[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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email: michelle at murrain.net
web: http://murrain.net
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