This basic circuit is used in a wide range of applications, including many consumer appliances.
The formula to use to calculate the correct resistance for resistor to use is:
RESISTANCE(Ohms) =
(POWER SUPPLY VOLTAGE-LED VOLTAGE DROP)/LED CURRENT RATING
where:
Power supply voltage is the voltage of the power supply (such as a 9 volt battery)
LED voltage drop is the voltage drop across the LED (typically about 1.7 - 3.3 volts; this varies by the color of the LED)
LED current rating is the manufacturer rating of the LED (usually given in milliamperes such as 15 mA)
LED-Introduction
History
The phenomenon of solid state junctions producing light was discovered in the crystal detector era. In the 1960s commercial red LED's became available, and by the 1970s these were in widespread use as indicators in a very wide range of equipment. These early LED's had much too small an output to be useful as lighting. They replaced the previously widely used indicator types of filament lamps and neon. Compared to neon, indicator LED's have longer lifetimes and run on lower voltage; compared to miniature filament lamps, indicator LED's have much longer lifetimes, such that they do not require replacement, and consume less power. The lack of need for replacement also eliminates the need for bulb sockets and a user access port.
Commercial amber (yellow) and orange LED's followed, and were used where differentiation of multiple LEDs was required. For many years LED's came in infra-red, red, orange, yellow, and green. Blue, cyan, and violet LEDs finally appeared in the 1990s.
To produce a white SSL device, a blue LED was needed. In 1993, Shuji Nakamura of Nichia Corporation came up with a blue LED using gallium nitride (GaN). With this invention, it was now possible to create white light by combining the light of separate LED's (red, green, and blue), or by placing a blue LED in a package with an internal light converting phosphor. With the phosphor type, some of the blue output becomes either yellow or red and green with the result that the LED light emission appears white to the human eye.
In 2008, SSL technology advanced to the point that Sentry Equipment Corporation in Oconomowoc, Wis. was able to light its new factory almost entirely with LEDs, both interior and exterior. Although the initial cost was three times more than a traditional mixture of incandescent and fluorescent bulbs, the extra cost will be repaid within two years from electricity savings, and the bulbs should not need replacement for 20 years.
Technology overview
A single LED diode can produce only a limited amount of light, and only a single color at a time. To produce the white light necessary for SSL, light spanning the visible spectrum (red, green, and blue) must be generated in approximately correct proportions. To achieve this, three approaches are used for generating white light with LEDs: wavelength conversion, color mixing, and most recently Homoepitaxial ZnSe.
Wavelength conversion involves converting some or all of the LED’s output into visible wavelengths. Methods used to accomplish this feat include:
Blue LED & yellow phosphor – Considered the least expensive method for producing white light. Blue light from an LED is used to excite a phosphor which then re-emits yellow light. This balanced mixing of yellow and blue lights results in the appearance of white light, but produces poor color rendition (i.e., has low CRI).
Blue LED & several phosphors – Similar to the process involved with yellow phosphors, except that each excited phosphor re-emits a different color. Similarly, the resulting light is combined with the originating blue light to create white light. The resulting light, however, has a richer and broader wavelength spectrum and produces a higher color-quality light, albeit at an increased cost.
Ultraviolet (UV) LED & red, green, & blue phosphors – The UV light is used to excite the different phosphors, which are doped at measured amounts. The colors are mixed resulting in a white light with the richest and broadest wavelength spectrum.
Blue LED & quantum dots – A process by which a thin layer of nanocrystal particles containing 33 or 34 pairs of atoms, primarily cadmium and selenium, are coated on top of the LED. The blue light excites the quantum dots, resulting in a white light with a wavelength spectrum similar to UV LEDs.
Color mixing involves using multiple colors of LEDs in a lamp to produce white light. Such lamps contain a minimum of two LEDs (blue and yellow), but can also have three (red, blue, and green) or four (red, blue, green, and yellow). As no phosphors are used, there is no energy lost in the conversion process, thereby exhibiting the potential for higher efficiency.
Homoepitaxial ZnSe is a technology developed by Sumomito Electric where a LED is grown on a ZnSe substrate, which simultaneously produces blue light from the active region and yellow emission from the substrate. The resulting white light has a wavelength spectrum on par with UV LEDs. No phosphors are used, resulting in a higher efficiency white LED.
