What are LEDs?
LEDs are semiconductor devices. Like transistors, and other diodes, LEDs are made out of silicon. What makes an LED give off light are the small amounts of chemical impurities that are added to the silicon, such as gallium, arsenide, indium, and nitride.
When current passes through the LED, it emits photons as a byproduct. Normal light bulbs produce light by heating a metal filament until it's white hot. Because LEDs produce photons directly and not via heat, they are far more efficient than incandescent bulbs.
Not long ago LEDs were only bright enough to be used as indicators on dashboards or electronic equipment. But recent advances have made LEDs bright enough to rival traditional lighting technologies. Modern LEDs can replace incandescent bulbs in almost any application.
LED Circuits
To build LED circuits, it helps to be familiar with Ohm's law, and the concepts of voltage, resistance, and current.
LEDs do not have resistance like a resistor does. LEDs have a dynamic resistance, that is their resistance changes depending on how much current passes through them. But it's easiest to think of them as having NO resistance. This means that if you just connect an LED to a battery, you'll have a short circuit. That's bad. You would probably ruin your LED.
So an LED circuit needs some resistance in it, so that it isn't a short circuit. Actually we need a very specific amount of resistance. Among the specifications for LEDs, a "maximum forward current" rating is usually given. This is the most current that can pass through the LED without damaging it, and also the current at which the LED will produce the most light. A specific value of resistor is needed to obtain this exact current.
There is one more complication. LEDs consume a certain voltage. This is known as the "forward voltage drop", and is usually given with the specs for that LED. This must be taken into account when calculating the correct value of resistor to use.
So to drive an LED using a voltage source and a resistor in series with the LED, use the following equasion to determine the needed resistance:
Ohm's = ( Source Voltage - LED Voltage Drop ) / Amps
For example, to drive an LED from your car's 12v system, use the following values:
| Source Voltage | = | 13.4 volts (12v car systems aren't really 12v in most cases) |
| Voltage Drop | = | 3.6 volts (Typical for a blue or white LED) |
| Desired Current | = | 30 milliamps (again, a typical value) |
So the resistor we need is:(13.4 - 3.6) / ( 30 / 1000 ) = 327 ohms
Don't forget to pay attention to the difference between amps and milliamps. Ohms law calculations require units in amps, although the specification is usually given in milliamps. Just divide milliamps by 1000 to get amps.
Also remember that it's acceptable to round the resistance UP to the next closest standard resistor value. Rounding up is never dangerous, as it will result in slightly less current. You will never find a 327 ohm resistor, but 330 ohm resistors are quite common, so just use a 330 ohm resistor.
For a very thorough discussion of how LEDs work, check out this article at How Stuff Works.
For a good roundup of all the popular LEDs and LED devices out there, check out The LED Museum.
Also check out http://science-ebooks.com/electronics/basic_electronics.htm,
online basic-electronics. |