Introduction: All You Need to Know About LEDs

A Light Emitting Diode is an electronic device that emits light when current is passed through it. LEDs are small, extremely efficient, bright, cheap, electronic components. People think that LEDs are just common light emitting components & tend to overlook the Interesting Facts & Features of LEDs. In this instructable I will teach you 'All You Need to Know About LEDs' which includes their Working, Current and Power Ratings, Builds, Types, Resistor Calculator for LEDs, Uses, Testing & a Simple LED circuit.

Here's a Link to the 'LED Resistor Calculator' free android app: LED Resistor Calculator . This app helps you calculate the appropriate resistor value required for an LED.

History Of The LED

Captain Henry Joseph Round was one of the early pioneers of radio and received 117 patents. He was the first to report observation of electroluminescence from a diode, leading to the discovery of the light-emitting diode. Vladimirovich Losev observed light emission from carborundum point-contact junctions. In course of his work as a radio technician, he noticed that crystal diodes used in radio receivers emitted light when current was passed through them. In 1927, Losev published details in a Russian journal about his work on light-emitting diodes. A couple of years later Nick Holonyak, Jr. invented the first visible-spectrum (red) LED in 1962 while working as a consulting scientist at a General Electric Company laboratory in Syracuse, New York.

Step 1: Composition & Working

IMAGE:

  1. Dremeling an LED.
  2. Top view of the Electodes of the LED. (larger- cathode, smaller- anode).
  3. Closeup of Anode and Cathode of LED. (LED sliced in half).
  4. Anode and Cathode of LED removed from plastic shell.

Composition

Most common LED's are made up of Gallium (Ga), Arsenic (As), and Phosphorus(P). Modern LEDs are not just GaAsP types - other semiconductor brews are abound! These Semiconductors are also used in various other Electronic components.

Working

A LED is a P-N Junction diode that emits Light. When an LED is in forward bias it emits light instead of heat generated by a normal diode. When the P-N junction is in forward bias, in case of a LED some of the holes combine with the electrons of N- Region and some of the electrons from N combine with hole from the P- region. Each recombination radiates light or Photons.

LED's do have a polarity and hence do not work if they are connected in Reverse bias. The easiest method to check the polarity of common LED is by holding the LED close to your eye. You will see that there are two electrodes. The thicker one is the Cathode(-). Light is emitted from the Cathode. The thinner electrode is the Anode(+).[Although this method of checking polarity won't work for some LED's like high efficiency LEDs,etc where the opposite is true]. Generally LED's are manufactured so that the length of the leads of the Cathode and Anode differ. Due to this LED's are manufactured with the Anode(+) lead longer than the Cathode(-) lead. This also makes it easier to determine the polarity.Note: Some manufacturers do keep both electrode leads the same length. Inorder to TEST the polarity you will need to use a Multimeter.

Step 2: Current & Power Ratings, Haitz's Law

IMAGE: LED Symbol.

Common IR LEDs may work down to ~1.5V but common Red LED's need ~1.8V, Common Green LED's need ~2V & common blue & whites LED's (which of course are blue with a phosphor coating) need a good 3V.

LEDs don't have a "voltage rating"; they are current-driven. The brightness is roughly proportional to the current, and not directly proportional to the voltage. At any particular current, they will have a forward voltage, but that is secondary to the current, which is the main factor that must be controlled.

Current Ratings

Current ratings of LED's are similar too Voltage Ratings. LED's generally have a standard current rating. Most LED's require about 5-25 mA. The current required by an LED sometimes depends on the Colour of the LED. If you supply excess current the LED will burn and get damaged. On the other hand if you supply very low current the LED will not produce its maximum output. Modern ultrabright red/green LEDs can give acceptable output (for status use etc) on as little as 1mA

Power Ratings

An LED's can have various power ratings depending upon their Type, Build and Current Ratings, etc. LED's also come in 'High Power LED' packages. LED's are less ineffecient than conventional light bulbs such as CFL's and Incandescent Bulbs.