To be considered SSL, however, a multitude of LEDs must be placed close together in a lamp to add their illuminating effects. This is because an individual LED produces only a small amount of light, thereby limiting its effectiveness as a replacement light source. In the case where white LEDs are utilized in SSL, this is a relatively simple task, as all LEDs are of the same color and can be arranged in any fashion. When using the color-mixing method, however, it is more difficult to generate equivalent brightness when compared to using white LEDs in a similar lamp size. Furthermore, degradation of different LEDs at various times in a color-mixed lamp can lead to an uneven color output. Because of the inherent benefits and greater number of applications for white LED based SSL, most designs focus on utilizing them exclusively.
Driving LEDs
LEDs have very low dynamic resistance, with the same voltage drop for widely varying currents. Consequently they can not connect direct to most power sources without causing self destruction. A current control ballast is normally used, which is sometimes constant current.
Indicator LEDs
Miniature indicator LEDs are normally driven from low voltage DC via a current limiting resistor. Currents of 2mA, 10mA and 20mA are common. Some low current indicators are only rated to 2mA, and should not be driven at higher current.
Sub-mA indicators may be made by driving ultrabright LEDs at very low current. Efficacy tends to reduce at low currents, but indicators running on 100uA are still practical. The cost of ultrabrights is higher than 2mA indicator LEDs.
LEDs have a low max repeat reverse voltage rating, ranging from apx 2v to 5v, and this can be a problem in some applications. Back to back LEDs are immune to this problem. These are available in single color as well as bicolor types. There are various strategies for reverse voltage handling.
In niche applications such as IR therapy, LEDs are often driven at far above rated current. This causes high failure rate and occasional LED explosions. Thus many parallel strings are used, and a safety screen and ongoing maintenance are required.
Alphanumeric LEDs
These use the same drive strategy as indicator LEDs, the only difference being the larger number of channels, each with its own resistor. 7 segment and starburst LED arrays are available in both common anode or common cathode forms.
Lighting LEDs on mains
A CR dropper (capacitor & resistor) followed by full wave rectification is the usual ballast with mains driven series-parallel LED clusters.
A single series string would minimise dropper losses, but one LED failure would extinguish the whole string. Parallelled strings increase reliability. In practice usually 3 strings or more are used.
Operation on square wave and modified sine wave (MSW) sources, such as many inverters, causes heavily increased resistor dissipation in CR droppers, and LED ballasts designed for sine wave use tend to burn on non-sine waveforms. The non-sine waveform also causes high peak LED currents, heavily shortening LED life. An inductor & rectifier makes a more suitable ballast for such use, and other options are also possible.
Lighting LEDs on low voltage
LEDs are normally operated in parallel strings of series LEDs, with the total LED voltage typically adding up to around 2/3 of the supply voltage, and resistor current control for each string.
In resistor-drive devices, LED current is then proportional to power supply (PSU) voltage minus total LED string voltage. Where battery sources are used, the PSU voltage can vary widely, causing large changes in LED current and therefore color and light output. For such applications, a constant current regulator is preferred to resistor control. Low drop-out (LDO) constant current regs also allow the total LED string voltage to be a higher percentage of PSU voltage, resulting in improved efficiency and reduced power use.
Torches run one or more lighting LEDs on a low voltage battery. These usually use a resistor ballast.
In disposable coin cell powered keyring type LED lights, the resistance of the cell itself is usually the only current limiting device. The cell should not therefore be replaced with a lower resistance type, such as one using a different battery chemistry.
Finally, an LED can be run from a single cell by use of a constant current switched mode inverter. While adding additional expense, this method provides a high level of color and brightness control, and ensures longer LED lifetime.
Comparison to other lighting technologies :
Incandescent lamps (light bulbs) create light by running electricity through a thin filament, thereby heating the filament to a very high temperature so that it glows and produces visible light. A broad range of visible frequencies are naturally produced, yielding a pleasing warm yellow or white color quality. The incandescing process, however, is highly inefficient, as over 98% of its energy input is emitted as heat.[citation needed] A standard 100 watt 120 VAC light bulb produces about 1700 lumens, about 17 lumens per watt. Incandescent lamps are relatively inexpensive to produce. The typical lifespan of a mains incandescent lamp is around 1,000 hours.[citation needed] They work well with dimmers. Most existing light fixtures are designed for the size and shape of these traditional bulbs.
Fluorescent lamps (light bulbs) work by passing electricity through mercury vapor, which in turn produces ultraviolet light. The ultraviolet light is then absorbed by a phosphor coating inside the lamp, causing it to glow, or fluoresce. While the heat generated by fluorescent lamps is much less than its incandescent counterpart, energy is still lost in generating the ultraviolet light and converting this light into visible light. If the lamp breaks exposure to mercury can occur. Linear fluorescent lamps are typically five to six times the cost of incandescent lamps[citation needed], but have life spans around 10,000 and 20,000 hours. Lifetime varies from 1,200 hours to 20,000 hours for compact fluorescent lamps.
