Most LEDs have their characteristics specified at a current of 20 mA. If you want really good reliability and you are not certain you don't have worse-than-average heat conductivity in your mounting, heat buildup in wherever you mount them, voltage/current variations, etc. then design for 15 milliamps.
Now for how to make 15 milliamps flow through the LED:
First you need to know the LED voltage drop. It is safe enough to assume 1.7 volts for non-high-brightness red, 1.9 volts for high-brightness, high-efficiency and low-current red, and 2 volts for orange and yellow, and 2.1 volts for green. Assume 3.4 volts for bright white, bright non-yellowish green, and most blue types. Assume 4.6 volts for 430 nM bright blue types such as Everbright and Radio Shack. Design for 12 milliamps for the 3.4 volt types and 10 milliamps for the 430 NM blue.
You can design for higher current if you are adventurous or you know you will have a good lack of heat buildup. In such a case, design for 25 ma for the types with voltage near 2 volts, 18 ma for the 3.4 volt types, and 15 ma for the 430 NM blue.
Meet or exceed the maximum rated current of the LED only under favorable conditions of lack of heat buildup. Some LED current ratings assume some really favorable test conditions - such as being surrounded by air no warmer than 25 degrees Celsius and some decent thermal conduction from where the leads are mounted. Running the LED at specified laboratory conditions used for maximum current rating will make it lose half its light output after rated life expectancy (20,000 to 100,000 hours) - optimistically! You can use somewhat higher currents if you heat-sink the leads and/or can tolerate much shorter life expectancy.
Next, know your supply voltage. It should be well above the LED voltage for reliable, stable LED operation. Use at least 3 volts for the lower voltage types, 4.5 volts for the 3.4 volt types, and 6 volts for the 430 NM blue.
The voltage in most cars is 14 volts while the alternator is successfully charging the battery. A well-charged 12 volt lead-acid battery is 12.6 volts with a light load discharging it. Many "wall wart" DC power supplies provide much higher voltage than specified if the load is light, so you need to measure them under a light load that draws maybe 10-20 milliamps.
Next step is to subtract the LED voltage from the supply voltage. This gives you the voltage that must be dropped by the dropping resistor. Example: 3.4 volt LED with a 6 volt supply voltage. Subtracting these gives 2.6 volts to be dropped by the dropping resistor.
The next step is to divide the dropped voltage by the LED current to get the value of the dropping resistor. If you divide volts by amps, you get the resistor value in ohms. If you divide volts by milliamps, you get the resistor value in kilo-ohms or k.
Example: 6 volt supply, 3.4 volt LED, 12 milliamps. Divide 2.6 by .012. This gives 217 ohms. The nearest standard resistor value is 220 ohms.
If you want to operate the 3.4 volt LED from a 6 volt power supply at the LED's "typical" current of 20 ma, then 2.6 divided by .02 yields a resistor value of 130 ohms. The next higher popular standard value is 150 ohms.
If you want to run a typical 3.4 volt LED from a 6 volt supply at its maximum rated current of 30 ma, then divide 2.6 by .03. This indicates 87 ohms. The next higher popular standard resistor value is 100 ohms. Please beware that I consider the 30 ma rating for 3.4-3.5 volt LEDs to be optimistic.
One more thing to do is to check the resistor wattage. Multiply the dropped voltage by the LED current to get the wattage being dissipated in the resistor. Example: 2.6 volts times .03 amp (30 milliamps) is .078 watt. For good reliability, I recommend not exceeding 60 percent of the wattage rating of the resistor. A 1/4 watt resistor can easily handle .078 watt. In case you need a more powerful resistor, there are 1/2 watt resistors widely available in the popular values.
You can put LEDs in series with only one resistor for the whole series string. Add up the voltages of all the LEDs in the series string. This should not exceed 80 percent of the supply voltage if you want good stability and predictable current consumption. The dropped voltage will then be the supply voltage minus the total voltage of the LEDs in the series string.
Do not put LEDs in parallel with each other. Although this usually works, it is not reliable. LEDs become more conductive as they warm up, which may lead to unstable current distribution through paralleled LEDs. LEDs in parallel need their own individual dropping resistors. Series strings can be paralleled if each string has its own dropping resistor.
