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post #15 of (permalink) Old 04-04-2012, 06:10 AM
Planted Tank Enthusiast
Join Date: Apr 2011
Location: North Jersey
Posts: 514

I did some work using the equations and posts you’ve made in this thread and elsewhere along with the excellent spreadsheet tool. I think my questions below belong in this thread because they address a topic that will be common among anyone with wider tanks.

I am working on designing a fixture for a 40 breeder which is 18” front to back. As far as I can find online I haven’t found anyone making one yet so breaking some new ground, in a way.

I don't mean the below to ask, "design me a fixture." Instead I am asking for some clarifications or suggested assumptions in using the equations and sheets to account for cases of widely spread LED rows of 4-5" spacing between rows (and >8" spacing between first and third rows)

As previously suggested by yourself and common sense, I went into this with some “knowns” and requirements for the fixture, to fix some of the variables. I know that I want the recommended middle PAR range of 40-60 as you have frequently referenced, I want even lighting over my tank, no more than 2 LED drivers, dimmable for fine-tuning PAR, passive cooling via aluminum channel or other heat sink, and I wanted to build a canopy that is less than 15" above the tank rim - the canopy height bit is one of the more important variables that can be fixed because it most directly impacts how the LED will be designed of course.

For use in the spreadsheet I chose 20” above substrate as the height. Pending actual builds etc I could vary this up and down within reason, as the canopy itself won’t be built prior to the lights.

I know that an 18” wide 40 breeder with lights 20” above the substrate will require multiple rows of LED lights if I use aluminum channel passive cooling. With 40 degree optics (more on this in a second), at a height of 20”, the “high intensity” inner-half cone diameter will have a 3.5” radius at the substrate surface. To avoid high intensity glass strike, I figured that the starting point for my rows would be to have one directly centered in the tank, one 5 inches from the front glass and one 5 inches from the back glass.

I read with great interest a post you made elsewhere regarding that rows of LEDs greater than a few inches apart don’t add up when kept at a low height, and thus it probably isn’t accurate to use “n = 3” for number of LED rows in this design.

So, knowing that my front and back rows would be 8” apart, and the center row is 4” from the row on either side of it, it seemed I was at the point where it’s just a guess to select any number above 1 as the “number of rows” combining for PAR.

Using 1.25 as “n” rows to account for the large spread from front row to back row but still account for “some” combination, I came up with the following:

27 Cree XPGs
3.333” Spacing per row, 4-5 inches between center row, front row and back row
20" above substrate
60 PAR
40 Degree optics

Would let me run the XPG at .6-.7 Amps – within the passive cooling range and if I adjust the PAR down it gets better from there.

This seems like a high number of XPGs, and with quite focused Optics, compared to what I’ve seen elsewhere used for tank lighting applications. Do you think I have made drastically wrong assumptions at some point in the process?

If I change to 60 degree optics the calculated LED current required to hit 60PAR with the 27 XPG (3 rows of 9 at 3.33”) goes to 1.1 A. Way too high for passive cooling. But why would I need 40 degree optics only 20 inches above the substrate?

edit: Of course if I change my assumption to "n=2" which is still of course only 66% of my actual number of rows to account for the spread from front to back, it decreases the number of LEDs required (4" spacing) and amperage as well. Seems like I might need to just experiment. Which isn't so bad anyway - the two worst cases are that I either end up having to go higher above my tank than I wanted to do and possibly rethinking using a canopy, and/or having an extra row of LEDs on a heat sink I can use for another tank. I can't find it but I am sure you have posted somewhere the graph of Cree XPG output vs distance from light (horizontal) at one or various mA? If you have, I would be able to look at that and figure out how much each spot on substrate should get with this setup.

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