The efficacy of fluorescent tubes with modern electronic ballast commonly averages 50 to 67 lm/W overall. Most compact fluorescents 13 watts or more with integral electronic ballasts achieve about 60 lumens/watt. They should be recycled rather than disposed to prevent mercury release. Some flicker at 100 or 120 Hz, and the quality of the light tends to be a harsh white due to the lack of a broad band of frequencies. Most are not compatible with dimmers.
Neon lamp (light bulbs) used like night-lamp in children's room. Typically a 230 V (in Europe) is rated 0.5 W of power.
SSL/LEDs LEDs come in multiple colors, which are produced without the need for filters. A white SSL can be comprised of a single high-power LED, multiple white LEDs, or from LEDs of different colors mixed to produce white light. Advantages include:
High efficiency - LEDs are now available that reliably offer over 100 lumens from a one-watt device, or much higher outputs at higher drive currents
Small size - provides design flexibility, arranged in rows, rings, clusters, or individual points
High durability - no filament or tube to break
Life span - in properly engineered lamps, LEDs can last 50,000 - 60,000 hours
Full dimmability – unlike fluorescent lamps, LEDs can be dimmed using pulse-width modulation (PWM - turning the light on and off very quickly at varying intervals). This also allows full color mixing in lamps with LEDs of different colors.[1]
Mercury-free - unlike fluorescent and most HID technologies, LEDs contain no hazardous mercury or halogen gases
However, some current models are not compatible with standard dimmers. It is not currently practical to produce high levels of room lighting. As a result, current LED screw-in light bulbs offer either low levels of light at a moderate cost, or moderate levels of light at a high cost. In contrast to other lighting technologies, LED light tends to be directional. This is a disadvantage for most general lighting applications, but can be an advantage for spot or flood lighting.
Because individual LEDs are low-voltage DC devices, implementing SSL to operate from mains AC requires well designed circuitry and a thermal case to dissipate the heat.
Applications
This garden light can use stored solar energy due to the low power consumption of its LED
Traffic lights
Automotive lighting
Stage lighting
Bicycle lighting
Flashlight (Electric torches)
Domestic lighting
Public Transit Vehicle Destination signs
Billboard displays
Floodlighting of buildings
Display lighting in art galleries to achieve a low heating effect on pictures etc.
Train lights (Now common on nearly all modern and most older MU's and Loco's in the UK)
Challenges
The current manufacturing process of white LEDs has not matured enough for them to be produced at low enough cost for widespread use. There are multiple manufacturing hurdles that must be overcome. The process used to deposit the active semiconductor layers of the LED must be improved to increase yields and manufacturing throughput. Problems with phosphors, which are needed for their ability to emit a broader wavelength spectrum of light, have also been an issue. In particular, the inability to tune the absorption and emission, and inflexibility of form have been issues in taking advantage of the phosphors spectral capabilities.
More apparent to the end user, however, is the low Color Rendering Index (CRI) of current LEDs. The current generation of LEDs, which employs mostly blue LED chip + yellow phosphor, has a CRI around 70, which is much too low for widespread use in indoor lighting. (CRI is used to measure how accurately a lighting source renders the color of objects. Sunlight and some incandescent lamps have a perfect CRI of 100, while white fluorescent lamps have CRI varying from the 50s to 95.) Better CRI LEDs are more expensive, and more research & development is needed to reduce costs. End user costs are still too high to make it a viable option, for instance, Maplin's website quotes comparable LED spots at £9.99 GBP[4] against the standard Halogen lamp twin pack which comes in at £6.49 GBP (or roughly £3.25 GBP each).
Variations of CCT (color correlated temperature) at different viewing angles present another obstacle against widespread use of white LED. It has been shown, that CCT variations can exceed 500 K, which is clearly noticeable by human observer, who is normally capable of distinguishing CCT differences of 50 to 100 K in range from 2000 K to 6000 K, which is the range of CCT variations of daylight.
LEDs also have limited temperature tolerance and falling efficiency as temperature rises. This limits the total LED power that can practically be fitted into lamps that physically replace existing filament & compact fluorescent types. R&D is needed to improve thermal characteristics.
The long life of SSL products, expected to be about 50 times the most common incandescent bulbs, poses a problem for bulb makers, whose current customers buy frequent replacements.
Research and developmet
Feb 2008 - Bilkent university - Turkey reports 300 lumens of visible light per watt luminous efficacy (not per electrical watt) and warm light by using nanocrystals [2].