Haitz's Law

It states that every decade, the cost per lumen (unit of useful light emitted) falls by a factor of 10, and the amount of light generated per LED package increases by a factor of 20, for a given wavelength (color) of light. It is considered the LED counterpart to Moore's law, which states that the number of transistors in a given integrated circuit doubles every 18 to 24 months. Both laws rely on the process optimization of the production of semiconductor devices.

Step 3: Build

IMAGE:

  1. Basic LED.
  2. Dome LED.
  3. SMD LED (Large).
  4. SMD LED (Small).
  5. Display LED used in 7-Segment Display.

LEDs are produced in a variety of shapes and sizes. The color of the plastic lens is often the same as the actual color of light emitted, but not always. For instance, purple plastic is often used for infrared LEDs, and most blue devices have colorless housings. Modern high power LEDs such as those used for lighting and backlighting are generally found in surface-mount devices (SMD) packages. Some LED's have diffused plastic lenses.

Basic LED

The basic LED is one of the most widely used LED's. Due too it's popularity its pice is relatively cheaper compared to othe LED's. It looks very basic and the design is very simple.

Dome LED

This is a type of LED which is shaped liked a 'Dome'. This shape is designed too increase the area to which the light is transmitted. In other words the Angle of Emission( Circumfernce) of Light from the LED is greater than the Basic LED. This is generally controlled by how far they place the light emitter from the dome. The spec sheets almost always give you a "half-power angle" (the angle off-axis at which you're seeing only half the brightness). If you want a much wider emission angle you can cut the dome off with a dremel tool. If you care, you can then file or polish the end, but it's not necessary. The closer you cut it to the emission device, the wider angle you'll get. But be careful not to cut too close because there's a tiny wire in there that usually cannot be seen by eye. Although this type of LED is slightly more expensive than a basic led.

SMD LED

This type of LED is generally very small in size. SMD means Surface Mounted Device. And as its name suggests, this LED is soldered onto the surface of the PCB unlike conventional 'through- hole' components. These LED's are Generally soldered by Machines( Precise Soldering Robots) and are extremely difficult to solder by hand (Although it isn't impossible to Solder SMD LED's by Hand). Inorder to solder SMD LED's by hand you just need a fine tipped soldering iron, some thin solder, a bright light, and possibly a magnifier and some good and precise soldering skills.

Display LED

This type of LED is mainly used in displays as its shape is flat.

Step 4: Types

IMAGE:

  1. Dome LED's.
  2. IR LED's.
  3. 7 Segment Display LED
  4. Tri colour LED (colour changing LED).

Colour LED

Coloured & White LED's are mainly used in Indicators, Lamps, Lighting Equipment, etc. They are one of the most commonly used LED's

Colour Changing LED (Tri/Bi Colour LED)

In this type of LED, the colour emitted by the LED changes within a specific period of time. A tiny Integreated Circuit (IC) is embedded into this LED inorder to control the time delay between transitioning the various colours. Tri/bicolor LEDs do not change color they are actually two separate LEDs (often a red and a green) in one package. You turn one or the other to produce two colors and both to make a third.

Infrared (IR) LED

This type of LED beams infrared rays of light. These infrared rays cannot be seen by the Human Eye. This type of LED generally works on a transmission frequency of 38KHz.The designer modulates the LED as a way for the receiver to discriminate it from other IR sources. LED's are also modulated at very low frequencies to simply show a blinking LED, and are often modulated at relatively high frequencies with varying duty-cycle to effectively control their brightness. And then some are modulated at much higher frequencies to send data (as used in fiber optics for example). Its mainly used in Remote Controlled and Small Range Communication devices. You can test a IR LED by viewing it under a Camera whilst a current is appliead across the LED. In other words cameras can detect IR rays emitted from the LED. Cameras that don't have an IR block filter can generally see near IR quite well (and tend to be cheap cameras and particularly security cameras). But it should be mentioned that even some cell phone cameras don't see IR LED's very well at all because of their IR block filter.

