leds

LEDs(Light emitting diodes) are little lights used in many products. You can find them in toys, synoptic panels, car lights, garden lights, house lights, remote controls etc.
You can find them in the classic colors red, green, yellow,orange,infrared but also the newer colors blue, white, UV, pink.
The famous history of the leds begun in the 60's.


Leds are current driven devices. The brightness is proportional to the current through the led. Usually a resistor connected to a voltage source limits the current in most applications. Mostly this is the easiest and cheapest solution.
A current of 20mA for a 5mm led is the nominent current for most devices, but always check datasheets or application notes before connecting leds to power supplies. Because one little mistake can send your led to heaven. Never go higher then 25mA else the lifetime of your led will be shorter

Also important is the direction of the current thus also the polarity of the voltage. You can compare leds with diodes. In one direction they conduct current and will be illuminated, in the inversed direction they stop the current and remain dark. Now how can you see what the positive lead(anode) is and wich the negative lead(cathode). Well the longest lead is the anode the other the shortest is the cathode(flat side on the package).
A bad connected voltage higher than the backward voltage typical 4 to 5V will damage the led)
The forward voltage of the led differs from color to color. Some typical voltages are: Calculate it:
Now the most important formula to connect the led with a series resistor at a voltage:

      R = ( U - Uf ) / If

Example: White led with a nominal current of 20mA and forward voltage of 3.2V will be connected to a voltage of 12V
what resistor do i need?

     R = ( 12V - 3.2V ) / 0.02A
     R= 440 ohm
So the resistors must be at least 440ohm this means we take a real resistor of 470ohm

Some advice: Don't raise the input voltage too high, because all the energy will be wasted in the resistor.
When the power dissipation is high, the resistor needs to be bigger and more expensive.

The power rating of the resistor needs to be at least:       Pmin = ( U - Uf ) * If

Example:When you take the values of the example above, our resistor of 440ohm must be rated:

      Pmin = ( 12V - 3.2V ) * 0.02A

     Pmin= 0.176Watt
This means we take a resistor with a nominal power rating of 1/4 Watt
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