Philips Lighting has ceased research on compact fluorescents, and is devoting the bulk of its R.& D. budget, 5 percent of the company’s global lighting revenue, to SSL
The phenomenon of solid state junctions producing light was discovered in the crystal detector era. In the 1960s commercial red LED's became available, and by the 1970s these were in widespread use as indicators in a very wide range of equipment. These early LED's had much too small an output to be useful as lighting. They replaced the previously widely used indicator types of filament lamps and neon. Compared to neon, indicator LED's have longer lifetimes and run on lower voltage; compared to miniature filament lamps, indicator LED's have much longer lifetimes, such that they do not require replacement, and consume less power. The lack of need for replacement also eliminates the need for bulb sockets and a user access port.
Commercial amber (yellow) and orange LED's followed, and were used where differentiation of multiple LEDs was required. For many years LED's came in infra-red, red, orange, yellow, and green. Blue, cyan, and violet LEDs finally appeared in the 1990s.
To produce a white SSL device, a blue LED was needed. In 1993, Shuji Nakamura of Nichia Corporation came up with a blue LED using gallium nitride (GaN). With this invention, it was now possible to create white light by combining the light of separate LED's (red, green, and blue), or by placing a blue LED in a package with an internal light converting phosphor. With the phosphor type, some of the blue output becomes either yellow or red and green with the result that the LED light emission appears white to the human eye.
In 2008, SSL technology advanced to the point that Sentry Equipment Corporation in Oconomowoc, Wis. was able to light its new factory almost entirely with LEDs, both interior and exterior. Although the initial cost was three times more than a traditional mixture of incandescent and fluorescent bulbs, the extra cost will be repaid within two years from electricity savings, and the bulbs should not need replacement for 20 years.
Technology overview
A single LED diode can produce only a limited amount of light, and only a single color at a time. To produce the white light necessary for SSL, light spanning the visible spectrum (red, green, and blue) must be generated in approximately correct proportions. To achieve this, three approaches are used for generating white light with LEDs: wavelength conversion, color mixing, and most recently Homoepitaxial ZnSe.
Wavelength conversion involves converting some or all of the LED’s output into visible wavelengths. Methods used to accomplish this feat include:
Blue LED & yellow phosphor – Considered the least expensive method for producing white light. Blue light from an LED is used to excite a phosphor which then re-emits yellow light. This balanced mixing of yellow and blue lights results in the appearance of white light, but produces poor color rendition (i.e., has low CRI).
Blue LED & several phosphors – Similar to the process involved with yellow phosphors, except that each excited phosphor re-emits a different color. Similarly, the resulting light is combined with the originating blue light to create white light. The resulting light, however, has a richer and broader wavelength spectrum and produces a higher color-quality light, albeit at an increased cost.
Ultraviolet (UV) LED & red, green, & blue phosphors – The UV light is used to excite the different phosphors, which are doped at measured amounts. The colors are mixed resulting in a white light with the richest and broadest wavelength spectrum.
Blue LED & quantum dots – A process by which a thin layer of nanocrystal particles containing 33 or 34 pairs of atoms, primarily cadmium and selenium, are coated on top of the LED. The blue light excites the quantum dots, resulting in a white light with a wavelength spectrum similar to UV LEDs.
Color mixing involves using multiple colors of LEDs in a lamp to produce white light. Such lamps contain a minimum of two LEDs (blue and yellow), but can also have three (red, blue, and green) or four (red, blue, green, and yellow). As no phosphors are used, there is no energy lost in the conversion process, thereby exhibiting the potential for higher efficiency.
Homoepitaxial ZnSe is a technology developed by Sumomito Electric where a LED is grown on a ZnSe substrate, which simultaneously produces blue light from the active region and yellow emission from the substrate. The resulting white light has a wavelength spectrum on par with UV LEDs. No phosphors are used, resulting in a higher efficiency white LED.
To be considered SSL, however, a multitude of LEDs must be placed close together in a lamp to add their illuminating effects. This is because an individual LED produces only a small amount of light, thereby limiting its effectiveness as a replacement light source. In the case where white LEDs are utilized in SSL, this is a relatively simple task, as all LEDs are of the same color and can be arranged in any fashion. When using the color-mixing method, however, it is more difficult to generate equivalent brightness when compared to using white LEDs in a similar lamp size. Furthermore, degradation of different LEDs at various times in a color-mixed lamp can lead to an uneven color output. Because of the inherent benefits and greater number of applications for white LED based SSL, most designs focus on utilizing them exclusively.
Driving LEDs
LEDs have very low dynamic resistance, with the same voltage drop for widely varying currents. Consequently they can not connect direct to most power sources without causing self destruction. A current control ballast is normally used, which is sometimes constant current.
