Choosing proper resistor value for LED anodes

[ViperDoc]

OK, I just want to make sure I've got this right:

Let's say I have:

1) a 5VDC relay power supply
2) and an LED with a 2V forward voltage rating and a 20 mA forward current rating.

like this: https://www.mouser.com/ProductDetail/bi ... cycode=USD

I use Ohm's law as R = V/I where V = 3 volts for a desired 3 volt voltage drop, and I = 0.02 A, which gives me 3/0.02 = 150 ohm. Slap a 150 ohm resistor on the LED anode and I should be good, yes or no?

Thanks.

[xtian]

No, you plug in the source voltage for R, 5v. You want a 250 ohm resistor to allow 20mA of current. Coincidentally, I just did this exact thing in my Black Butte Desktop #2, because I had an unused 5vAC winding.

[martin manning]

Viperdoc is correct. Current is set by the resistor value and the source voltage minus the LED junction drop. Usually the LED is run at a fraction (say 1/2 to 1/4) of its rated forward current.

[ViperDoc]

Cool, so a range of 150-600 ohm is acceptable? Thanks!

[thetragichero]

modern leds are super bright even with high resistance. I've been using them in place of the standard 6.3v lamps and even using 10k they're still way too bright. you should be fine using what you've got a lot of

[ViperDoc]

That’s great news. Thanks!