What are the electrical characteristics of LEDs?

It's useful to think about two main types of LEDs—the familiar indicator LEDs that come in 5mm and 3mm epoxy packages, and “illumination-grade” LEDs, which are high-output devices, designed for lighting.

A typical indicator LED has a forward voltage rating between 2 and 4 Volts of DC. You may see maximum ratings above that. A typical drive current for indicator LEDs, even high-brightness ones, is 20 milliamperes (mA). From this you can see an indicator LED dissipates a modest amount of power—a few tens of milliwatts compared to the few tens of Watts a familiar incandescent bulb uses. In other words, the power used by an indicator LED is one thousandth of that used by a familiar light bulb.

Arrays are constructed to take advantage of this low power consumption. A series string of ten blue LEDs will take a 33 VDC forward voltage to light, but still only draw 20 mA of current from the source. So the supporting wiring can be less expensive for an LED array compared to a light bulb (which may draw an Ampere of current—fifty times as much as the LED). For parallel arrays, the current combine. So you can drive 10 blue LEDs in parallel from a 3.3V source, but the current drawn will be 200 mA. This flexibility in array construction is part of what makes LEDs very popular in mobile, battery-powered devices. The designer can arrange LEDs to take best advantage of the power source that's available.

Illumination-grade LEDs have comparable forward voltages to indicator LEDs. This is a reflection of the fact that the junction material is the main determinant of the forward voltage. But the junctions in illumination-grade LEDs are typically larger, and can draw more current, and dissipate more power (while producing more light). A Luxeon Star LED has a drive current of 350 mA, and dissipates about 1 Watt of power.　　

Another important LED spec is maximum reverse voltage. A diode conducts current when a forward voltage is applied, but will not conduct if a reverse voltage is applied, up to a point. Reverse voltages in excess of the maximum can cause the diode to fail.

What happens if I overdrive an LED?

A typical indicator LED will have a spec that represents a typical point along the operating curve. That would look something like “3.3V @ 20mA typical.” Driving this LED above that point will shorten the useful life. You may also get a maximum rating for either current or forward voltage. Exceeding those ratings will dramatically shorten the useful life, generally ending it suddenly in the process. But driving an LED “hot” will make it burn more brightly for a shorter time. The heat dissipated by the junction has to be conducted through the leads, which aren't very big and aren't designed to heatsink the package. When the junction runs hot, the light output will also degrade much more rapidly. You may find that your LED won't light up at all without being overdriven somewhat. If the package of an indicator LED (the 5mm or 3mm types) feels hot, you are definitely overdriving the LED. Illumination-grade LEDs are designed with heat sinks in the package, and it's normal for these to run hot. The 1W dissipated by the Luxeon Star will warm up a heat sink quickly at room temperature.

Then there's thermal runaway. The current through the junction will tend to increase as the temperature rises; so current-regulated drivers are preferable to voltage-regulated drivers. But proper thermal management is important too.

How do they make white LED light?

There's two main ways to get white light from LEDs: The first method is to combine light from red, green, and blue colored LEDs. If you get the right mix, the effect is white light. This is the same way your television works—a white object on the screen is really depicted using dots of red, green and blue lit up in proportions that form an impression of white. White formed this way can be “tuned” to look warm or cool by adjusting the amounts of colors in the mix.

The second method uses a blue LED with a phosphor coating. The coating emits a yellow light when the blue light from the LED shines on it. The mix of the yellow light with the blue light forms a white light. Inefficiency in the phosphor conversion is one reason that a white LED is less efficient overall than a colored LED. Some of the light energy is lost in the conversion to yellow.

What makes an LED such a good colored light source?

If you want a colored light, LEDs can't be beat for efficiency. The process that gives off the light makes light of a certain wavelength, which is a function of the junction material. The efficiency of that process is much higher than with incandescent lamps—15% for LEDs compared with 5% for incandescent. Then when you filter an incandescent light down to a single color, you give up as much as 90% in the process. So when you need a colored light, such as a traffic signal, or an exit sign, LEDs have a significant advantage.

Conversely, LEDs aren't yet as efficient at making white light. LEDs commonly make white light by the process of exciting a phosphor, which gives off other wavelengths of light, giving a combined effect of a whiten light. The conversion of the phosphor isn't perfect, so some efficiency is lost.