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### Topic: Blue LEDs & Resistors (Read 1 time)previous topic - next topic

#### tcoop25

##### Oct 17, 2011, 02:25 pm
If I am powering a blue LED (with a voltage drop of 3.4V)off of a 3.3V pin (on something like an Arduino Fio), do I still need to include a current limiting resistor?

-TC

#### Noorman

#1
##### Oct 17, 2011, 02:42 pm
There is nothing to limit in this case, the led is powered with less than it's forward voltage and will use almost it's nominal current.
You have to check if the current the led uses is not exceeding the current limits of the port it is connected to.

Cheers

#### Grumpy_Mike

#2
##### Oct 17, 2011, 03:24 pm
Quote
do I still need to include a current limiting resistor?

You ALWAYS need a current limiting device, the simplest of which is a resistor.
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

#### tcoop25

#3
##### Oct 17, 2011, 03:57 pm
Thanks Grumpy Mike.  So, a simple 330 ohm resistor would suffice?

-TC

#### Grumpy_Mike

#4
##### Oct 17, 2011, 06:15 pm
Well it depends on how much current you want down it.
For a 3.4V LED being run off 5V then you would use:-
5 - 3.4 = 1.8V across the resistor
1.8 / 0.02 = 80 Ohm resistor

So anything larger will allow less current to flow. Normally 10mA is fine so 160 R resistor would be good.

#### retrolefty

#5
##### Oct 17, 2011, 06:29 pm
Quote
If I am powering a blue LED (with a voltage drop of 3.4V)off of a 3.3V pin (on something like an Arduino Fio), do I still need to include a current limiting resistor?

You simply cannot drive a led with a Vf or 3.4vdc from a 3.3vdc logic pin, the led will never be forwarded biased to be able to turn on reliably. However once a led is forward biased some form of current limiting is always required.

Lefty

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