pg17
Here's the catch 22. If you have the background to swap LEDs, you will probably be building your own units instead of using commercial ones.

Remember each kind of LED has a different voltage drop/forward current curve. The V-drop can even vary in the same model LED depending on the mfg. bin. Also good heat sinking does not readily lend itself to easily switching out LEDs. Plus you need fast and good soldering skills.

I looked at the possibility of being able to change LEDs when I first starting building. The only practical idea I could think of was to mount an entire series string ( or a portion of the string) on a strip of copper or aluminum and fasten it to the bottom of the heat sink. I didn't think it was worth the effort especially considering it would be introducing another thermal junction

I have been using mostly K2s since efficiency is better then the IIIs and most Rebels. I do use SSCs for my cool whites. I have noticed a price drop of 30% this month from Future Elec.

To my knowledge most of the medium to large LED drivers are primarily voltage regulated at 24 V. I am only seen current regulated in the smaller DC to DC bucks and boosts. If you look at the Xitanium part numbers, the 4th character from the right designates voltage or current control. That is why I try to leave about 1 to 2 volts to drop in each series string and use a resistor.
I don't know if that is overkill or not, but at least it gives me a convenient way to monitor individual current to each string without going to the hassle of inserting an ma meter.
------- found some 60 watt drivers on Ebay for $20 plus shipping.

you might want to recheck your physics texts regarding
"Closer is not actually good because light only drops off with distance if the inside of the room is absorbing light".
Everytime you double the distance from a light source, it diminishes by a factor of four, whether it is in a grow room or outer space. Just play around with a light meter to verify.

At this stage of LED development, I believe the only significant edge LEDS have over HID is the ability to get very close to the plant. The only disadantage to getting closer is your area of coverage decreases. I put mine 4 to 6 inches max above the scrog.

If you run at 4.5 inches that gives you a 16X advantage over an equivalant light source run at 18" which is typical of HID.

more corrections.
Dont mean to beat up on you, but If I don't Physicsnole will.
The top of the UV range is 400nm. 435 is some of kind of blue and it is one of the peak absorption points along with 660nm for Chlorphyll (either A or B, I can't rememberwhich). That is why everyone is trying to hit those wavelengths.
The big question will it be worth the effort in practical results?
By the way, I have heard the same thing from good sources that UV helps the quality but not the quantity.

Something everyone needs to keep in mind is why are we messing around with LEDS. I think the obvious reason is to get more light energy per watt of household current. I can understand why everyone wants to drive them at max currents because they are so expensive but that is really defeating the purpose of what we are trying to achieve.

If you want to make better units then the Pycron or the Megablaster 2000, you can't drive LEDs at their max rated currents. Look at the LEDengin red 660nm for example. When you increase current from 700 ma to 1000 ma, power is increased by 56%. However, output only increases 25%. That means that the additional watt output is only 50% as efficient as the first two watts. If you used that extra watt instead to power 1/2 of an additional LED, you would be getting twice the light output from that watt.
And from a practical standpoint it becomes much harder to keep the LEDS cooled to a normal level when you push them towards max rating, which causes the output efficiency to drop a bit more.

I'm not buying that about the reflectors . Maybe Physicsnole can offer an understandable explanation, I'm not that swift with physics.
My thinking is that the reflected light energy has to travel an additional distance to the relective floor or wall with its energy decreasing by the square, take a really big hit with the relective inefficiency of whatever reflective material is used and then lose more energy traveling to the plant. Plus it is going to be tough getting the angles of reflection where they bounce right back at the plant. Most of the reflected light is probably going to miss the plant and hit another reflective surface.
I don't use focus lenses on the LEDs, so I have a typical 140 degree view angle, I do use a small hood on the edge lights in order to keep a consistent pattern.

If I had enough leds to give me good coverage, I would have them almost touching the tops

Yes, those are the two trade offs: cost or energy.
Each person will have their own criteria. I lean more towards the energy efficient, not so much for saving energy but to maintain the a good profile with my electric bill. Also in the spirit of research, since the goal most people are working on is trying to make them more energy efficient then HIDs.

Trying to make LEDs $$$ efficient is a losing battle at this time. HPS is so much cheaper. Of course in a year or two or three, it will be a different story. We are all just playing around now trying to figure out the best configurations until we can get our hands on the "good stuff" that is hopefully in the pipe line.
However, I do see your point. I am driving the LEDengin 660nms at
over 4 watts instead of 2.5 because they are so damn expensive.
When I am spending $2 to $3 on K2s, not that big a deal. On the K2 blues if you run 100 at one watt instead of 50 at 2 watts like the Procyn, you will put out about 25% more light from the array, with the added benefits of twice the light sources and easier to manage temps.

If you have a small grow it is not worth the expense of doubling up on your LEDs to get the extra efficiency. If you are running over a KW, energy efficiency starts to be an issue.

I am going to play around with some water cooling designs in a few weeks and see if that can prove useful.

Thanks for the link: I was going to solder some copper tubing to copper plate screw it to the bottom of a heat sink and mount the LEDs right on the copper tubing, but those are cooling plates are muy bueno.
I bet those puppies are expensive, they sure look nice. Have you seen any pricing? I guess you still have to run a bong cooler in order to get below ambient temp or have some other way to cool your water.
However, on my 25 watt arrays. They only run 2 or 3 degrees hotter then ambient temp with the fans on, so I am not really feeling much pressure to water cool.

If you are using K2s, dont sweat the PCB. I just attach the bare emitter directly to the heat sink using Artic Alumina Adhesive. It is not electrically conductive. It looks a little funky having connecting wires on the bottom of the light, but I really didn't want to screw around with PCBS and you get a very direct thermal path.

I only use the 25 watt Xitanium for test purposes. I got a bunch of
180 watt, 24 volt drivers used for the sign and lighting industry. I posted a picture of one a day or two ago. I run six 25 watt arrays per drivers. I do have some extras that I will be disposing of soon.

I still don't get it about the reflective room and the distance. You guys are saying that if I had a perfectly reflective room I could turn the light upside down and it wouldn't make any difference or move it an infinite distance away as long as it was inside a perfectly reflecting sphere. How come you are not taking into account the extra distance the reflected light has to travel?
Lets say you got a bulb that emits light evenly in all directions and it is in the exact middle of a sphere coated with a material that reflects with 100% efficiency. Lets put the target at the out edge of the sphere. All the light that does not hit the target directly is going to travel a longer distance before it hits the target bouncing off at least one surface but most likely multiple surfaces. That light will be diminished by the inverse square law.

That is why you want to get those lights right on frikken top of the plants, not 18 inches away.

I presume you are using rebel stars instead of the bare rebel emitter?
If you are using the bare emitters you can get a pcb from Asia Signals to mount them on. I believe you do have to have a hot air setup to solder them, I don't know if an iron would work. I have been wanting to try some Rebel emitters with the Asia Signals PCB, since the emitters are cheap and some colors have better efficiency then the K2s. The Rebel stars are just too darn expensive at this time.