Lighting FAQ
- What does "Candela", "Lumen", etc. mean?
- Footcandle to Lux Conversion
- What does "inverse square law"
- What does "cosine law" mean
- What is the difference between Lumen and Watt
- How to calculate beam angle
- How to calculate lumens output
What does "Candela", "Lumen", etc. mean?
The formal definition can be found in many handbooks, so here are informal and, hopefully more useful, definitions:
Luminous intensity (or candlepower) is the light density within a very small solid angle, in a specified direction. In other words, this is the total number of lumens from a surface emitted in a given direction. The unit of measure is candela. In modern standards, the candela is the basic of all measurements of light and all other units are derived from it. Candlepower measurements are often taken at various angles around the source and the results plotted to give a candlepower distribution curve. Such a curve shows luminous intensity (how "bright" the source seems) in any direction.
Luminous flux is the time rate of flow of light. The unit of measure is the Lumen. One lumen may be defined as the light flux emitted in one unit solid angle by a one-candela uniform-point source. The lumen differs from the candela in that it is a measure of light flux irrespective of direction. The lumen is used to express a quantity of light flux: total output of a source, output within a specific angular zone, amount of absorbed light, etc.
However, if you need to calculate something which is not related to the human eye, for example temperature increase due to absorbed light, do not use luminous flux, instead we need to use the correct unit of power, the Watt (see below).
Illumination is the density of luminous flux on a surface This parameter shows how "bright" the surface point appears to the human eye. The appropriate units of measure are Footcandle and Lux. One footcandle is the illumination produced by one lumen uniformly distributed over one square foot of a surface, or conversely this is the illumination at the point of a surface which is one foot from, and perpendicular to, a uniform point source of one candela. So, footcandles incident on a surface=Lumens/Area(sq.feet). Lux is used in the International System. Both have a similar objective, but meters are used for Lux and feet are used for Candelas. Therefore, one lux=0.0929 footcandles. Or, approximately, 1 Fc=10 Lux.
Luminance or Brightness is a luminous intensity of a surface in a given direction per unit of projected area of the surface. Luminance can be expressed in two ways: in candelas per unit area or in lumens per unit area. I don't want do go too into this subject, because it is so seldom used. There are many different standard units of measurement. For example: Candela per square inch (cd/in²) , Footlambert (luminance of a surface emitting one lumen per square foot), Lambert (similar, but per square cm).
1 cd/in.² =452 Footlamberts
1 Lambert=929 Footlamberts=2.054 cd/in².
Actually, our eye sees brightness, not illumination. Every visible object has brightness. Usually, brightness is proportional to the object's illumination, so a well illuminated object seems brighter. For a perfectly diffusing reflecting surface:
Footlamberts = Footcandles * Surface Reflectance
Footcandle to Lux Conversion
One footcandle is the illumination produced by one lumen uniformly distributed over one square foot of surface, and lux is the illumination over one square meter of surface. Therefore, one lux=0.0929 footcandles. Or, approximately, 1 Fc=10 Lux.
What does "inverse square law"
The inverse square law tells is that the illumination is inversely proportional to the square of the distance between the point source and the surface, i.e.:
What does "cosine law" mean
Effective illumination is proportional to the cosine of the angle of incidence of the light on the surface (angle between the direction of the light and the perpendicular to the surface)
Illumination at the O point on surfaces 1 and 2:
Here are a few cases:
When the surface is tilted by an angle of 30º, the illumination is reduced by a factor of 0.87
45º - 0.71
60º - 0.5
What is the difference between Lumen and Watt
Lumen is a unit of the photometric system and Watt belongs to the radiometry system.
Both characterize a power of light flow. However, lumen is power "related" to the human eye sensitivity. Therefore, lights with the same power in watts, but different colors have different luminous fluxes, because the human eye has different sensitivity at different wavelengths. At a wavelength of 555 nm (maximum eye sensitivity) 1 Watt equals 683 Lm.
Very powerful sources of infrared radiation produce no lumen output, because the human eye can’t see it. However, if you need to calculate total power absorbed by a surface (to estimate temperature increase, for example), you have to transfer lumen flux to watt. This can be done by using a spectral luminous efficiency curve, which can be found in many photometry handbooks.