7 Segment Display LED

A 7 segment display LED is an LED consisting of 7 display LED's connected in the form of an 8. Its used in calculators, displays, etc. An LED similar to this is also used to display alphabets.

UV LED

UV LED's emit Ultra Violet rays of light. These rays have various applications such as Sterilization, Water purification, etc.

Step 5: Resistor Calculator for LED's

IMAGES:

  1. Various resistors and a LED.
  2. LED Resistance Calculator App Logo.

So the most common question asked about LED's is the appropriate resistor to use along with. The reason a resistor is used along with LED's is to protect them from excess current which can burn and damage the LED. But choosing the right LED isn't that simple. Why? Well if you choose a very high resistance, the LED will not emit its maximum light. And if you a low resistance there are chances of the LED getting Damaged.

So a simple formula was invented:

Resistance = (Source Voltage - LED Voltage) / (LED Current / 1000).

*Keep in mind, the LED Current is in milliamps(mA).

Inorder to make this calculation easier you can use this free Android App LED Resistance Calculator. It is an app designed especially for this Instructable. Other features and more electronics related functions and calculators will be added to this app. The app was developed by BluBot Technologies . You can check out his Instructables and contact him through his Orangeboard @Nathan Neal Dmello .He also undertakes various other projects in developing Apps, Websites, Computer Programs, etc. You can contact him through his website.

Step 6: Uses

IMAGE:

  1. TV Remote without button pressed.
  2. TV remote with button pressed and IR LED flash detected.
  3. Strip of Dome LED's from an Emergency Flashlight.
  4. LED Flash of a Smartphone Camera.
  5. LED power indicators of a Laptop.

LED's are used everywhere. From your phone flash, to your cars music system, to your garden lights, to your TV display. Basically their adaptive nature and effeciency has given them a place in most electronic gadgets.

Some of the most known uses are:

  1. Lighting.
  2. Displays.
  3. Indicators.
  4. Decorative Lights and Objects.
  5. Remote Control.
  6. Sterilization.
  7. Purification of Water.
  8. Dentistry & other Medical applications.

Step 7: Testing & Circuit

IMAGE:

  1. Multimeter used to test LED.
  2. Simple circuit using LED.

Testing

A classic quick tester for colour,brightness & polarity is just a 3V lithium coin cell(e.g. CR2032). Only touch lower voltage LEDs to this briefly of course, or they may overheat !

Some LED's can be tested inorder to check whether it is working properly using a multimeter & by following the steps:

  1. Set the dial of the multimeter to the 'Continiuity' function.
  2. Now connect the Anode(+) of the LED to the RED/Positive/(+) probe of the multimeter & connect the Cathode(-) of the LED to the BLACK/Negative/(-) probe of the multimeter.
  3. If the LED is working the Multimeter will start to make a 'Beep' sound. And a value will display on the screen of the multimeter. In addition to this the LED should light up.

*Testing an LED using the continuity function of a multimeter usually won't work because most multimeters apply only a low voltage, less than 1V, for the resistance and continuity tests. If it does, the multimeter won't make a continuous beep; it may make one short beep. Many multimeters have a diode test function, indicated by a diode symbol, which applies up to 2V across the diode. This will reliably tell you the polarity of many LEDs but not necessarily blue and white LEDs with high forward voltages.

You can also Test the LED and any other component with the help of this circuit:- Electronic Sensor Component Tester

Circuit

This is one of the most basic and versatile circuit you can find which uses an LED in it. The reason it is a great circuit to start of with is that it can also check the working of any other Electronic components or Electronic Sensors. You can also check out a detailed tutorial which will help you to make this circuit: Electronic Sensor Component Tester

Comments

author
Jonathanrjpereira (author)2016-02-07

Thanks to the entire Instructables Community for reading my Instructables. It just goes to show that even the most simple Instructables and go along way & can spread knowledge to an entire community.