Indicator LEDs
Miniature indicator LEDs are normally driven from low voltage DC via a current limiting resistor. Currents of 2mA, 10mA and 20mA are common. Some low current indicators are only rated to 2mA, and should not be driven at higher current.
Sub-mA indicators may be made by driving ultrabright LEDs at very low current. Efficacy tends to reduce at low currents, but indicators running on 100uA are still practical. The cost of ultrabrights is higher than 2mA indicator LEDs.
LEDs have a low max repeat reverse voltage rating, ranging from apx 2v to 5v, and this can be a problem in some applications. Back to back LEDs are immune to this problem. These are available in single color as well as bicolor types. There are various strategies for reverse voltage handling.
In niche applications such as IR therapy, LEDs are often driven at far above rated current. This causes high failure rate and occasional LED explosions. Thus many parallel strings are used, and a safety screen and ongoing maintenance are required.
Alphanumeric LEDs
These use the same drive strategy as indicator LEDs, the only difference being the larger number of channels, each with its own resistor. 7 segment and starburst LED arrays are available in both common anode or common cathode forms.
Lighting LEDs on mains
A CR dropper (capacitor & resistor) followed by full wave rectification is the usual ballast with mains driven series-parallel LED clusters.
A single series string would minimise dropper losses, but one LED failure would extinguish the whole string. Parallelled strings increase reliability. In practice usually 3 strings or more are used.
Operation on square wave and modified sine wave (MSW) sources, such as many inverters, causes heavily increased resistor dissipation in CR droppers, and LED ballasts designed for sine wave use tend to burn on non-sine waveforms. The non-sine waveform also causes high peak LED currents, heavily shortening LED life. An inductor & rectifier makes a more suitable ballast for such use, and other options are also possible.
Lighting LEDs on low voltage
LEDs are normally operated in parallel strings of series LEDs, with the total LED voltage typically adding up to around 2/3 of the supply voltage, and resistor current control for each string.
In resistor-drive devices, LED current is then proportional to power supply (PSU) voltage minus total LED string voltage. Where battery sources are used, the PSU voltage can vary widely, causing large changes in LED current and therefore color and light output. For such applications, a constant current regulator is preferred to resistor control. Low drop-out (LDO) constant current regs also allow the total LED string voltage to be a higher percentage of PSU voltage, resulting in improved efficiency and reduced power use.
Torches run one or more lighting LEDs on a low voltage battery. These usually use a resistor ballast.
In disposable coin cell powered keyring type LED lights, the resistance of the cell itself is usually the only current limiting device. The cell should not therefore be replaced with a lower resistance type, such as one using a different battery chemistry.
Finally, an LED can be run from a single cell by use of a constant current switched mode inverter. While adding additional expense, this method provides a high level of color and brightness control, and ensures longer LED lifetime.
Comparison to other lighting technologies :
Incandescent lamps (light bulbs) create light by running electricity through a thin filament, thereby heating the filament to a very high temperature so that it glows and produces visible light. A broad range of visible frequencies are naturally produced, yielding a pleasing warm yellow or white color quality. The incandescing process, however, is highly inefficient, as over 98% of its energy input is emitted as heat.[citation needed] A standard 100 watt 120 VAC light bulb produces about 1700 lumens, about 17 lumens per watt. Incandescent lamps are relatively inexpensive to produce. The typical lifespan of a mains incandescent lamp is around 1,000 hours.[citation needed] They work well with dimmers. Most existing light fixtures are designed for the size and shape of these traditional bulbs.
Fluorescent lamps (light bulbs) work by passing electricity through mercury vapor, which in turn produces ultraviolet light. The ultraviolet light is then absorbed by a phosphor coating inside the lamp, causing it to glow, or fluoresce. While the heat generated by fluorescent lamps is much less than its incandescent counterpart, energy is still lost in generating the ultraviolet light and converting this light into visible light. If the lamp breaks exposure to mercury can occur. Linear fluorescent lamps are typically five to six times the cost of incandescent lamps[citation needed], but have life spans around 10,000 and 20,000 hours. Lifetime varies from 1,200 hours to 20,000 hours for compact fluorescent lamps.
The efficacy of fluorescent tubes with modern electronic ballast commonly averages 50 to 67 lm/W overall. Most compact fluorescents 13 watts or more with integral electronic ballasts achieve about 60 lumens/watt. They should be recycled rather than disposed to prevent mercury release. Some flicker at 100 or 120 Hz, and the quality of the light tends to be a harsh white due to the lack of a broad band of frequencies. Most are not compatible with dimmers.
