While all diodes release light, most don't do it very effectively. In an ordinary diode, the semiconductor material itself ends up absorbing a lot of the light energy. LEDs are specially constructed to release a large number of photons outward. Additionally, they are housed in a plastic bulb that concentrates the light in a particular direction. As you can see in the diagram, most of the light from the diode bounces off the sides of the bulb, traveling on through the rounded end.
LEDs have several advantages over conventional incandescent lamps. For one thing, they don't have a filament that will burn out, so they last much longer. Additionally, their small plastic bulb makes them a lot more durable. They also fit more easily into modern electronic circuits.
But the main advantage is efficiency. In conventional incandescent bulbs, the light-production process involves generating a lot of heat (the filament must be warmed). This is completely wasted energy, unless you're using the lamp as a heater, because a huge portion of the available electricity isn't going toward producing visible light. LEDs generate very little heat, relatively speaking. A much higher percentage of the electrical power is going directly to generating light, which cuts down on the electricity demands considerably.
Up until recently, LEDs were too expensive to use for most lighting applications because they're built around advanced semiconductor material. The price of semiconductor devices has plummeted over the past decade, however, making LEDs a more cost-effective lighting option for a wide range of situations. While they may be more expensive than incandescent lights up front, their lower cost in the long run can make them a better buy. In the future, they will play an even bigger role in the world of technology.
WARNING - SCIENCE AHEAD!
What is a Diode?
A diode is the simplest sort of semiconductor device. Broadly speaking, a semiconductor is a material with a varying ability to conduct electrical current. Most semiconductors are made of a poor conductor that has had impurities (atoms of another material) added to it. The process of adding impurities is called doping.
In the case of LEDs, the conductor material is typically aluminum-gallium-arsenide (AlGaAs). In pure aluminum-gallium-arsenide, all of the atoms bond perfectly to their neighbors, leaving no free electrons (negatively-charged particles) to conduct electric current. In doped material, additional atoms change the balance, either adding free electrons or creating holes where electrons can go. Either of these additions make the material more conductive.
A semiconductor with extra electrons is called N-type material, since it has extra negatively-charged particles. In N-type material, free electrons move from a negatively-charged area to a positively charged area.
A semiconductor with extra holes is called P-type material, since it effectively has extra positively-charged particles. Electrons can jump from hole to hole, moving from a negatively-charged area to a positively-charged area. As a result, the holes themselves appear to move from a positively-charged area to a negatively-charged area.
A diode comprises a section of N-type material bonded to a section of P-type material, with electrodes on each end. This arrangement conducts electricity in only one direction. When no voltage is applied to the diode, electrons from the N-type material fill holes from the P-type material along the junction between the layers, forming a depletion zone. In a depletion zone, the semiconductor material is returned to its original insulating state -- all of the holes are filled, so there are no free electrons or empty spaces for electrons, and charge can't flow.
At the junction, free electrons from the N-type material fill holes from the P-type material. This creates an insulating layer in the middle of the diode called the depletion zone.
To get rid of the depletion zone, you have to get electrons moving from the N-type area to the P-type area and holes moving in the reverse direction. To do this, you connect the N-type side of the diode to the negative end of a circuit and the P-type side to the positive end. The free electrons in the N-type material are repelled by the negative electrode and drawn to the positive electrode. The holes in the P-type material move the other way. When the voltage difference between the electrodes is high enough, the electrons in the depletion zone are boosted out of their holes and begin moving freely again. The depletion zone disappears, and charge moves across the diode.
When the negative end of the circuit is hooked up to the N-type layer and the positive end is hooked up to P-type layer, electrons and holes start moving and the depletion zone disappears.
If you try to run current the other way, with the P-type side connected to the negative end of the circuit and the N-type side connected to the positive end, current will not flow. The negative electrons in the N-type material are attracted to the positive electrode. The positive holes in the P-type material are attracted to the negative electrode. No current flows across the junction because the holes and the electrons are each moving in the wrong direction. The depletion zone increases.
When the positive end of the circuit is hooked up to the N-type layer and the negative end is hooked up to the P-type layer, free electrons collect on one end of the diode and holes collect on the other. The depletion zone gets bigger.
The interaction between electrons and holes in this setup has an interesting side effect -- it generates light!
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More Useful Information
When selecting an LED for your mod, it's important to note the LED's viewing angle. This is the 'focus' of the light. A wider viewing angle will spread the light across more area, whereas a narrow viewing angle will be like a spotlight. If two LEDs have the same luminous intensity value (lv), the lamp with the larger viewing angle will have the higher total light output.
For example, if you're looking for LEDs for underglow, do not use one with a narrow angle, or it will look something like this:
LEDs with a wide viewing angle will diffuse the light more and look something like this:
I personally like to use a wide viewing angle for headlights, and a more narrow angle for fog lamps. I've also found that for police/fire light bar applications, a wider angle is best.
A few things you'll need to know if buying 'bulk' LEDs:
- Source Voltage
Diode Forward Voltage
Diode Forward Current
Diode forward voltage is the typical voltage rating at which the LED operates at. A white LED typically operates at around 3.3v.
Diode forward current is the amount of power the LED will dissipate during normal operation. This is usually around 20-30mA
The above items are required in order to calculate your current limiting resistor. See links at the bottom of the page for resistor calculators. Depending on where you buy your bulk LEDs, the supplier may be able to supply you with this data. (Of course this is irrelevant if you're buying pre-wired LEDs designed to operated at a specific voltage. They have a current limiting resistor circuit built in)
LEDs are current-driven devices, not voltage driven. Although drive current and light output are directly related, exceeding the maximum current rating will produce excessive heat within the LED chip due to excessive power dissipation. The result will be reduced light output and reduced operating life. Using the appropriate size resistor is critical. The benefit of this is that your LEDs can be just as bright at 4v as they would be at 24v with the appropriate resistor. Also, I like to put my resistors on the negative side of the LED, which is the side with the shorter lead.
Relevant links and interesting diagrams:
LED Resistor calculator for Single LEDs
LED Resistor calculator for Multiple LEDs
Oznium - A good source for bright, prewired LEDs
PM me to have your LED pictures and projects included here!