LEDs2,00,000.jpg
author
SherylinRM (author)2016-03-17

Any chance of an app for the pc and not the phone?

author
DenethS (author)SherylinRM2017-03-12

U can download MEMUI in your laptop or desktop and your device will be like a android phone and then u can download that app.THE MEMYI has playstore ,so there u go......

author
Mark 42 (author)2016-02-08

Somebody needs to design an LED "lightbulb" which turns amber as you dim it.
I like LEDs, but I want the warmth of the color change as I dim it (like an incandescent)

author
GaryF36 (author)Mark 422016-02-08

This is the #1 challenge for the LED industry. Unfortunately LED is pure white light, so any light changes need to be made via filters. I do believe however if all combinations of the elements when tested may come up with the yellow light mmm maybe Au will throw a yellow light? (actually yellow/orange light is preferred by humans at night due to evolutionary reasons and the color of fire. White light is un-natural at night and leads to insomnia and proven increases in cancer in the long term.)

author
static (author)GaryF362016-03-05

Please give us break. Do people proof read what they make up or repeat? Humans spent more time in day light than than they did in the light cast by a fire and fire is offered as greater influence in the evolution of the humans? Ignoring that the lower life forms evolved from most likely lacked the mastery of fire for a greater time period than human has mastery of fire. and used fire as a source of illumination, further romoving fire's importance in evolution as far a vision is concerned..\

author
starphire (author)Mark 422016-02-26

It's been done a number of times before, in fact patents with methods of doing exactly go back many years, and products have been created that do this. Not by changing the wavelength of a single kind of LED, but by mixing different colors of LED in changing proportions. Most of these products simply didn't make it, or haven't reached the consumer market yet. You can also use a programmable LED light bulb (RGB or RGBW, such as Philips Hue) and just create the effect in the app that controls them.

author

Instructable Idea Detected!

author
JonathanB115 (author)Mark 422016-02-08

http://www.usa.philips.com/c-m-li/warm-glow-dimmable-led-lighting

author
AnubisAndIsis (author)2016-02-26

You missed Linear 1's LED wizard, a staple for anyone who builds with LED's. It draws out printable diagrams for you and proposes multiple layouts depending on what array you want to build.

http://led.linear1.org/led.wiz

author
DavidW173 (author)2016-02-08

It looks as though LED lighting will be the only choice in the near future:

http://www.nytimes.com/2016/02/02/business/energy-...

author
Electrospark (author)DavidW1732016-02-13

field-induced polymer electroluminescence has been around for quite some time now but in the recent past years professor David Carroll has been experimenting and improving them to make more light. It's still a technology that need lots of improvements before being able to use it for usable house lighting.

author
ScrappyGirl (author)2016-02-07

We have to be careful about LED lights, they are turning up everywhere. The blue wave length of LED's is shorter, more frequent and can penetrate the retina of your eye. This is especially true for blue, grey and green eyed people. Also, because everything from TV's, computers and some car headlights have the blue light, light eyed people need to protect their eyes from this shorter wave length. Opthalmologists are studying this because humans are using and staring at these lights from a younger age. If you wear glasses you can get a blue blocker filter to help block the shorter wave length of LED's.

author
M2aestro (author)ScrappyGirl 2016-02-11

This is important for people to know with respect to LED home lamps; most of those use phosphors that tend not to radiate the undesired wavelengths just as a matter of efficiency requirements, rather than to generate directly the spectrum advertised for the particular lamp assembly (given in degrees K equivalent). None-the-less, there are LED arrays that give out a large amount of light in the very short blue/violet/some ultraviolet part of the spectrum, and I suspect that most folk would not choose them because the color appears so cold.

author
GaryF36 (author)ScrappyGirl 2016-02-08

Interesting points you raise here. I use a tool to red shift the light on my computers at night to remove the blue light. It make a big difference on eyestrain and reduction of insomnia,

author
ScrappyGirl (author)GaryF362016-02-08

Here is one article by Zeiss, the article I spoke of is not in any public publication, but Zeiss gives pretty much the same science on blue light. http://www.zeiss.ca/vision-care/en_ca/better-vision/understanding-vision/eye-and-vision/blue-light-the-good-and-the-bad.html

author

Interesting point. On a side note; The inventor of the Blue LED won the Nobel Prize in Physics in 2014.

