I think most of you members in this site have been working/worked with several kinds of
micro-controllers. And most probably you will have situations where that you needs to connect a
LED to the GPIO pin(s). When you connecting a LED to your GPIO pin it's essential to connect it
through a series resistor of approximately 100-Ohm.
But many people ignore that. But this is a serious flaw in your design. Want to know why?
Well this short article is all about explaining why?
LED is like a diode it has exponential VI curve. Which is normally given by,
.
Which means a very little linear increment of Vr (like 10mV) will double the forward current on it's operating
point. So that's why we need to consider this issue more terrible and should not ignore it.
Since these are the very basic things that we learnt in grade 9 or 10. But Even well experienced design engineers
are doing this mistake. Because why, forgetting and don't care attitude. It's really really bad and sad in electrical
engineering.
This same mistake is also found in when we driving transistors using microcontrollers. There too we have to use that.
Otherwise microcontroller's output drive will get damaged.
So how to calculate the appropriate serial resistor value?
Well the simple answer is V=IR. for a example if your micrcontroller GPIO pin have a 5.0 V output.Then,
Normally for normal LED's operating point Vf =2.0 V and If= 10mA.
For a transistor , you need to use it's maximum base current (Ib) and put Vf as 0.7 V.
--Happy Designing and Circuit Analysis--
--Merry Xmas too--