Neon lamp (light bulbs) used like night-lamp in children's room. Typically a 230 V (in Europe) is rated 0.5 W of power.
SSL/LEDs LEDs come in multiple colors, which are produced without the need for filters. A white SSL can be comprised of a single high-power LED, multiple white LEDs, or from LEDs of different colors mixed to produce white light. Advantages include:
High efficiency - LEDs are now available that reliably offer over 100 lumens from a one-watt device, or much higher outputs at higher drive currents
Small size - provides design flexibility, arranged in rows, rings, clusters, or individual points
High durability - no filament or tube to break
Life span - in properly engineered lamps, LEDs can last 50,000 - 60,000 hours
Full dimmability – unlike fluorescent lamps, LEDs can be dimmed using pulse-width modulation (PWM - turning the light on and off very quickly at varying intervals). This also allows full color mixing in lamps with LEDs of different colors.[1]
Mercury-free - unlike fluorescent and most HID technologies, LEDs contain no hazardous mercury or halogen gases
However, some current models are not compatible with standard dimmers. It is not currently practical to produce high levels of room lighting. As a result, current LED screw-in light bulbs offer either low levels of light at a moderate cost, or moderate levels of light at a high cost. In contrast to other lighting technologies, LED light tends to be directional. This is a disadvantage for most general lighting applications, but can be an advantage for spot or flood lighting.
Because individual LEDs are low-voltage DC devices, implementing SSL to operate from mains AC requires well designed circuitry and a thermal case to dissipate the heat.
Applications
This garden light can use stored solar energy due to the low power consumption of its LED
Traffic lights
Automotive lighting
Stage lighting
Bicycle lighting
Flashlight (Electric torches)
Domestic lighting
Public Transit Vehicle Destination signs
Billboard displays
Floodlighting of buildings
Display lighting in art galleries to achieve a low heating effect on pictures etc.
Train lights (Now common on nearly all modern and most older MU's and Loco's in the UK)
Challenges
The current manufacturing process of white LEDs has not matured enough for them to be produced at low enough cost for widespread use. There are multiple manufacturing hurdles that must be overcome. The process used to deposit the active semiconductor layers of the LED must be improved to increase yields and manufacturing throughput. Problems with phosphors, which are needed for their ability to emit a broader wavelength spectrum of light, have also been an issue. In particular, the inability to tune the absorption and emission, and inflexibility of form have been issues in taking advantage of the phosphors spectral capabilities.
More apparent to the end user, however, is the low Color Rendering Index (CRI) of current LEDs. The current generation of LEDs, which employs mostly blue LED chip + yellow phosphor, has a CRI around 70, which is much too low for widespread use in indoor lighting. (CRI is used to measure how accurately a lighting source renders the color of objects. Sunlight and some incandescent lamps have a perfect CRI of 100, while white fluorescent lamps have CRI varying from the 50s to 95.) Better CRI LEDs are more expensive, and more research & development is needed to reduce costs. End user costs are still too high to make it a viable option, for instance, Maplin's website quotes comparable LED spots at £9.99 GBP[4] against the standard Halogen lamp twin pack which comes in at £6.49 GBP (or roughly £3.25 GBP each).
Variations of CCT (color correlated temperature) at different viewing angles present another obstacle against widespread use of white LED. It has been shown, that CCT variations can exceed 500 K, which is clearly noticeable by human observer, who is normally capable of distinguishing CCT differences of 50 to 100 K in range from 2000 K to 6000 K, which is the range of CCT variations of daylight.
LEDs also have limited temperature tolerance and falling efficiency as temperature rises. This limits the total LED power that can practically be fitted into lamps that physically replace existing filament & compact fluorescent types. R&D is needed to improve thermal characteristics.
The long life of SSL products, expected to be about 50 times the most common incandescent bulbs, poses a problem for bulb makers, whose current customers buy frequent replacements.
Research and developmet
Feb 2008 - Bilkent university - Turkey reports 300 lumens of visible light per watt luminous efficacy (not per electrical watt) and warm light by using nanocrystals [2].
Philips Lighting has ceased research on compact fluorescents, and is devoting the bulk of its R.& D. budget, 5 percent of the company’s global lighting revenue, to SSL
LED - Technology Inside
LEDs may be little, but new high-brightness models are producing a considerable amount of light.
