Verifying resistance math

[Rayataz]

Hey guys,

I just wanted to make sure my resistor math is sound before accidentally burning any components out. I'm planning on using the Nano Biscotte sound board with a 3.7v li-ion battery for my lightsaber. Now according to the Nano Biscotte manual, it comes with 3.3v pad for powering the LED and I intend to use it to power the LED of a 16mm momentary switch (http://www.thecustomsabershop.com/16mm-Anti-Vandal-Short-Momentary-Blue-Ring-Switch-P905.aspx). According to the specs on the site, the LED has a forward voltage of 3.0v and uses 20mA.

So here is my math -
R = (Vs - Vf) / I
R = (3.3 - 3.0) / 0.02
R = 15 ohms

P = (Vs) * I
P = 3.3 * 0.02 = 0.066

Thus, the closest resistor I can find is an 18ohm/0.25w resistor (http://www.thecustomsabershop.com/18ohm-14w-resistor-P506.aspx).

Can someone make sure this is correct? And if I'm wrong, please tell me why. :confused: Thank you!

[Greenie]

I believe this is most commonly used for the led of switches :)

http://www.thecustomsabershop.com/20mA-DynaOhm-Variable-Resistor-P847.aspx

[Silver Serpent]

Your math is correct.

There are 15 ohm resistors, but the 18 ohm resistor in the store will work just fine. The blue and green accent LEDs are usually a little bright for my tastes, so I will often use an even higher ohm value for them, to dim them a bit more.

[Rayataz]

Thanks guys! I appreciate the input!

And the DynaOhm variable resistor looks really useful! lol (Except if the LED is too bright, I should opt for a higher resistor for a lower amperage)

[Rayataz]

So I have a follow-up question: Let's say I want to use that same 3.3v pad to power both my switch LED and a 5mm LED. Since the voltage isn't enough for the two to be lit in series, I would have to set them up in parallel, correct? Do I use the same math for each individually or is there something else I should take into account?

[Forgetful Jedi Knight]

You can use the same math for both, just keep in mind that the pad will only put out 20mA which will be cut in half if you try to run 2 LEDs in parallel.

[Rayataz]

You can use the same math for both, just keep in mind that the pad will only put out 20mA which will be cut in half if you try to run 2 LEDs in parallel.

Ah, got it. Thank you! :)