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Cultivated Reef

Dimmable LED Driver… Dimmable How?


cocojakes

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I've been looking around on e-bay and the like, trying to build myself a cheap LED light for my frag tank. I'm thinking I'm going to make an array of 1W LEDs, instead of a few small clusters of 3W, as the tank is only 8" deep, so I don't need a ton of penetration power, just want to be able to spread the light out more, so 3x 1W can be spread out more than 1x3W.

 

Anyway, the idea is two strips, each strip has:

 

5x warm white

10x royal blue

5x blue

5x violet

 

all 1W LEDs

 

the issue is in the drivers. I want to just have 2 drivers, one for RB/V and one for W/B. so one driver needs to drive 20x 1W LEDs, and the other needs to drive 30. I want to be able to dim the two channels separately, just with simple dimmer knobs. but this is where I'm getting stuck

 

what do they mean when they say dimmable driver, as I still only see two input leads, and 2 output leads, no leads for a dimmer. Do they mean I can use a standard dimmer switch, to dim the 120V coming from the wall going into the driver? how does this work, does it still allow the driver to dim the current while keeping the voltage strong enough to drive the whole string of LEDs?

 

If someone could give some input, on any aspect of this build, I would appreciate it.

 

 

PS, do people think I can get away with 25x 1W LEDs on a 20" heat sink, without needing Fans? The whole point behind this is to avoid fans, otherwise I would just get a standard chinese black box LED light similar to the reef breeders value fixture.

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You won't need fans, provided the heatsink profile is adequate. You only mentioned the length, so I can't guarantee that you can get away with it.

 

As for the drivers, it sounds like they are triac dimmable. Yes, that means dimmable via a wall type dimmer switch. That doesn't mean that any dimmer will work. The cheap ones are strictly resistive, and won't work (the driver will get very grumpy). The next option is called a triac dimmer. A triac is a solid state AC switch, and is used to chop up the AC waveform. It's also known as phase dimming. These are only a little more expensive than resistive dimmers (you don't see resistive dimmers very much any more), but usually required a higher load to operate more efficiently. Then you have reverse phase dimmers. Triac dimmers chop the leading edge of the AC waveform. reverse phase dimmers chop the trailing edge. These don't require as high of a load to operate efficiently, and usually are less noisy (less buzzing at certain dimming points). They are also the preferred method for LED dimming.

 

Anyway, there are a lot of articles online describing more about how dimmers work.

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okay, so a driver that says it is dimmable, suitable for 18-25 x 1W LEDs, I could hook up a dimmer to the AC input, then hook the output to 24x 3W LEDs (the driver sends 54-90V DC at 300mA) and be set?

 

the heatsinks (I have 8 of them) are 150x20x6, thinking 6 LEDs per heat sink, running at max 300mA, most likely lower than that since they will be less than 6 inches off the water, and an incredible number of LEDs for such a small tank size. just to get good even spread.

 

Does this seem reasonable? Or am I asking for trouble, gonna find out when I get it all together that theres some kind of mismatch between the dimmer/driver/LEDs, or that even at such low power they way overheat such tiny heat sinks.

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You should be fine on the drivers, but just double check by looking at the forward voltage for each LED at 300mA, and make sure that it's within the limits of the driver. Don't take "18-25 LEDs" as gospel.

 

As for the heatsinks, they seem a little on the small side to run passively. A small fan blowing over them would help immensely.

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I'm thinking I might only put 3-4 LEDs per heat sink instead of 6 like originally planned. Also whats the best way to figure out forward voltage of an LED string? the driver says 300mA 45-90V DC as the output, seems like most high powered LEDs have somewhere around 3V drop each? so 24 LEDs in a string isn't too unreasonable?

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It's dependent on the LEDs used. Companies like Philips and Cree have their datasheets posted so you can pretty reliably estimate the forward voltage of the LEDs in question. Chinese LEDs though, generally don't have their datasheets posted, and when they do, they aren't always the most accurate.

 

If you don't have a reliable source of data for the LEDs, then I would suggest hooking up an array that would fall somewhere near the middle of the voltage range of the driver based on your estimations. Once you have the array fired up, measure the voltage drop across each LED color. That will allow you to estimate better whether or not your full array will play nice with the driver.

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