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This is my first post here. I haven't done anything with electronics/circuitry since college way back in the 80's. So, go easy on me.

I want to put together a string of LEDs for a moonlight type thing for my aquarium. There will be 4 LEDs in 3 separate fixtures. However, I want to power all 12 from one source. Make sense?

An acquaintance gave me (20) LiteOn LTL17KCBH5D-002A LEDs. The data sheet is here (if allowed) http://optoelectronics.liteon.com/upload/download/DS20-2013-0089/S_110_LTL17KCBH5D-002A.pdf

These have a max Vf of 3.6v and a peak forward current of 100mA. So, in a perfect world, I should be able to run 3 parallel groups of 4 (wired in series) off of a single 12v - 1.5A source. Correct?

Or would I be better off running them all in parallel?

Makes sense! Two issues:

First, datasheet says "peak forward current of 100mA" but that's only for 1/10th duty cycle. Peak power dissipation is 108 mW per LED, which is closer to 30mA continuous at absolute max before it fries itself:

$$(30 \ \text{mA}) (3.6 \ \text{V}) = 108 \ \text{mW}$$

Somewhere between 20-30 mA should probably be right for continuous drive.

Second, you may not be able to get the LEDs to turn on at all if you put four in series and drive from a 12V source!

$$4 \cdot 3.6 \ \text{V} = 14.4 \ \text{V}$$

This is greater than the power supply voltage. They'll probably turn on but very weakly.

You'll probably have to do 3 in each series chain, plus a resistor. Or perhaps two LEDs and a resistor if you're set on four per fixture.

Here's what happens if you do 4 in series -- click this circuit and run the DC Sweep Simulation:

As you can see, there'll barely be any current at all with four LEDs in series powered by a 12V source.

Instead, use three LEDs in series and a resistor:

You may have to adjust the resistor value based on your actual LEDs and power supply voltage.

Thank you. I'll have to look at this again tomorrow with a clearer head. But I think I see where you're coming from. Much appreciated.

Well, I've had some time to look this all over with a clearer head. Makes perfect sense. And you caught the fact that these LEDs are not 1w, but rather .1w. Not sure they'll put out as much light as I want and may have to get others. But, what the hay, they were free. So, I can learn with these regardless.

I thank you for taking the time and effort to set me on the right path.

Excellent. Glad to help!

I had some free time this weekend, so I wired up the LEDs. They were very dim. Way more dim than I thought they would be. After sitting there stating at them for a time, I realized I grabbed 10k ohm resistors. Duh! I swapped them out for the 120's and they were much brighter. But not quite bright enough. So, I wired up 6 more for a total of 18. 3 groups of 6 spaced about 14 inches apart did a really good job of mimicking moonlight. THAT part of the project is now planned out and awaiting the final build. Thanks again!

Now for the next stage.... Maybe ...

I'll start a new thread for bit though.