Most basic general questions are handled in Craig Johnson's LED FAQ, located at http://ledmuseum.home.att.net/reserved.htm.
Info on using and determining dropping resistors is in my LEDs 101 File. That file also explains why it is usually not good to connect an LED directly to a fixed voltage source.
Now for the other frequently asked questions:
Where do I get blue or white LEDs with a voltage drop less
than 3 volts?
How do LED flashlights work with just one battery?
How do LED keychain flashlights get away with no dropping
resistor?
Where do I get infrared LEDs with oddball wavelengths such
as in the 700's of nm?
Where/how do I get LED lights for motor vehicles?
Where do I buy XXXX?
How do I power LEDs from household line voltage?
Converting millicandelas to lumens!
How long is the phosphor persistence in white LEDs?
LEDs for growing plants?
I was sent here to look for goodies not mentioned above!
This one is not easy. Low voltage figures in some Hosfelt Electronics catalogs are typos - those LEDs drop generally 3.3 to 3.6 volts at a usual amount of current.
More likely your solution will be using a boost converter.
An LED flashlight bulb with a boost converter built in to utilize 3 volts
is described in Craig
Johnson's Starlite Flashlight and Night Pearl Flashlight Bulb Page.
UPDATE 9/23/2001 - Craig Johnson added a Night Pearl Bulb Page.
There are 1.5V and 3V versions.
UPDATE 3/24/2008 - Lumileds Luxeon K2 TFFC blue, white and green LEDs have lower voltage drop than usual. They usually conduct a significant amount of current at 3 volts. However, I would not consider these reliably drawing sufficient current to be bright from two AA, C or D cells. Rechargeable cells mostly provide 1.25-1.3 volts per cell, and non-rechargeable ones only provide about 1.3-1.4 volts in "average condition" with very light load.
Lumileds home page,
http://www.lumileds.com
UPDATE 8/10/2003 - Cree has lower-voltage-drop blue LED chips known as
their "Razor Thin" ones. As of 8/10/03 you can get to the index page for
Cree's LED products (chips/dice, not ready-to-use LED "lamps") Here.
Typical voltage drop is 2.9 volts at 5 mA and as I see from a graph on the datasheet, about 3.3 volts at 20 mA. Sorry, this is still a little high for two 1.5V cells!
UPDATE 3/24/2008: Nichia's latest blue, white and green LEDs (with 20 mA characterization current) have lower voltage drop than older versions - typically 3.2 volts.
UPDATE 3/13/2004 - zinc selenide white LEDs have a typical voltage drop of 2.65 volts at 20 mA. They can be made in aqua-blue. White ones have chips that emit aqua-blue light out their tops and orangish light out their sides, and the light has to be mixed inside the LED. Roithner Laser is selling a few zinc selenide models. Craig Johnson has an online review of these in his Page on LEDs available from Roithner Laser.
UPDATE 12/28/2005 - Lumex has blue LEDs with boost converters so as to work at 1.5 volts. One is available from Digi-Key with catalog number 67-1876. However, this is not a very bright one - it appears to be intended to have low power consumption, close to 7 mA at 1.5V in my experience (with small sample size), which leads me to think that the LED chip receives about or a little over 2 mA, while LEDs of this style are usually specified for 20 mA.
UPDATE 12/3/2006 - there is the related white one, with DigiKey catalog number 67-1877.
LED flashlight "bulbs" with boost converters built in are available - check out Craig Johnson's Night Pearl Bulb Page.
UPDATE 8/10/2003 - A higher power LED flashlight bulb that supposedly accepts an amazing range of voltages is now available. Craig Johnson has a web page on this one here.
One old traditional way of powering LEDs from a single 1.5 volt cell was
to use National Semiconductor's LM3909 LED flasher IC. One can pulse the
LED fast enough to appear continuously on, or one can add a diode and a
filter capacitor. However, the LM3909 is generally not the best way and
will not boost 1.5 volts to a voltage that will power blue, white, or
non-yellowish green LEDs. Adding a diode and a filter capacitor is
recommended if you use this to boost 3 volts for blue, white or
non-yellowish green LEDs, since those LEDs are generally more efficient
with steady current than with pulsed current.
UPDATE 3/24/2008 - The LM3909 is discontinued, but there is now a published
LM3909 emulator circuit at
Red Circuits.
Another IC that looks better for this is the Texas Instruments TPS61010DGS.
Craig Johnsom reviews some of these at http://ledmuseum.home.att.net/ledir.htm
Another supplier of oddball wavelength IR LEDs is Epigap, http://www.epigap.de/ They have the odd wavelengths 700, 740, 770, 810, 840, and 870 nm and some less-common longer wavelengths.
Another is Epitex,
http://www.techmark.nl/epitex/products.htm#irled
Another link: Epitex Home Page.
