LEDs

From TheBestCaseScenario

[edit] LEDs

Light emitting diodes, commonly called LEDs, are real unsung heroes in the electronics world. They do dozens of different jobs and are found in all kinds of devices. Among other things, they form the numbers on digital clocks, transmit information from remote controls, light up watches and tell you when your appliances are turned on. Collected together, they can form images on a jumbo television screen or illuminate a traffic light.

LEDs have been around for longer than you may think, invented during the dawn of computers and electronics for that matter, LED's emit light when a current passes through them,

[edit] Connecting LEDs

© Copyright 2006 - John Hewes"

LEDs must be connected in the correct way round or they will not work, the diagram may be labeled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method). Bi-colour LEDs A bi-colour LED has two LEDs wired in 'inverse parallel' (one forwards, one backwards) combined in one package with two leads. Only one of the LEDs can be lit at one time from a DC source, but AC will appear to light both simultaneously. Overall, they are less useful than the tri-colour LEDs described below.

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[edit] Calculating LED resistor values

© Copyright 2008 - Sebastiaan Pierrot " LED's allways need an resistor, this is a simple tutorial how to calculate the right resitor values for your LEDs. LEDs are current driven components and always have a datasheet with their specs on it. I'll try to explain with the use of an example.

For this example I chose a nice red Led http://www.farnell.com/datasheets/70655.pdf

Now you have to look for 2 values:

(Vf) Forward Voltage, this is the voltage that drops over the LED.

(If) Forward Current, this is the current at which the LED shines at its best

It is best to look for the typical values of the LED and not the maximum values as this will result in shorter life span for the LED.

Now you have to wonder At which voltage you want to use your led, We will call it (Vs)Voltage source I will use 12 volts

With this formula you will be able to calculate your resistor values (this is just ohms law) (Vs - Vf)/If = resistor value

our example: Vs = 12 V

Vf = 1.90 V

If = 20 mA (you have to specify in A) so it is 0.020 A

(12V - 1.9V)/ 0.02 A = 505 ohm

This resistor value does not exist so you will have to look for the nearest value in the E12 series: http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/comp/passive/resistor/e12/e12.html

The nearest value for this resistor will be 470 ohm, this is a bit smaller but that won't affect the LED very much because we used the typical (safe) values of the led specs. "

[edit] Testing LEDs

Never connect an LED directly to a battery or power supply! , It will be destroyed almost instantly because too much current will pass through and burn it out.

LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1kohm resistor is suitable for most LEDs if your supply voltage is 12V or less. Remember to connect the LED the correct way round!

For an even quicker method, they can be connected directly to certain batteries. 3x1.6V watch batteries in series will work. Most keychain flashlights use this principle, since the battery is unable to produce enough current to damage the LED.

[edit] Tri/Bi Colour LEDs

The most popular type of tri-colour LED has a red and a green LED combined in one package with three leads. They are called tri-colour because mixed red and green light appears to be yellow and this is produced when both the red and green LEDs are on. The diagram shows the construction of a tri-colour LED. Note the different lengths of the three leads. The centre lead (k) is the common cathode for both LEDs, the outer leads (a1 and a2) are the anodes to the LEDs allowing each one to be lit separately, or both together to give the third colour. Bi-colour LEDs A bi-colour LED has two LEDs wired in 'inverse parallel' (one forwards, one backwards) combined in one package with two leads. Only one of the LEDs can be lit at one time and they are less useful than the tri-colour LEDs described above.

[edit] LED-Stripes