Even though it is a
basic thing for an Electronics Engineer, I have personally seen that many
people get confused on choosing the ‘right’ resistance value when connecting LED
in a circuit. Some prefer choosing a small resistance around 220 ohm and if LED
glows they consider it to be correct. However, choosing right resistance value
for LED has a simple logic.
Each LED has a rating.
For example, the LED shown below is very commonly used:
This has a rating given
as 2V, 20mA. This rating should be considered in choosing the resistance value.
Suppose you are considering a LED which has rating same as above, and want to connect it to a source
voltage of 5V. So the source is providing a voltage of 5V, but the LED needs around
2V. So the remaining (5-2=3v) has to be dropped across resistor.
So now you have the
voltage across resistor as 3V, and current to be given to LED is 20mA. So using
Ohm’s law,
Resistance R= V/I=
3v/20mA= 150 ohm is the required resistance.
See
it’s simple J