They have 700, 735, 750, 760, and 810 nm and a few longer wavelengths.
Other suppliers will be added here when I find out about them.
I have seen some units where the current reached levels that I would call adventurous - especially in Photon models with blue, blue-green or white LEDs. However, you need really heavy use with constantly fresh batteries to damage the LEDs - and any significant LED damage if such currents are sustained will probably take hundreds or thousands of operating hours. I consider it a safe bet that few users of these lights will log 500 hours of use with highly fresh batteries in a lifetime. Normally, the battery can only provide current in excess of the LED's maximum rating for a few minutes. Also, it is not easy to notice if the LED deteriorates to even half its original performance. But if you do notice any fading, chances are something like 99 percent that it will be due to the condition of the battery rather than the condition of the LED.
Lights for cars, especially other than center high-mount brake lights,
are more customized and specialized. Availability is mainly as a
replacement part for a specific car, all too often at a very high price
through a dealer for that make of car. If the light is not at an edge or a
corner of the vehicle, you may get away with a truck light but I do not
guarantee this and I discourage replacing any light unit on your vehicle
with something else unless it is approved by DOT (in the USA that is, or
whatever authority has jurisdiction in your country) and the manufacturer
intended it to be used in your vehicle.
Replacing an automotive incandescent bulb with an LED "bulb" of the same
base style and of the appropriate color will appear to work, but light
output will often fall short of requirements. Expect hype in claims of light
output from LED "bulbs" that fit where incandescent bulbs normally go.
In addition, the light distribution pattern will be different and even in
the highly unlikely event you have adequate total light output the amount
of light output into some, probably even most directions will almost
certainly not be in the allowable range.
Homebrewing a vehicle light requires knowing the lower limit and upper
limit for amount of light into something like 40 different directions so I
do not recomend this.
My LED Top Page, http://www.misty.com/~don/ledx.html for links to manufacturers and suppliers.
Craig Johnson's Where To Buy Page, http://ledmuseum.home.att.net/buy.htm.
My Bright/Efficient LED Page, http://www.misty.com/~don/led.html has a bit of supplier info.
Craig Johnson's "Punishment Zone" flashlight review page, http://ledmuseum.home.att.net/zone.htm, mentions more LED flashlights than perhaps even he could remember! Plus a few items other than just flashlights.
UPDATE 2/24/2004: This page is no longer the way to get there! Instead, try one of these:
The main "front page" for Craig Johnson's "LED Museum" and "Punishment Zone" site.
Craig Johnson's "What's New" page, linking to pages of his added or updated roughly within the past month.
Persons who are properly qualified and experienced at building line voltage powered electronic devices can:
a) Hack an LED night light.
b) Build a circuit to power an LED from line voltage, which would typically for low power LEDs (up to 30 mA) consist of:
* A brigde rectifier, with the LED connected to the DC output leads. The voltage rating of the bridge rectifier is not important.
* A capacitor in series with one of the AC leads of the bridge rectifier to limit the current. Use .47 microfarads to get approximately 19 milliamps of LED current (assuming 120 volts 60 Hz). The capacitor must have an actual AC voltage rating well above the line voltage. A DC rating well above 1.414 times the line voltage is not sufficient - an actual AC rating is required.
* A resistor in series with either AC lead of the bridge rectifier to limit peak current. If you have a capacitor across the LED (recommended around 220 microfarads), then this resistor only needs to be perhaps 33 to 100 ohms (for 120 volts AC). Otherwise this resistor needs to be at least 1.5 Kohms unless the LED is known to reliably repeatedly withstand current peaks well over 100 mA. This resistor should be a composition or wirewound type rather than a film type or otherwise known to reliably handle the current inrush through the current-limiting capacitor when power is applied.
* A fuse in case things go wrong.
* A resistor across either the current-limiting capacitor or the line terminals of this circuit, so that the line terminals do not present a shock hazard from charge of the current-limiting capacitor after this circuit is disconnected. The resistor needs to discharge the current-limiting capacitor from the peak line voltage (typically 169 volts) to no more than 28 volts (almost 2 time constants) within a fraction of a second. A 220K resistor discharges a .47 microfarad capacitor from 169 volts to 28 volts in about .19 second (max. of .23 second with 10% tolerances of resistor and capacitor) and will dissipate approx. .065 watt at 120 volts.
As of early 2008, the most suitable LEDs for growing plants are only about as efficient at producing the desired light as better fluorescent lamps of types made for this. LEDs also cost a lot more than fluorescent lamps of the same output.
Now, for wavelengths and how much light is needed:
Chlorophyll has two spectral ranges of utilization - red and blue.
Plants also have two kinds of chlorophyll - A and B.