How to calculate beam angle
This is easy. If you know the distance from a fixture to the screen (much larger than fixture length) and the image diameter, then:
In most practical cases the following approximation is true:
Of course, both measurements must be in the same units (meters, feet, inches, etc.)
(Example: distance = 20 feet, image diameter=5 feet. Exact formula gives 14.25 º, second – 14.32º)
In the case of "soft edge" light image diameter, usually, is measured at point where illumination is 50% (beam angle) or 10% (field angle) of the center illumination.
How to calculate lumens output
The best way is to use a photometric sphere, however the number of people who have one is much less than the number of people who want to know total lumen output (luminous flux) of a fixture.
Another way is to measure illumination (which is the density of luminous flux on a surface) at a number of points and then integrate the resulting values.
Assuming that the beam has axial symmetry (if not – you’re in trouble. You have to measure many points all over the beam) and fixture beam angle is small (we can neglect cosine-cube coefficient from cosine-law and inverse-square-law, that is less than 5% for the 20º beam and 1% for the 10º beam), we have the following formula:
Beam radius is divided into n equal part (radiuses and illumination reading values are indexed from 0, at the beam center, to n-1, at the beam edge).
2 points (center and edge readings only):
3 points (center, middle, and edge):
4 points:
5 points:
8 points:
Here:
P - total lumens
R - beam radius
E - illumination
There is nothing magical about these equations. They are obtained by using integrating rules over the beam.
In the case of "soft edge" fixture, where the image size is taken at 10% of the center illumination (field angle), the first formula becomes very simple:
To get the result in lumens, you should use proper units. If you use footcandles, then the radius must be in feet. If you use lux, then the radius must be in meters.
And last, it doesn't make any sense to calculate luminous flux with 2-3 digits after the decimal point by this method. Assumptions which were made (illumination distribution is perfectly symmetrical, etc.) inevitably result in some error in the final calculation, so instead of 14231.41 Lm it is more practical to use 14KLm.
LED DefinitionsAmbient:The surrounding light level in a given area. It is also the temperature in which a LED light source is expected to operate in. Referring to light, it is the light given off by the Sun, Moon, other light fixtures nearby or even within the same space.
Amperage: The strength of an electrical current measured in amperes. The higher the amperage number, the higher the ability to place more devices on a circuit that will be driven by that amperage.
Amp: The basic unit of electric current adopted under the System International d'Unites; "a typical household circuit carries 15 to 50 amps"
Bulb: It is not a LED. A bulb is a light bulb, a flashlight bulb, a MR16 or E27 light bulb. A LED light bulb is a finished product that has the LEDs installed, electrical components installed and is ready to be used by the consumer. A LED light bulb is screwed in place, twisted and locked in place, pressed into sockets or contact terminals. This is a LED bulb.
Bulb Base: The part of the bulb that is used to set it into place and to make contact with electricity. There are many types and sizes. Most common are E26/27, USA and European standard household size, or medium base as it often is called. The 26 or 27 equates to the diameter measurement in millimeters of the threads of a screw-in bulb base. This type of bulb also includes the PAR 20, PAR 30 and PAR38 types, and a few others. There are MR16 and MR11 type base LED bulbs as well as GU10s.
Chromaticity: Chromaticity tells you what the lamp itself or a neutral surface illuminated by a lamp will look like. Chromaticity sets the "tone" or atmosphere of a room: warm, cool or something in between. Chromaticity (sometimes called color temperature) is usually measured in Kelvins. It can also be defined by using x and y coordinated against a standard chromaticity scale developed by the Commission Internationale de l'Éclairage (CIE). Here is a Chromaticity Graph that is commonly used.
Color Rendering Index (CRI): Also CCT or Correlated Color Temperature. It is a measure of the quality of light. A measurement of the amount of color shift that objects undergo when lighted by a light source as compared with the color of those same objects when seen under a reference light source of comparable color temperature. LED light CRI values generally range from 60(average) to 90(best). High CRI equates to sharper, crisper, more natural colored pictures while at the same time reducing glare.
