LED's 101
Posted Wed, 05/14/2008 - 22:54 | by 1500W
LED’s can be an excellent choice for accent lighting and ambiance. Since the relatively recent invention of the high output white LED, they are now well suited for direct lighting. Having said that, LED’s behave very differently than conventional incandescent lights.
First and most obvious difference is LED’s have a polarity. They will conduct current in only one direction. Further, they don’t conduct until there is sufficient voltage, usually 2-3 volts, they immediately conduct very well. If unchecked, LED’s will conduct more current then they can handle and burn out. For this reason, it is important to include a current limiting device, usually a resistor, to protect the LED. LED’s are not really specified by voltage but instead by current. Most discrete(low intensity) LED’s I have seen typically have an operating current of 20mA.
As rig builders we have several power options to power LED’s. First, the Power can be pulled right from the PC power supply. This could take the form of 12v, 5v or possibly 3.3v. While taking power from the PC power supply is convenient, it does limit the amount of power you can use for LED’s although not significantly for most small LED array applications. The second drawback to using the PC power supply is control. The lights can only be on when the computer is on. To avoid this I suggest purchasing a separate power supply. If funds are short, they are easy to find at the local Salvation Army or Goodwill store. Power supply capacity depends on the number of lights in the array as well as the voltage each LED requires. As I said before, most LED’s require 2-3 volts. This excludes most blue and white LEDs which can require up to 5 volts. Always ask for a voltage and current spec when buying LED’s.
Once you have the LED’s and know their specs, its time to start building arrays. If you are using common LED’s, those requiring approximately 2 volts, I recommend stringing five in series. This means chaining five LEDs together by connecting the positive on one to the negative of the next and so on. At either end put a resistor (I find 110ohm works best) and connect to a 12v power source. If they don’t light up, its typically because one of the LED’s is backwards. It is best to connect a ammeter in series to the circuit to ensure the LED’s are not conducting more than 20mA. If, at first, you make a mistake and over current the LED’s, rejoice knowing LED’s are cheap! If in doubt, start with 200ohm resistors and work down. Once the first string is built, it can be placed in parallel with other strings. Remember, to ensure uniform brightness, it is best that each string have its own resistor. I have found that you can run about 1,000 common LED’s in this manner on a 2-amp 12-volt power supply. For LED’s requiring higher voltage, try using 4 LED’s in series along with a 110ohm resistor. This seems to work for blue and white 3 volt LED’s purchased from Digikey.
There are many ways to physically build an array. On favorite is to use ¼” pegboard and hot glue. The LED’s drop in and you can fit approximately 150 per square foot.
i wonder if there is
i wonder if there is something so that you can have scrolling text with those on your desk, that would be really cool.
On the bag of some it says
On the bag of some it says that the flat edge of the led is either positive or negative
Beautiful shot of the
Beautiful shot of the LED:)
Great info too.
New LEDs have one lead
New LEDs have one lead longer than the other. In most cases, the longer lead is positive.
Also in most cases, the smaller metal conductor thats inside the epoxy lens housing, is positive.
The larger base with the parabolic dish (which mounts the chip die) is negative.
Otherwise, just hook it up randomly to find out, it’s only 1 out of 2 chances…
can’t take too long…
It won’t hurt the LED, just make sure not to over voltage them in either direction.
You can use an LED resistance calculator to find out the right resitstance you’ll need:
For ONE LED:
http://led.linear1.org/1led.wiz
For TWO or more LEDs:
http://led.linear1.org/led.wiz
At one point in development
At one point in development I looked at a device call the IOWorrior40. This is a USB interface that could do what you are describing. It was too much of a development challenge for me to tackle at the time. PM me for detailed info.