Basic photosynthesis will proceed if only one chlorophyll is utilizing light. Chlorophyll A is the basic one for making chemical energy from light energy. Chlorophyll B is the main "accessory pigment" in green algae and "higher plants", and serves to utilize light at wavelengths not absorbed well by Chlorophyll A and transfer the energy from such wavelengths to the process that uses Chlorophyll A. Other accessory pigments are mainly carotenoids, which mainly utilize blue and violet light.
Although stimulating only one photopigment is sufficient for photosynthesis, many plants have some requirement of stimulating more than one photopigment for proper growth regulation, flowering and fruiting.
Most published spectral curves do not show well actual utilization, and a few show well ratio of absorption to transmission, or ratio of light utilized to light not utilized. If these curves are redrawn to show ratio of light utilized (or absorbed) to incident light, the peaks become wider and flattened. Many of these curves are also inaccurate, showing the peaks as more symmetric than they actually are. The peaks are asymmetric, with wavelengths shorter than peak being used better than wavelengths longer than peak.
The red region has a utilization peak around 660-670 nm for Chlorophyll A and around 635-645 nm for Chlorophyll B, depending on the source of information. Plants generally make good use of all red wavelengths except for ones much longer than 670 nm. 700 nm is close to useless for plants. Most plants actually make good use of orange and even yellow-orange light, to such an extent that high pressure sodium lamps have been used for growing plants.
The blue utilization peaks of chlorophyll are mostly reported to be around 430-440 nm for Chlorophyll A and around 453-470 nm for Chlorophyll B. Unlike the red peaks, the blue peaks are usually shown with substantial asymmetry indicating that wavelengths shorter than peak are used well while wavelengths longer than peak are not used well. Beta carotene is a major blue-absorbing accessory pigment, with a double peak at about 450 and 480 nm.
GaAlAsP red LEDs (peak wavelength typically 660 nm) and Agilent's similar "TS AlGaAs" (peak wavelength near 655 nm) match the Chlorophyll A red peak well. However, Lumileds InGaAlP red LEDs with peak wavelength in the upper 630's nm and dominant wavelength (color specification) in the 620's of nm are better because they are so much more efficient. The shorter wavelength is not utilized much less than more optimum red wavelengths are.
Keep in mind that red Lumileds Luxeons will achieve a much higher visual level of illumination than fluorescent lamps made for growing plants will at the same amount of watts of light per square meter. This is because red Luxeons produce shorter wavelengths of red, selected because the human eye is more sensitive to wavelengths closer to 555 nm.
The blue response bands of both chlorophyll A and chlorophyll B and also beta carotene are served well by Lumileds royal blue LEDs, which typically have peak wavelength near 450 nm.
Photosynthesis works most fundamentally from red light and secondarily from blue light. However, many plants have some need for blue light for proper growth regulation and/or flowering and/or fruiting.
As for how much light is needed:
This page at Wisconsin Center for Space Automation and Robotics describes a plant growth unit illuminated by 670 nm red and 470 nm blue LEDs. Illumination intensity is adjustable, with maximum values (my translation to watts of light per square meter) of about 98 watts per square meter for red and about 18 watts per square meter for blue. If you need to get by on much less than this, I would make these figures more equal in case your plants have specific blue light requirements that need to be satisfied. I suspect plants that do not have especially high needs can fare reasonably well with 25 watts per square meter of red and 9 watts per square meter of blue.
Keep in mind that red Luxeons are typically about 26% efficient when the junction temperature is 25 degrees C, and typically about 16% efficient in the more reasonable situation of heatsink temperature of 35 degrees C.
The most efficient royal blue Lumileds Luxeons currently available are LXML-PR01-0225 (of the Rebel series), which are typically 20% efficient at 350 mA with a 25 degree C junction temperature. Thankfully, blue LEDs have a much lower temperature sensitivity than red ones have.
It is likely that a Luxeon K2 TFFC royal blue version and a more efficient royal blue Rebel will become available in the near future, with efficiency around 30%.
Cree already has royal blue XRE series LEDs that are around 22-30% efficient at 350 mA, with a typical peak wavelength around 455 nm - still good.
Keep in mind that plant-growing fluorescents are about 25% efficient at producing desirable wavelengths of light for F40T12, and 20% efficient for the F20T12.
Keep in mind that blue LEDs operate less efficiently with more power, even if the amount of power and current is well within their ratings.
First, explore My LED Top Page, http://www.misty.com/ledx.html and all links therefrom even if it takes several hours.
If this completely fails so badly that you think you are better off e-mailing a sort-of webmaster with a 50-hour-per-week unrelated fulltime job, then try e-mailing me, at don@misty.com Spammers beware - my sysadmin haas a hobby of punishing spammers and I encourage and cooperate with him! Also beware that my response rate for non-spam e-mail from strangers is down to about 60%, maybe even less.
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