Color Temperature: A measure of the color of a light source relative to a black body at a particular temperature expressed in degrees Kelvin (K). Incandescent lights have a low color temperature (approximately 2800K) and have a red-yellowish tone; daylight has a high color temperature (approximately 6000K) and appears bluish (the most popular fluorescent light, Cool White, ia rated at 4100K). Lamps with color temperatures below 5000K tend to be more yellow/red, lamps rated between 5000 and 6000K are viewed as white, while lamps above 6000K tend to have a blue cast.
Constant Current Driver:
Constant Voltage Driver:
Dimmer: 12vdc only. Used with LED lights powered by 12vdc - never 110/120vac. PWM built-in, as is an On/Off Switch. Will dim majority of 12vdc LED lights and a few 12vdc LED bulbs such as MR16s with the proper transformer/power supply. Dimmers are part of our LED control products.
Foot-Candle: The unit is defined as the amount of illumination the inside surface of an imaginary 1-foot radius sphere would be receiving if there were a uniform point source of one candela in the exact center of the sphere. Basically, the amount of light that a single candle would provide to a 1ft. radius sphere.
Full Spectrum: A light bulb or lamp that produces a light spectrum that covers the entire range of visible light (400-700nm) without gaps in its spectral output. White LEDs are inherently a full spectrum light source.
Intensity: Is a measure of the time-averaged energy flux or amount of light striking a given area. For bulbs alone this is measured in terms of lumens while for lighting fixtures it is measured in lux (lumens/sq. meter).
Kelvin Color Temperature: A measure of the color of a light source relative to a black body at a particular temperature expressed in degrees Kelvin (K). Incandescent lights have a low color temperature (approximately 2800K) and have a red-yellowish tone; daylight has a high color temperature (approximately 6000K) and appears bluish (the most popular fluorescent light, Cool White, is rated at 4100K). Today, the phosphors used in fluorescent lamps can be blended to provide any desired color temperature in the range from 2800K to 6000K. Lamps with color temperatures below 5000K tend to be more yellow/red, lamps rated between 5000 and 6000K are viewed as white, while lamps above 6000K tend to have a blue cast.
L.E.D.: LED means light emitting diode. LEDs are a solid state device and do not require heating of a filament to create light. Rather, electricity is passed through a chemical compound that is excited and that generates light.
LEDs are not bulbs or lamps in the true sense of the word and application. LEDs require a lot of work to make them ready to be used by the consumer. They need to be placed on a circuit board or other material that will allow electricity to pass through it at a specific voltage and current, and with components required to operate them at specific voltages such as 12vdc, 24vdc or 120vac. They do not come ready to plug into a 12volt or 120 volt power source. These are LEDs.
LED Bar: Refers to a solid strip of material on which LEDs have been soldered to, along with resistors and other components that a specific product requires to make them operate at the stated operating voltage. The Bars are usually an enclosed strip of LEDs. Enclosures are plastics, or aluminum, or metal composites with various types of lens/cover plates
LED Cluster or Array: A group of LEDs set in a square, rectangular or linear pattern, and formatted to be operated at a specific voltage. They will always include two wires called leads. One is positive, the other negative.
LED Drivers: are current control devices that replace the need for resistors. LED Drivers respond to the changing input voltage while maintaining a constant amount of current (output power) to the LED as its electrical properties change with temperature.
LED Lighting: A general term used by those who do not know the specific type or category of LED lighting they are after. LED lighting includes LED bulbs and fixtures, flashlights, strips, clusters and other LED light sources.
LED Strip: LED Strips are usually printed circuit boards with LEDs soldered to the board. The strip can be rigid, or flexible and without any enclosure to protect the LED and circuit.
Low Voltage: With LEDs, that means 12vDC 24vDC or 48vDC, as opposed to 110/120vac which is high voltage. With LEDs, low voltage is commonly 12vdc sometimes at 24vdc. To run these low voltage lights, power will have to be sent to the light through a power supply/transformer/adapter that is hooked up to 110/120/240vac power lines. The actual voltage reaching the light will be at 12vdc.
