total noob with basic question.

DangerToMyself

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First off, thanks for having a forum like this where someone like me might be able to get pointed in the right direction.

I'm working on a little "moonlight" project for my aquarium that might lead me into doing some larger projects. It's been nearly 30 years since I took electronics in college. So, go easy on me. I've been doing online research into this project and some things are coming back to me. But, better safe than sorry. Thus, my noob questions.

Here's my goal: Make 3 pendant style fixtures each containing 4 blue leds. All powered by the same source.

Here's what I have: A pack of 20 LiteOn LTL17KCBH5D-002A. And a 12VDC - 1.5A power supply.

Now, like I said, it's been awhile. And looking at that data sheet, I'm thinking these are 1w leds. I was told by the acquaintance that gave them to me that they were 3w. But, I don't see it. They have a Vf of 3.2V and If of 30mA.

So, if I am right, and they are 1w, I could get a boost converter and run all 12 in series from that one PS. Correct? Now, I admit, I know little about boost/buck converters (might go without saying). So, maybe I need a larger PS from the start. I have another that's 30VDC~830mA

Any insight into this is greatly appreciated.
 

iamlucky13

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They are not 1W, but 0.1W. The datasheet has the power rating on page 2.

Their nominal rating is 20 mA, at 3.2V, for 64 mW (0.064 W).

Their maximum rating is 30 mA. Voltage increases a little bit at that current level, so the maximum power, according to the datasheet, is 108 mW.

I think the simplest way to do this is going to be to use your existing 12V power supply to run them in parallel strings of 3 LED's in series on each string, plus a resistor on each string to limit the current so you don't burn the LED's out. The resistor should be about 120 ohms if you want to run them at their nominal rating. Ideal size might vary slightly depending on the exact LED's you get.

You might be able to get away with running 4 of them in series in each string, with no resistor, but you could end up finding them either very dim, about right, or over-driven, leading to early burnout.

If you want to ensure the power supply has an easy time driving these LED's, keep the total current below half its rating. Since that would be 750 mA, you can power several dozen strings of these with that power supply.

Since this also keeps you at 12 V, there is less chance of creating a hazard around water, since you said you're using these near an aquarium.
 

DIWdiver

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+1 for KISS.

The only thing iamlucky13 didn't mention is that you can use cheap-as-dirt 1/4W resistors.

Resistors shouldn't cost a dollar total, and it couldn't be easier to build.
 

DangerToMyself

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Thank you both. I think I may have to rethink these LEDs if they are only .1w. I don't think they'll be bright enough for my needs.

Shoot, I've got the stuff to do it. So, might as well put them to the test. If nothing else, it'll be a learning experience.

Thanks again.
 

iamlucky13

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I'd suggest you wire up one string temporarily to test and see how bright it is. If this is just accent lighting for your aquarium, what you're describing might actually be enough, and it's a good way to test out your idea in general. A simple solderless breadboard like this makes it really simple to test, because you can make temporary connections in seconds:
https://www.amazon.com/dp/B004RXKWDQ/?tag=cpf0b6-20
 

DangerToMyself

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I'd suggest you wire up one string temporarily to test and see how bright it is. If this is just accent lighting for your aquarium, what you're describing might actually be enough, and it's a good way to test out your idea in general. A simple solderless breadboard like this makes it really simple to test, because you can make temporary connections in seconds:
https://www.amazon.com/dp/B004RXKWDQ/?tag=cpf0b6-20

That is kind of what I'm thinking. I ordered a decent sized breadboard a few days ago. Hopefully, I'll be able to get at least 4 going to see how bright they are. I really don't need a lot of light to achieve what I want. Just enough to simulate moonlight. I've never messed with led lights before. So, it's all learning, or relearning some aspects, for me. Who knows? Maybe I'll be able to get by with only 2 or 3 in each fixture.

Thanks for the advice!
 

DangerToMyself

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As suspected, I tested out these LEDs last night. Not even close to bright enough. No worries though. I'm learning a few things with them that will carry over. I will order some actual 1w blues today. They'll most likely be too bright. But I can dim them to where I want with the Arduino and PWM.
 

DangerToMyself

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I have a tube of 1 or 2 W blues on 25mm stars. PM me if interested.
Well, in my testing, I didn't realize that I had accidentally used 10k resistors. After switching to 120s, they were brighter. And after adding 6 more LEDs, they are bright enough. Thanks for the offer though. I would have taken you up on it.
 

iamlucky13

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Oops! Yeah, that will make a big difference. Good job spotting the mistake.
 
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