Since their introduction in early 2000s, Light emitting diodes (LEDs) have continued to cause waves in the electric industry. They are widely known for their uniqueness in generating light in diverse manners. The LEDs do have practical characteristics. Getting to know more about that will help you appreciate their function in the electric world. In this write-up, an attempt is therefore made in discussing the basic practical characteristics of LEDs.
LEDs are current devices
LEDs are never voltage devices as some people may think. They are rather current devices like all other diodes. This means that Ohm’s law doesn’t apply for LEDs. The law states that voltage and current are proportional. The case is different with LEDs. Voltage and current are not proportional in LEDs since they are simply current devices. The performance of LEDs is mainly determined by how much current that passes through them. LEDs are driven in a unique manner when they operate. Their optical output is usually specified in lumens or watt. The power supplies meant for LEDs are mainly designed to drive them with a steady current.
Another characteristics found in LEDs is forward voltage. Generally, the forward voltage of each LED is always constant. However, this may vary with the magnitude of current passing through some LEDs. Each LED is also different depending on its make-up. You can compare them in diverse ways. The Silicon diodes for instance are used as rectifiers. Some of the diodes with lesser forward voltage usually dissipate less power as they are used. The forward voltage characteristics also depend on the temperature level of the die. In most cases, this may also depend on how big the entire LED package is. A diode in a big package is likely to stay cooler thereby having a higher forward voltage. Most LED die manufacturers have continued to research on how to reduce the forward voltage of the device for maximum efficiency. If this is reduced, there is a possibility of increasing the light per watt output of the LED.
Most diodes usually conduct current when they experience a voltage applied from anode to cathode. They will also conduct if they have enough voltage from cathode to anode. This can be the case whether the conduction is carried out intentionally or not. Avalanche and Zener diodes fall under the intentional category. You can easily use any of the diodes based on the actual voltage you want the conduction to occur at. On the other hand, rectifier diodes and LEDs fall under the unintentional category. When they conduct current in the reverse direction, they normally have the opportunity of being broken. However, if any of your diodes gets broken in such manner, you can still get it replaced with a higher voltage part. Most LEDs are very complicated and hence, they do have a reverse breakdown voltage of only 5V. This can actually cause serious problem in a practical circuit where the LED is used.
Meanwhile as semiconductors, LEDs of all kinds have absolute maximum current rating as stated by their manufacturers. These are 1 A or 700 mA although there may be variations since there are various kinds of LEDs and their manufacturers. Some white LEDs for instance are driven above their rated current. This can force their phosphors to saturate while the light output turns blue.
Efficacy - not Efficiency
Efficiency in the electric world refers to electrical conversion. It is the output power divided by the input power. Efficiency is usually expressed as a percentage. As for LEDs, efficiency is never the term you can use for them. Efficacy is rather the right term. The efficacy of white LEDs and many other types is usually measured in lumens per watt (lm/W). Lumens refer to the amount of light the human eye can perceive. However, this perception has strange effects. Usually the human eye peaks in the green part of the visible spectrum. If for instance a green LED and a red LED comes with the same efficacy, the red will also be emitting a lot more light than the green. However, you can’t compare two LEDs based on their efficacy except if they are having the same color. Most LEDs don’t emit the same wavelength since they are not lasers.
White LEDs for instance are produced by the addition of 2 or 3 different colors. Hence, their efficacy depends on the exact mixture of colors. It also depends on the brightness of the colors mixed and the direction where the spectrum of each color is emitting. Most white LED vendors do consider efficacy as the parameter for their sales. Hence they play games with the spectral composition of the white LEDs.
In any case, you have to be careful if you are concerned about efficacy. To run an LED at more current level than it produces will make its efficacy to reduce.
Optical Spectra in LEDs
Actually, the main goal of LEDs is to product light. However, their optical spectra are key performance parameters found in them. The optical spectrum of an LED gives you more information about the lights it produces. However, there is no real practical way of looking at the spectrum before you decide the color output of the light in a given application. Hence, a set of numbers are usually used in characterizing the vital features of the optical spectrum in LEDs. Such numbers include CCT and CRT. The CCT gives an idea on how cold the light may be while the CRI gives idea on how well the colors are reproduced by the light.
In all, the above discussed subheadings throw more light on the practical characteristics of LEDs. You need to digest them well in order to appreciate LEDs and their functions. You can also learn more about the designs and interior mechanisms of LEDs as you keep on researching on the devices. The future of LEDs is indeed very bright. LEDinside expect more breakthroughs in the area as the technology evolves.