Lumen Maintenance: How well a LED light bulb is able to retain its intensity when compared to new. Typically a high power smd LED bulb will retain 70% of its intensity for 40,000-50,000 hours. That means a good quality LED bulb will run 8 hours a day for 13 years at 70% of its new condition. No other light source can do this.
Lumens: The unit of luminous flux in the International System, equal to the amount of light given out through a solid angle by a source of one candela intensity radiating equally in all directions. Used to measure light bulbs as stand alone light sources. Lighting fixtures are measured by lux output which is lumens per square meter. See for a better(?) understanding - Lumens, Illuminance,
Lux: Typically used to measure the light intensity produced by a lighting fixture. The higher the lux reading the more light the lighting fixture is producing over a given area. Known as lumens per square meter
mA: stands for milliamp. 1000mA equals 1.0 amp. All LEDs run on current and current is measured in milliamps. All LED products have a mA rating at which they are to be powered at.
Max Rated Temperature:, or Operating Temperature is the ambient temperature where the LED light source is installed at and should be maintained at. In most case that is around 40-50° Celsius. That is comparable to 104° F to 122° F. Operating a LED light source beyond the Operating temperature will lower the LED's life span or kill it.
MCD: or Millicandela, is used to rank/denote the brightness of an LED. 1000mcd is equal to one Candela. The higher the mcd number, the brighter the light the LED emits.
Nanometers: or nm. Used to measure the wavelengths of light. The lower the wavelength eg. 400nm the bluer and stronger the light source. Longer wavelengths above 600nm are red. Above 680nm, they fall into the InfraRed category, which is colorless to our eyes. White LEDs have no specific wavelength. They are measured by the color of white against the chromaticity scale.
Operating Life: usually refers to the number of hours a specific type of LED is expected to be operational. With high powered LEDs, that usually means life after it loses 10-15% or more rated output after 1000 or more hours of run time. The Cree and Luxeon LEDs are rated for 50,000 at 75% maintenance for 50,000 hours.
PCB/Printed Circuit Board: are made from various materials including fiberglass and aluminum. The pcb has an electrical circuit imprinted in silver etching. That circuit says how the LED will operate. The pcb is also the platform by which LEDs are employed in various applications. It can be a rigid board or flexible to twistable.
Power Supply: and Transformer and Voltage adapter apply to the electrical conversion of 110/120/240vac line power into 12vdc that will then be applied directly to the LED light product. Power Supplies are rated according to the current/amperage load capacity each will handle. It is an electrical or electro-mechanical device and is sometimes referred to as a LED Driver.
PWM: Pulse Width Modulation with regards to LEDs means that the LED will be pulsed or strobed at a rate so fast that the eye will see the light as being constantly on. In fact it is not. This pulsing or turning the LED on and off lowers the potential heat stress on the chemical that makes the light, thus allowing the LED to perform longer than anticipated. This is why we strongly recommend a dimmer/PWM with every purchase of a 12vdc LED product.
RGB: RGB stands for Red, Blue, Green, the 3 primary colors that make white light and all other colors. It can be a pre-programmed 7 color automatically changing LED bar or strip that is non-adjustable. It also means a RGB color changing system that allow adjustment of color change frequency, strobing, chasing and other action modes by using a color controller. Click here for more illustration..
SMD/SMT: A type of low profile LED that is surface mounted to a PCB. These type LEDs are very powerful and range in lumen output from 35 up to 170 lumens. With the latest LED technology being applied today, these have shown to have the most promise in delivering light levels and coloring that we are used to having. Those smd LEDs we talk about, use and sell are in the .5 watt, 1 watt, 3 watt and 5 watt power range. When you see a 7 watt or 9 watt LED light, it will contain 1 watt LEDs x 7, or 1 watt LEDs x 9, or 3 watt LEDs x 3.
SSL: SSL means Solid State Lighting. It does not use heating of a thin fragile filament to create light. Rather it uses electrical current passing through a chemical that will get excited and thus emit light.
Task Lighting/Lamp: A LED light used to specifically light a particular area used for work or reading. Typically found in the form of a desk, floor, or clamp-on lamp, it can be a high powered LED light in any form.
UV-A: (380–315 nm), also called Long Wave or "blacklight" because it is invisible to the human eye. Can cause skin irritation and fading of fabrics.