First used as status and indicator lamps, and more recently in under-shelf illumination, accent lighting, and directional marking applications, high-brightness LEDs have emerged within the last six years. But only recently have they been seriously looked upon as a feasible option in general purpose lighting applications. Before you recommend or install this type of lighting system, you should understand the basic technology upon which these devices are based
Light-emitting diodes (LEDs) are solid-state devices that convert electric energy directly into light of a single color. Because they employ “cold” light generation technology, in which most of the energy is delivered in the visible spectrum, LEDs don't waste energy in the form of non-light producing heat. In comparison, most of the energy in an incandescent lamp is in the infrared (or non-visible) portion of the spectrum. As a result, both fluorescent and HID lamps produce a great deal of heat. In addition to producing cold light, LEDs:
-Can be powered from a portable battery pack or even a solar array.
-Can be integrated into a control system.
-Are small in size and resistant to vibration and shock.
-Have a very fast “on-time” (60 nsec vs 10 msec for an incandescent lamp).
-Have good color resolution and present low, or no, shock hazard
The centerpiece of a typical LED is a diode that is chip-mounted in a reflector cup and held in place by a mild steel lead frame connected to a pair of electrical wires. The entire arrangement is then encapsulated in epoxy. The diode chip is generally about 0.25 mm square. When current flows across the junction of two different materials, light is produced from within the solid crystal chip. The shape, or width, of the emitted light beam is determined by a variety of factors: the shape of the reflector cup, the size of the LED chip, the shape of the epoxy lens and the distance between the LED chip and the epoxy lens. The composition of the materials determines the wavelength and color of light. In addition to visible wavelengths, LEDs are also available in infrared wavelengths, from 830 nm to 940 nm.
The definition of “life” varies from industry to industry. The useful life for a semiconductor is defined as the calculated time for the light level to decline to 50% of its original value. For the lighting industry, the average life of a particular lamp type is the point where 50% of the lamps in a representative group have burned out. The life of an LED depends on its packaging configuration, drive current, and operating environment. A high ambient temperature greatly shortens an LED's life.
Additionally, LEDs now cover the entire light spectrum, including red, orange, yellow, green, blue, and white. Although colored light is useful for more creative installations, white light remains the holy grail of LED technology. Until a true white is possible, researchers have developed three ways to deliver it:
Blend the beams. This technique involves mixing the light from multiple single-color devices. (Typically red, blue, and green.) Adjusting the beams' relative intensity yields the desired color.
Provide a phosphor coating. When energized photons from a blue LED strike a phosphor coating, it will emit light as a mixture of wavelengths to produce a white color.
Create a light sandwich. Blue light from one LED device elicits orange light from an adjacent layer of a different material. The complementary colors mix to produce white. Of the three methods, the phosphor approach appears to be the most promising technology.
Another shortcoming of early LED designs was light output, so researchers have been working on several methods for increasing lumens per watt. A new “doping” technique increases light output several times over compared to earlier generations of LEDs. Other methods under development include:
-Producing larger semiconductors.
-Passing larger currents with better heat extraction.
-Designing a different shape for the device.
-Improving light conversion efficiency.
-Packaging several LEDs within a single epoxy dome.
One family of LEDs may already be closer to improved light output. Devices with enlarged chips produce more light while maintaining proper heat and current management. These advances allow the units to generate 10 times to 20 times more light than standard indicator lights, making them a practical illumination source for lighting fixtures.
Before LEDs can enter the general illumination market, designers and advocates of the technology must overcome several problems, including the usual obstacles to mainstream market adoption: Industry-accepted standards must be developed and costs must be reduced. But more specific issues remain. Things like lumen-per-watt efficacy and color consistency must be improved, and reliability and lumen maintenance should be addressed. Nevertheless, LEDs are well on their way to becoming a viable lighting alternative.
Audi R8 - First full-LED headlamps
Audi announced that effective immediately, options available for the Audi R8 will also include the first full-LED headlamp. The daytime running light now features 24 LEDs per headlamp along with light-emitting diodes for turn signals and low-beam and high-beam headlights.
Audi introduced a strip-shaped fog lamp at the Detroit Auto Show in 2003. Following which, the Audi Nuvolari quattro featured the first full-LED headlamp at the Geneva Auto Show
The option will run you €3,590 (or $5,585 on the stateside), which we think is way too expensive. We doubt anyone would look at your R8 and go “Oh man you didn’t opt for the full-LED headlamps?” Well, unless we’re checking out your car - then we’d just make you feel bad about not cashing out almost $6,000 for full-LED headlamps.
First full-LED headlamp for the Audi R8
- 54 high-performance LEDs provide all lighting functions
- Color similar to daylight offers advantages at night
- Innovative lighting technology as an option
Ingolstadt – Effective immediately, the range of equipment available for the Audi R8 sports car now also includes the first full-LED headlamp. In addition to the daytime running light which now features 24 LEDs per headlamp, light-emitting diodes are also used for the turn signals, the low-beam and the high-beam headlights. The €3,590 option rounds out the list of equipment available for the high-tech, mid-engine sports car.
