I tried do some examples on Arduino using LED's and resistance but not the normal value (for some little diferences of ohm value).
But I would like to understand better, which type of resistance/value we should use, I have no much idea, I readed on google but I still can't understand easily which value use in which case
can someone provide if it's even a link with some good explanation or soemthing like that ?
Hi, you mean work out the correct resistor to use with an led?
An led needs a certain voltage to light. Its called the forward voltage, written Vf. The brightness depends on the current (up to the maximum of course). Red leds have a Vf around 1.8-2.2V. Amber/yellow around 2.2-2.6V. Blue and white around 3.2-3.8V. There are two types of green, some have similar Vf to yellow but others similar to blue. Too much voltage will damage the led.
So if you are running an led with a Vf of 3V with a 5V supply, you need to "loose" or "drop" the extra 2V that the led does not need. This can be done with a resistor in series with the led. The led drops 3V and the other 2V by the resistor.
Unlike leds, resistors follow Ohm's law, so the voltage drop accross it depends on the current flowing through it. The law says R = V / I.
if we want 20mA (= 0.02A) of current to flow through the led and resistor, we need R to be 2V / 0.02A = 100Ohms.
So have a go. What resistor would you need to get 15mA to flow through an led with Vf of 2.2V?
Paul
Nice clear explanation Paul 8)
Pedro147:
Nice clear explanation Paul 8)
Agree, was good explanation.
Thank you Paul for your time, you made me understand it finally! I readed a couple of articles but was too much deep and didn't covered the part you explained, or then, the way was written I couldn't get it.
biscottoesse:
you made me understand it finally!
OK, so what's the answer to my question?
Using what source voltage?
So have a go. What resistor would you need to get 15mA to flow through an led with Vf of 2.2V?
Is one to assume 5V? Or the lowest Vhigh from an arduino pin, 4.2? Or 3.3V
I suppose you did mention a 5V supply:
(Vs - Vf)/current = resistance
(5V-2.2V)/.015A = 187 ohm, use a standard 180 ohm resistor.
(5V-2.2V)/180 ohm = 15.6mA
Aww Bob! The question was meant as an exercise for biscottesse!