author

Yes, that is true. But science is still studying the long term effects of these lights. Our eyes were designed to see with light, not look into light constantly as we are doing now. It will be interesting how this all turns out.

author

Can yoi send a link to the research/report. I really want to read it. I could also include any viable research in this Instructable.

author

I will, it's at work. So I will bring it home tomorrow.

author
Phil_S (author)Jonathanrjpereira2016-02-07

Yes - I bet he never thought that most of that effort would end up as as Christmas decorations. A huge amount of what started as a breakthough in lighting gets dumped in landfill. Everywhere you look now is blue, red and green are passe.

author
EnricoC3 (author)ScrappyGirl 2016-02-07

Fluorescent lamps are much more dangerous than LEDs, as they emit UV light, which is then converted to visible by the phosphor. LEDs start off with blue wavelength, which is less dangerous than UV. Also, sunlight contains much more hazardous content compared to LEDs and CFLs, so after all, LEDs are a pretty safe light source.

author
pmshah (author)EnricoC32016-02-07

"sunlight contains much more hazardous content compared to LEDs and CFLs," True. But unlike staring straight into the LED - TV how many look directly at the sun at all?

author
ScrappyGirl (author)EnricoC32016-02-07

Working in the Optical industry, I would like to say the jury is still out on that. However, I do know that fluorescent is dangerous and yes, we are getting more UV because of fluorescent and the higher amounts of UV allowed in our atmosphere now. It's about the wave length of the blue light, it's much shorter, faster and penetrates the retina much further especially in light eyed people. They are studying the long term effects of this now. Time will tell, but in the mean time the Optical industry has created a 'blue blocker' on the lens of eye wear. We are looking into this kind of light at a much younger age and of course for longer periods. I just think people need to be aware of this, it's up to the individual. As humans we great at developing technology then marketing it and it takes time to understand the effects of what we are doing. All the best!

author

Lets hope that research is in favor of LED's

author
M2aestro (author)2016-02-11

I bit misleading title for the article; this is really just an introduction to the LED for future LED use hackers. It hits the basics for those who might need those, however.

author
gizmologist (author)2016-02-07

It would have been nice, since you mentioned it, if you had explained terms like "P-N junction" for folks that don't understand semiconductors. By the way, a lithium coin cell won't burn out an LED because the battery is not capable of more than about 20 mA of current. You can prove this to yourself by taking apart some cheap LED keychain lights - They contain only an LED and lithium battery.

author
GaryF36 (author)gizmologist2016-02-08

The P-N junction is also at the core of Solar cell electrical energy production. Photons knock the electrons over the P-N junction causing an electrical cycle to flow. Effectively the reverse happens with an LED.

author

I guess I could add some points about the PN junction. Thanks

author
schabanow (author)2016-02-08

Hi Jonathan, thank you for your impressive article! Some remarks are below:

1. https://en.wikipedia.org/wiki/Oleg_Losev

2. How to calculate the proper serial resistor's value next to your LED in order not to blow the latter away:

R ser = (U pwr-U drop) / I led;

where

R ser = serial resistor next to LED to be lighted; Ohms

U pwr = pwr supply voltage; Volts

U drop = how much volts drop across your LED (see manufacturer's DataSheet); Volts

I led = how much Ampers go through your LED; Ampers

For example: you have LED with 3,4 V nominal drop voltage, 300 mA nominal forward current. You have 5 V from your USB socket. How much Ohms should you put there in serial in order not to loose your LED and just have fun with it?

R ser = (5 V - 3,4 V) / 0,3 A = 5,33 Ohm

Thank you!

author
Lee Wilkerson (author)2016-02-07

Great work and write-up.