UV-B: (315–280 nm), also called Medium Wave radiation. Can cause severe damage to the skin and human eye through exposure.
UV-C: (← 280 nm), also called Short Wave or "germicidal" for its ability to destroy even bacterial life forms. Extremely hazardous to all life forms due to its ability to cause immediate damage to cellular DNA.
View Angle Degree: Also referred to as directivity, or the directional pattern of a LED light beam. The expressed degree dictates the width of the light beam and also controls to some extent, the light intensity of a LED. View angles range from 8 to 160 degrees, and are provided through the use of optics, special lenses made to collimate light to into a desired view angle.
Voltage: The rate at which energy is drawn from a source that produces a flow of electricity (amperage) in a circuit. The difference in electrical charge between two points in a circuit is expressed as volts.
Voltage Regulator: A device which limits or controls and stabilizes the voltage being applied to a using unit such as LED lights and motors. Regulators also take higher voltages than required and reduces it to the working voltage that makes a specific product run correctly. In many instances a lack of a Voltage Regulator will allow higher voltage than a product can work with and will cause irreparable damage.
Volts: The International System unit of electric potential and electromotive force, equal to the difference of electric potential between two points on a conducting wire carrying a constant current of one ampere when the power dissipated between the points is one watt.
Waterproof: meaning the LED product can be submerged into calm water but there is a limited depth as stated for each specific product, and must be specifically stated for "submersion". It also means that the product is made to withstand water being splashed onto it like rain, or having snow on it. These products will be made of high grade stainless steel, aluminum, or high strength plastics.
Watts: The unit for measuring electrical power. It defines the rate of energy consumption by an electrical device when it is in operation. The energy cost of operating an electrical device is calculated as its wattage times the hours of use. In single phase circuits, it is related to volts and amps by the formula: Volts x Amps x PowerFactor = Watts.
Watt per LED: It can be confusing when two watt numbers are used in product specifications. For the application to smd high powered LEDs, the 1 watt, 3 watt, 5 watt, etc, refers to the power consumption of that specific LED installed in that product. The watt numbers expressed as light output are a comparison to an incandescent light bulb light output, eg; a 60 watt light output is equal to a 60 watt incandescent light bulb. The Watt Output is equipment measured.
Weatherproof: meaning the product will take water splashing and high humidity without deterioration to the LED or circuit. LED product cannot be submerged into water.
White: White is defined by Kelvin Temperature or Degrees Kelvin. Most will say that a Kelvin Temperature of 6000k plus is white with a bluish tint. And let's say that 5000k -5500k is daylight/sunlight white. At 4200k-4500k, it is called cool white. At 2800-3300k, it's warm white, which is the color temperature most incandescent light bulbs emit.
From 5500k on down the scale, the color becomes "warmer" due to the dominance of red and yellow hues. In the opposite direction, whites will have cooler colors like blues and green becoming more apparent, thus they are called cool whites.
Summery of LED Lighting
The lighting efficiency of the new high power LED (light-emitting diode) light bulbs is more than eight times that of incandescent lights, and twice as high as compact fluorescent lights. Led bulbs also emit a much higher percentage of light in the desired direction. This makes them even more efficient compared to either incandescent or fluorescent for task lighting, desk lamps, reading lights, night lights, ect... LED night lights use only about 25 cents' worth of electricity per year and generate very little unwanted heat. The energy savings may be doubled in air-conditioned environments where each watt of incandescent lighting can add another watt or more to the power needed for air conditioning.
Led bulbs can operate for 30,000 hours or more. One led lightbulb can easily outlast 30 incandescent bulbs, or 6 compact fluorescents! Led bulbs cost less than the bulbs they replace. Operating 8 hours per day, led light bulbs can last 10 years or more!
Led light bulbs are less sensitive to shock, vibration, and the extreme temperature changes that can quickly ruin fragile incandescent bulbs. And unlike fluorescent bulbs which wear out much faster if they are frequently turned on and off, led bulbs are not affected by frequent on-off switching.
home led lights Choosing the right lighting for your needs will ensure that you have a fun and festive ambiance. This is where battery powered lights come in.and in the end thank you for sharing it with us
ReplyDelete