[Techtalk] matching LEDs to resistors

Michelle Murrain michelle at murrain.net
Mon Dec 5 02:47:03 UTC 2011


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.
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