Lighting CREE project for micro grow box

[captain budseye]

So, I've been thinking. I will try to change the XP-Ls for XHP50 (native 12vdc). The XHP70 "scare" me as I expect them to be too strong (4022lm/32w a piece = 125lm/w) for such a small space and bleach my plants. Plus the XHP50 has a great lumen output for the watts => 2546lm/19w = 134lm/w!
Now I wonder, given the very limited volume of my space, should I install 2 or 3 leds? According to another website, 5000lm per square foot is average and 7500lm per square foot is high. I would expect 2 XHP50 (about 5000lm) to suffice as that already would be at least twice what my present fixture outputs. Light bleaching is a concern because of the fact that the fixture will be immobile and I already have light burning problems with my present fixture when the plants come closer than 5cm to the leds.

What do you guys think? 2 or 3 XHP50 in my 52cm L / 23cm D / 44cm H box?

Sorry I was going to get back to you earlier with some kind of answer to your previous questions but I go side tracked by life. I had a look at the XP-L and XPE I-V characteristic and if you want to go down that route I can try to help figure out what ratings of resistors you'd need, although it would be less than ideal and possibly impractical. One big problem with using those LEDs (I'm assuming the other cree 3w were XPEs?), is that if you drive the XP-Ls at 3v they draw over 1A of current, which is well within their rating, whereas the XPEs only draw around 200-300ma, if I'm remembering correctly. This could cause problems if one of the XPLs died, because that current has to go somewhere and could blow the XPE strings. However if the XPEs went the XPL string might well be able to take the surge current caused by one or both of the XPE strings blowing, but don't take my word on that I am no expert, not yet at least . The other issue is that the difference in voltage required to drive the XPEs at 300ma to 1A is only 0.5v, so without proper regulation the current going through them could vary massively depending on a lot of non-ideal things that could happen.

Another thought I had was that you could put fuses on each parallel string which would blow if one LED went, but at least it wouldn't destroy your whole set up. Another option is build a simple current source circuit out of a couple of transistors if you're comfortable with a soldering iron.

In all honesty I still think your best bet is to try to find a constant current boost converter that's in budget and wire all of your LEDs in series if that's at all possible. You can get ones on ebay from china for dirt cheap, I'd be more than happy to have a look and give you a hand finding the right kind of thing.

In terms of bleaching your plants I don't think that anyone who hasn't tested them empirically can really advise you on whether 2 or 3 of those LEDs is going to work, because a lot of that if not all is purely trial and error.
What I would recommend as a strategy if possible is using as many LEDs as possible but running them at a lower drive current. I don't know if you've already bought them, but you can get cree XPEs for cheap off of aliexpress, they won't be the best bin, but at very low drive currents the efficiency gain from driving them softly might make up for it.

 

[Jimmy McGill]

Hey, mate. Well, at this point, adapting the original plan just feels like too much work/money spent/more parts that can go wrong. I've been thinking a lot about this since yesterday and no matter how I turn it around, I have to stay within the parameters of my grow.
As I can't put my hand on a dc/dc converter, I'll just keep on using my power supply with native 12vdc leds. I trust it as I did a few grows with it and know it is stable at all times. No flickering, nor surges, it does a good job. Plus it is extremely discreet and discretion is a serious concern to me at the moment.
The XHP50's are by far my best option right now. The are very powerful and run on 12vdc which simplifies my problem immensely. I will probably take 3 and make separate circuits so I can switch them on and off independently if needed. I think that it's my best option since cobs are not a possibility for the moment. If one burns out, it will just stop working without affecting the rest of the rig, no hazards, no catastrophic failures unless the Corsair gives up, but seeing it can handle 52Amps, I don't see that happening anytime soon with such a "light" draw (9Amps).
Plus it will be a good test and see how those tiny XHP monsters performs in a grow box.

 

[mjau]

At what current do you run xhp50? I ordered some to try them, thinking of trying xhp70 also, I even might like them more. Will try to make panel of 9 XHP50s, 12V and run them on 700mA.

 

[Jimmy McGill]

At what current do you run xhp50? I ordered some to try them, thinking of trying xhp70 also, I even might like them more. Will try to make panel of 9 XHP50s, 12V and run them on 700mA.

Hey mjau. I don't really know and I actually would like to know it too. Since I am running them with a computer power supply, I would think they receive the current they "ask" for. But I don't know what that value is.
In my rig, they run without flickering or perceptible changes in light intensity, also they do not overheat, all that makes me think that they get a constant current that is adapted to proper functioning. But, again, I don't know if that value is the max 1.5amp they can take in a 12v circuit or a lower one.

The XHP70 look great too, I hesitated for a long time between 3 x XHP50 and 2 x XHP70. I finally settled on 3 x XHP50 for light distribution and heat management. I am happy with the way they work, not too hot and plenty of light from a very small package.