I use a 1.5V battery to test polarity. Current only flows one way through a diode (LED or any other type). Quick, simple, and effective. If you can't see visible light from either polarity, then it is probably an IR.

author

Thanks

author
KenConstruct (author)2016-02-07

I wanted to add what I think is one of the coolest uses of modern LEDs - an emergency crystal radio diode. In an emergency you don't generally have a chance to pick up a germanium diode, but red diodes are everywhere. Put enough forward voltage across a red diode to get a dim light (DC/battery) and wire the lit diode (with power) in place of the typical 1N34A. Then all you need to build a radio receiver is a bunch of wire!

author

Perfect for the Survival Content

author
fred_ (author)KenConstruct2016-02-07

Or a razor blade and a pencil if you don't have the led and battery.

author
KenConstruct (author)fred_2016-02-07

I've heard about the razor. I also heard you have to torch (heat with high temp flame) modern razors to "blue" the surface of the steel get the diode with the pencil to work - for those that want to try it.

author
fred_ (author)KenConstruct2016-02-07

The razor needs an oxide layer. A torch might help if it wasn't already there. It's finicky you need to move the graphite around on the razor with a spring like a pc of safety pin then leave it alone you can't hold it by hand.

Similar to a whisker (metal not hair) on a rock. Fools gold aka pyrite.

Another is the "voices in your head" radio from a bad old school tooth filling. That's not an old wives tail.

The germanium diode happens to be a better detector than an LED when you're trying to use the TINY power in rf to drive an earphone but any non linear junction is a possibility.

author
enemigo (author)KenConstruct2016-02-07

You should make and post an instructable concerning this, using the replacement diode instead of the 1N34A. Why no t?

author
KenConstruct (author)enemigo2016-02-07

I should. I'll put it on my bucket list. :-)

author
mickeypop (author)2016-02-07

Your history part is just a bit wrong, in that documenting the electroluminescence of Silicon Carbide is older. Early LEDs were basically Silicon Carbide crystal though modern ones use many other crystals now. Carborundum is a form of Silicon Carbide

The first electroluminescence reported from a piece of granite (Silicon Carbide) was from a french scientist in get this 1791, he was probing the stone with a battery and in places it would glow.

He wrote a paper on it and some letters one of which is in the archives of Benjamin Franklin.

It was ignored till 1852, an English scientist took it up and figured how to grow the crystal but it died again never taking off.

Though they did not know what a diode was yet and the Periodic Table did not exist so
they did not identify the elements it their papers, the "LED" perse is actually older than credited.

It wasn't till 1869 a Russian chemist Dimitri Mendeleev started the Periodic Table.

just a bit to think about

author
Dr Frugal (author)2016-02-07

Thanks for the lesson..! And for my fellow iPhone snobs here's a link to a resistor app in the App Store
iResistor Calculator by Dragon2Studios
https://appsto.re/us/scXUD.i

author
Thewiz453 (author)2014-10-10

Here's one most people don't know .......
Put a voltage meter across the leads of the led and apply light to it
There will be voltage generated
ENJOY

author
wiredcav (author)Thewiz4532016-02-07

Yup. I have used them that way as sensors for the light of a similar LED.

author
Tony--K (author)wiredcav2016-02-07

The following instructable has useful info about LEDs and IR emitters as light sensors, tested with different types of light sources:

https://www.instructables.com/id/Tests-of-LEDs-and-Various-Light-Sensors/

author

Thanks for the constructive advice. i will add that too the instructable.

author
DonP3 (author)2016-02-07

I've always had trouble overwintering my tropical pond plants, as conventional lighting was too expensive to use -- that is, until I discovered LED grow lights. I found lights that contain 60, 3 watt LEDs -- 20 white, 20 red and 20 blue. The lamps use 185 watts (they have a 5 watt fan) and the light is so intense, I need sunglasses to stay in the basement room with them. I have intensely green plants (Taro, Spider Lilies, Jasmine, Brazilian Firecracker) that need to be trimmed back several times during the winter as their growth is profuse. I'm even growing a very healthy tomato plant that is nearly 3 feet tall and intensely dark green.

author
Jonathanrjpereira (author)DonP32016-02-07

Do LED grow lights only have white,red&blue LED's; I was always under the impression that grow lights were of a specific spectrum of light.