The earlier launch of LED technology at the front of the car required a waiver by the European Union. The prominent advantages are lower energy consumption and a color that more closely resembles daylight, provides greater contrast and is easier on the human eye. LEDs are also non-wearing, require a lower voltage, are compact and offer greater design freedom.
The new full-LED headlamp represents the pinnacle of Audi’s lighting strategy. The use of LEDs already enjoys a long tradition. This innovative technology has previously been used in production vehicles for tail light functions, brake lights and daytime running lights. Since early 2003, Audi has also used the semiconductor technology in a number of concept cars to implement some or all of the lighting functions at the front of the car.
The brand with the four rings first introduced a strip-shaped fog lamp at the Detroit Auto Show in 2003. That same year, the Audi Nuvolari quattro showing the first full-LED headlamp was a highlight of the Geneva Auto Show. And the Le Mans quattro – a legitimate predecessor to the Audi R8 – illuminated the Frankfurt night with LEDs on the eve of the 2003 Frankfurt Auto Show.
Additional concept cars and many refinements to the light source and headlamp geometry followed. This technological highlight is now available as an option for the Audi R8, where it joins other very well-known high-tech components such as the aluminum Audi Space Frame, ceramic brakes and the mid-mounted V8 engine with FSI technology.
LED Head-lamp - Study by HELLA
Development : Luminous flux equal to xenon
Hella is developing complete headlamps using LED technology as a study provides information about possible series application from 2008.
The light and electronics specialist Hella, in cooperation with Volkswagen, has developed an LED headlamp which achieves low beam, high beam, direction indicator and daytime running light functions using cutting-edge LED technology only. The LED headlamp is a study using the VW Golf 5 as an example. It is to provide information about the performance ability of such systems and also offers the possibility of experiencing technical challenges in operation in the vehicle. A series solution is expected for the year 2008.
For the lighting functions, Hella uses both standard LEDs and LED assemblies especially designed for automotive applications. The light is directed onto the road with the aid of different optical systems. The most striking part of the headlamp is formed by seven pentagonal plastic lenses arranged in a honeycomb pattern. There is a shovel-shaped free-form reflector positioned next to these which produces the low beam together with four segments of the honeycomb. For the high beam, the other three segments of the honeycomb are also used. During the day, all seven segments of the honeycomb form the daytime running light. There are six standard LEDs arranged in a row beneath the shovel-shaped free-form reflector for the direction indicator function.
Light emitting diodes as a light source open up completely new possibilities for headlamp shapes and arrangements. This is made possible by the modular design as well as the large selection of different optical elements.
From the styling point of view, headlamps for future vehicle generations can be given a completely new look. The development of white LEDs in particular is creating application possibilities that would have been inconceivable only a few years ago. Alongside design aspects, the main reason for the development of this new headlamp technology is the reduced need for maintenance . The aim is to develop a headlamp which functions perfectly for the life of the vehicle.
Already, the LED headlamp prototype achieves a level of around 1,000 lumens luminous flux in the low beam, and has thus reached the level of a xenon headlamp. The luminous flux necessary for good high beam light cannot yet be attained on account of the lower luminance of LEDs. Hella's lighting specialists are convinced, however, that this will soon be achieved in the course of the further development of LEDs. The technical challenges that need to be solved by then involve thermal management in particular, as well as the development of new production processes and optical elements to accompany the LED technology.
n the USA headlamps with LEDs for main lighting functions are already approved according to the SAE standards valid there. In Europe or rather within the area of validity of ECE regulations, approval can be expected by 2008. Signalling functions in headlamps (direction indicator, position light and daytime running light) using LEDs are already approved in both the ECE and SAE regulated regions.
As early as in 2003, Hella achieved the world's first series application of white LEDs as a combined position and daytime running light in the headlamp of the Audi A8 W12. In the rear signal lamp sector, the company has successfully been using LEDs as light sources for more than ten years now, in high-mounted stop lamps, for example, or for tail light, stop light and direction indicator functions in combination rear lamps, such as in the case of the Volkswagen Golf Plus.
Hella has also been using LEDs for single-function lamps and combination rear lamps in the aftermarket and commercial vehicle sectors for quite some time. Thanks to their long service life, low power consumption and compact design, LEDs have already become standard for position and side marker functions. These properties also make LEDs particularly suitable for the current trend towards daytime running light applications.
I think you are right. Here is the only pic of a Towmotor that I could find.
I think you are right. Here is the only pic of a Towmotor that I could find.
chevrolet styleline cab 1952
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