I learnt that when we drive a LED, we should consider the working voltage. But then i asked myself about the current because i cant calculate the resistor i need then i read that you can take current as 20mA.
Here is the question:
The thing that runs the LED is the current ( or electrons ) not voltage. Voltage is like a pressure difference which moves the electrons here. Then why do we firstly consider the voltage or why do LEDs have a working voltage not current? ( like red led generrally works with nearly 2V )
LEDs run on energy which is the product of voltage and current, but if we supply it with a proper current it will assume the voltage that is right for the diode at the time. That voltage may change a little, but if you keep the current about the same all the time (like 10ma, 15ma, etc.) then the LED lights and the voltage drop is determined by the LED itself.
The voltages you read about are the characteristic voltages. That's like 1.2, 1.8, 2.2, 3.5, etc. These are the approximate voltages the LED will assume when driven with the current level it was designed to run at.
In a way this is the opposite from a light bulb that you screw or twist into a socket at home. The bulb requires a certain voltage, and if you apply that voltage it assumes a current that is right for the power of the bulb as it was designed. For the LED however we apply a current and let the device assume whatever voltage is appropriate given the internal chemistry, although we know from experience that the LED will choose a voltage that is close to the characteristic voltage.
So for the bulb, we choose the voltage and the bulb assumes the right current.
For the LED, we choose the current and the LED assumes the right voltage.
In each case the variable we dont really know at the time stays within a certain range given the normal operating power level of the device.