author
DonP3 (author)Jonathanrjpereira2016-02-07

Plants use primarily red and blue frequencies -- blue for vegetative growth and red for blooming and fruiting, though there are other factors involved such as dark/light lengths and temperature. The LEDs produce the frequencies needed and used by the plants and waste less energy producing frequencies that plants don't use. For me, they work better than anything I've ever used before and at lower cost.

author
fred_ (author)2016-02-07

1st thanks for the instructable. Some random musings on the article and some of the questions in the comments;

You can google for research on the "dangers of blue light" Basically dark blue light is damaging to the back of the eye, UV is damaging to the front of the eye. "white" LEDs tend to emmit more blue light than CFLs.

Note a "white" LED is really a blue LED with some phosphors over top to down convert some blue light to other colors. That's usually why the higher output bins have a higher color temperature (blueish). LEDs are all over the map when they are produced and get sorted into "bins" of dominant color temperature and light output. You could, and they do, put a red green blue set of LEDs ( multi color LED) in one package (like your tv) but this doesn't make a lot of $ sense for lighting applications. Color changing LEDs don't change color they just change the current in the individual red green blue LEDs buried in the one tiny package.

Side note. Higher color temperature means bluer light not more light. Avoid the blue tint auto bulbs with high color temperature. They just absorb other colors (less light output) and leave you with blue light that makes it harder to see.

LEDs are NOT efficient. They make a lot of heat instead of light and are very sensitive to it. (note they have a much better efficacy than a plain old light bulb which are about 95% heat generators) Say 6x better. Efficiency watts in vs watts out. Efficacy watts in vs lumens out. Lumens, how much visible light. Candela how bright it appears.

It's usefull to think of LEDs as current devices not voltage devices.
In the case of a "dome" LED. Aka Thru hole aka t-1 3/4 aka 5mm (diamter) LEDs or t-1 aka 3mm LEDs, that standard is 20mA. They don't tolerate more current because the epoxy dome and the metal legs don't dissapate heat very well. It's a bad idea to drive an LED with voltage because each LED and batch of LEDs have a range of voltages they drop when a current is passed thru them. A small change in voltage results in a large change in current. Since they are small you can get away with the resistor as a "ballast" to limit the current. Another LED calculator is http://led.linear1.org/led.wiz With larger lighting class LEDs it makes more sense to use a current regulator and not waste as much power (make as much heat) as a ballast resistor.

5mm leds are rated in candella (brightness) since they don't make a lot of light (lumens). Higher millicandella means the shape of the dome makes a narrower hence brighter beam of light. Lower candella ones make the same amount of light but spread it over a wider angle.

Lighting class LEDs are rated in lumens. You add optics to shape the beam depending on your application. One of the measures is efficacy lumens/watt. You can't directly convert lumen and candela without knowing the equation of the beam shape. These come in CoB or surface mount packages that better dissapate the waste heat and can be driven with more current. Some can be driven at 3amps.

For UV, dark blue or infrared LEDs etc. it doesn't make much sense to publish efficacy as your eye doesn't see the light very well. They are rated in mWatts of optical power which is efficiency.

It's fairly well know, at least for simplistic designs of solar trackers that LEDs also act (sort of in reverse) as solar cells. Note solar cells can be tested by the light they emit and plain old diodes in glass packages are light sensitive.
As for where to get em, there’s the small selection low qty places like https://www.superbrightleds.com/cat/component-leds/ or huge selection component distributors like Digikey.com

author
bertus52x11 (author)2015-01-04

Nice overview and clearly written!

Just a question: do you have a good source or brand for Dome LEDs?

I need a LED that emits wider than the common 20 degrees. Just wondering if Dome LEDs would solve my problem.

About This Instructable

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Bio: I'm Jonathan Pereira, a novice Electronics Engineer. I like to make Doze Lamps, Lumen Powered Thingamajigs, Almighty Brainy Buttons, Tweeting Fart Detectors and share ... More »
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