LED Resistor Question

[Hulldiver]

I do marine electronics for a living. I install and wire up equipment. I don't play with circuit boards. I install a lot of LEDs but ive never wired up my own. I drew up a quick schematic of what im trying to accomplish. I want 4 LEDs. 3 will be suffice if need be. Below is a link of the LEDs I want to use. I don't know what resistor I need.

Cool White 5050 SMD LED | Surface Mount (SMD) | Component LEDs | Super Bright LEDs

[offroadace]

I've always used these calculators with mine.
Just enter the required values and it should give you the correct resistor.

Single
Serial/Parallel

[Hulldiver]

Thanks. I'll try it out later tonight when I can sit down with the computer instead of my phone. The whole forward voltage, reverse voltage, voltage drop thing just confuses me.

[Hulldiver]

So... Heres the plan. Minus one led in the diagram. 4 would require 3 in series and 1 parallel with 2 resistors. 3 is much easier. the light bar will probably end up with 3 wires coming out of it. one center and one on each side. 1 pos, 2 neg = 2 resistors.



I took a picture of the laptop screen. Bad. I know. You get the idea though.

[Kingmeow]

You can't do that kind of wiring for a 3S, 11.1V pack.

According to your link, each of the LED you are using is 3.2V, 20mA. If you wire them in series, that's already a need for 3.2x4=12.8V which you can't supply with a 11.1V pack under load.

You can use 3 LEDs in series:

3.2x3=9.6V
11.1-9.6=1.5V difference.
Ohm's law V=IR, 1.5V=0.02mA x ?, so your resistor is 75 ohm or thereabouts.

Since you want 6 LEDs, you can construct two sets of 3 LEDs wired in series with the 75 ohm resistor. Then just wire the two sets in parallel.

(pos) - LED1 - LED2 - LED3 - 75ohm - (neg)
(pos) - LED4 - LED5 - LED6 - 75ohm - (neg)

[Hulldiver]

Right. I went ahead and figured for 12.5v, full charge, id rather it be a little dim than burnt. You pretty much hit the nail on the head though.