# What makes a good RGB LED

I am looking for supplier for RGB LEDs for an upcoming project RGB POV LED Globe. Normally I would go to my local electronics supply story, but this project requires lots (~1000) of LEDs and my local electronics store charges a premium for the LEDs, (@1.50 each)

## Current

An LED creates light by dissipating power across a semiconductor junction. Below a certain current, the electrons aren't boosted to the next shell, and you get no light. Above a certain current, you destroy your device. By modulating the (average) current between these two values, you can get varying degrees of intensity. This is a non-linear function with respect to light intensity, but you can get a more linear function by using a constant current and rapidly pulsing the LED on and off. This technique is known as pulse-width modulation, or PWM. If your PWM never exceeds a given duty cycle, and is sufficiently fast, you can set the constant current at which would, at steady-state, exceed the maximum power rating of your LED. This doesn't usually get you a brighter average, though.

You need to select LEDs whose current requirements are within the limits of your drive circuit and whose power requirements are sustainable using your chosen power source.

## Forward voltage

The forward voltage will be different for each color in the LED. This just complicates the calculation of the current a bit. If you're using a resistor to set the current, and the LEDs are common-anode, you should probably select LEDs with similar forward voltages to minimize power losses in the resistors. Be aware that the forward voltage is a function of the forward current!

# Standard stuff

Then, there are the generic properties that you need to select for any electronic device:

• Package
• Soldering temperature
• Manufacturer/Supplier

## Package

You may be tempted to use a standard T 1 3/4 5mm dome package. Don't accept this unless you're sure that it's what you want. To get 4 leads under this package, you need small, tight holes (soldering and PCB manufacturing will be hard), and the optical properties are less than optimal.

There are a plethora of surface mount packages which are lower profile and lower weight (which is desirable if you want to spin your project) and which have high viewing angles without using diffused lenses.

## Soldering temperature

LEDs are some of the most sensitive components to heat when soldering because of the optical requirements of their lenses and because of the unique semiconductors used to generate the light. Be careful if you're using anything but a temperature regulated soldering iron or oven for this.

## Manufacturer and distributor

For a one-off project or prototype, Adafruit or Sparkfun products are fine, but (1) you'll pay a premium for their selection and endorsement and (2) you're out of luck if they drop the product. The hobbyist sites are fine if you're making a one-off product, but if you want to distribute plans, make sure a compatible LED is widely available. Otherwise, contact Cree, Avago, or Lite-On (or your favorite manufacturer) directly, or use a major distributor like Digikey or Mouser. You'll have better luck and get better prices by buying in bulk and skipping the middleman.

# Color

One of the most important factors to consider is color, but RGB LEDs basically define that for you. You do need to consider the relationships between each color in your selection, but this can usually be accounted for in software. For instance, the human eye detects green much better than it detects blue, and red LEDs are usually more efficient than blue.

In addition to the relative power between the colors, you need to consider the spectral information. Many manufacturers have different definitions of each color - Red might be any light with a wavelength between 629nm (an orangey red) and 660nm, green could be from 515nm to 565nm, and blue could be anywhere from 430nm to 470nm (a greenish blue). And that's just the nominal peak! This isn't a laser, so not every ray of light coming from it has the same wavelength -there is an irregular distribution of the wavelength for each color. A red LED will emit a tiny amount of blue light, and vice versa.

• +1 for it being longer then I want to read with pretty headings and image – Kellenjb Mar 21 '11 at 21:36
• @Kellenjb - Not the reasons that I hoped it would earn upvotes, but I'll take it. :P – Kevin Vermeer Mar 21 '11 at 21:40
• Note that you can diffuse your own LEDs just by lightly sanding the outside of the case. More of the light will bounce back into the LED and out through other walls when the walls are of a rough texture. Just a tip for if you accidentally get stuck with water-clear LEDs when you wanted diffused ones. – Joe Mac Mar 21 '11 at 23:01
• @Joe Mac - For large quantities (anything greater than 1, really) ou'll be much better off doing this chemically (either with an acid, like the original manufacturer would use) or by spraying with a clear paint or adhesive. Both methods are hacks, and both will reduce the light output. – Kevin Vermeer Mar 21 '11 at 23:13
• @Joe Mac - Really, any modification of individual components is probably best left to the manufacturer, who already has them on an assembly line. There's no reason that you should accidentally get stuck with a component: if your supplier screws up, a phone call should fix the situation. – Kevin Vermeer Mar 21 '11 at 23:15