[Techtalk] matching LEDs to resistors

[Robyn Willison]

On 5/12/2011 1:17 PM, Michelle Murrain wrote:
> Hi,
>
> I'm an electronics newbie - so it would be great to learn more about
> this  - and I'm a bit confused.
>
> So an LED, for instance that has a  3.2-3.8V Forward Voltage, at 20mA current.
>
> Say the driving voltage is 5V:
>
> delta V = 2.8 V
>
> Ohms Law: V=IR
>
> 2.8 V = 20mA * R
>
> r = 2.8 V / 0.020 A = 140 Ohms?
>
> On my Arduino, I've been using 1K Ohm resistors - that's a lot more
> than 140 Ohms. So why are they OK? (one tutorial I saw suggested 300
> to 2K resistors for these LEDs.)
>
> What's missing?
>
> Peace,
> Michelle
>
> On Sun, Dec 4, 2011 at 5:03 PM, Tracey Hytry<shakti at bayarea.net>  wrote:
>>
>>> OK electronics gurus-- how do I know which resistors go with LEDs? Other than
>>> asking the vendor. Like this one
>>> http://search.digikey.com/us/en/products/WP7113SRD%2FE/754-1276-ND/1747675
>>
>> You need to know the voltage drop of the LED, subtract that from the driving voltage
(5V?); and decide how much current you can put through the LED.  Then 
use ohm's law to
find the dropping resistor value.

Hi all

5v----Resistor-----Led
        1.8v        3.2v

Led is a max 20mA

R = V/I

R = 1.8/.02

R = 90 Ohms

The current of 20mA is the Max this led can take, you want to keep it 
less than that so higher resistors are better.

5v----Resistor-----Led
        1.8v         3.2v

I = V/R

I = 1.8/1000

I = 1.8mA

The led wont be as bright with the larger resistor value.

Cheers

Robyn