How do I design a led array for my needs??? Please help

Adjuster

Joined Dec 26, 2010
2,148
...If the LEDs are of the same current rating, you can actually more or less mix and match them in a given string; it frequently works out that a higher voltage (like 24v, 28v, 32v, etc) works out to be more efficient, as you can make the forward voltages all total up to be nearly the same...
This is a good point to get hold of, and the converse of it may be as useful to think about: this is that going for single LEDs (plus series resistors) across the supply can lead to really low efficiencies. Since a very big current is then required at a low voltage, the voltage losses in things like rectifiers become quite large in proportion. That is not good news with any kind of supply, but with a linear one the result can be particularly dismal.
 

Wendy

Joined Mar 24, 2008
22,141
Actually I tend towards wall warts or modular power supplies (they are very common). Making a simple array (like LED grow lights) isn't complex, but it does tend to generate a little heat, some heat sinks may be required.

The LEDs themselves will definitely need heat sinks. If they are in direct sunlight you may need active cooling such as fans. If they are indoors a simple sheet of metal will probably work, but you may still need fans.

Did I read 125 LEDs? This is going to be a spot light, a big very bright spot light. A 3W is equivalent to a decent flashlight.

There are electronic circuits that are very, very efficient called puck bucks. They are switching power supplies configured as a constant current source. They run cool, but they are expensive. You could go with a simple LED regulator, but they will get very hot, and will need heat sinks and fans. They are also very cheap, and easy to build. There are transistor designs that are also cheap, and are slightly more complex.

Then there are resistors, they are the easiest, and cheapest, and if you are willing to solder several in parallel or series, do not require heatsinks. Overall they will still generate heat, it is just spread over 500 components or more. Resistors can be as cheap as 2¢ or 3¢ each, so don't let the numbers scare you. It is a math game, but it is simple algebra.

The color of the LED affect how much voltage they drop, this is true of low power and high power LEDs, it is pretty much a constant. Look at the datasheets of the parts you buy for exact numbers. You can mix colors in a chain, it is the current that is critical, the LEDs voltage drop is a function of the individual components.

The closer you get a chain of LEDs to the power supply voltage the more efficient it is. As a rule LEDs are much more efficient than florescent lights, so they will pay for themselves pretty quickly.

You don't need to show us the array, but if you draw a schematic we can help you optimize your design.
 

Adjuster

Joined Dec 26, 2010
2,148
If the OP really wants to run 125 3W LEDs, he could end up needing a lot of wall-warts. Certainly the little 5V ones used to charge mobile phones etc. would not be up to the job, unless he fancies using maybe 50 of them.

Modular supplies on at least the scale of the large lap-top adaptors running buck-pucks or similar would indeed be pricey, but have the advantage of requiring less design and building effort - but they do require sufficient knowledge to specify the right parts, and to understand things like heat-sinking.

I wonder in the end if the use of a commercial LED supply unit might be a better option. This would be even more expensive, but might be more realistic unless whoever is doing this has enough knowledge and practical experience build a system to handle several hundred watts.
 

bwack

Joined Nov 15, 2011
113
do u care to elaborate? these resistor methods seem functional, but i would like to go for maximum efficiency...
is there a way to wire 126 3 watt LEDs, with different foward voltage ratings, from the same power supply/driver?
You should think in terms of how much lumes (light radiated out of the thing) and not in terms of just watts. Efficiency is not only in the driver, but also in the leds. Some white leds, typically with 60 lm/W (lumen per watt), where other much more efficient LEDs have 110 lm/W.
So I will guess for you that you want maybe 126*3 W * 60 lm/ W = 22300lm. (I'm just guessing). Are you lighting a whole parking lot ? :D

That is 11 of the white 2100lm SST-90 leds
http://www.luminus.com/products/SST-90.html

Cree's X-Lamp series should have something efficient too.

The high efficient devices have better thermal paths and are more easy to parallell (if they are mounted close to eachother) so you could get away with a single CC switching driver. Get the number of leds down and you reduce failrate, possible cost too, especially if you need optics for this.
Please tell us what you will illuminate.2

Oh i forgot, efficiency also is not only in LEDs and drivers, but also in optics. Are you lighting outdoors, you could get street type optics which try to illuminate the surface below more evenly. Some leds even come embedded with optics (Osram's I think).
 

Wendy

Joined Mar 24, 2008
22,141
Depends, I picked up a 48V 3V power supply not too long ago. They make better, and if it comes to that a AT power supply would probably suffice.

Lets see, 125 X 3W is a 375W power supply. Larger than most, but doable. For that matter there is nothing wrong with multiple power supplies.

Rounding up to 450 watts, this will pull around 4 A from the wall outlet. Also doable.

The only question I would have is what is the color mix and the specs of the LEDs. I suspect fans are likely part of this final design.
 

Thread Starter

JoeJoe88

Joined Dec 23, 2011
12
JoeJoe,
Sorry we seem to be a bit slow in responding. The board is not interactive, and lots of people are wanting help. We try to help most everyone that comes in, but things usually don't get resolved in one single day.

I went through a rather lengthy monologue on how to calculate for # of LEDs in series, and resistors for the "left over" voltage to limit the current
thanks soo much for that, sorry if i was coming off ungrateful or impatient, i was just excited to finally be figuring this out.

i think im going to go with a buckblock
http://www.luxeonstar.com/2100mA-Externally-Dimmable-BuckBlock-Driver-p/a009-d-v-2100.htm

and all together 126 individual LED diodes

using 42 of the red, and 42 of the blue from this company

http://www.ebay.com/itm/20X-3W-RED-High-power-LED-Light-20PCS-3watt-LAMP-/170681500331?pt=LH_DefaultDomain_0&hash=item27bd6902ab

and 12 of these

http://www.luxeonstar.com/ANSI-White-3045K-Luxeon-A-Star-LED-165-lm-p/mr-3000-165-20s.htm

12 of these

http://www.luxeonstar.com/Deep-Red-655nm-20mm-Star-LED-720-mW-p/mr-d3350-20s.htm

12 of these

http://www.luxeonstar.com/Royal-Blue-447-5nm-20mm-Star-LED-890-mW-p/mr-r0800-20s.htm

and lastly, 6 of these

http://www.luxeonstar.com/Cyan-505nm-20mm-Star-LED-122-lm-p/mr-e0070-20s.htm

i was thinking about chaining the 6 cyan in a series with the 12 royal blue since the voltage drops r similiar

from my understanding, i would need to use a buckblock for each color
seems expensive but if its more efficient i dont mind, but i wonder if for the colors with less LEDs, would it be better to use resistors?

only thing im missing is the power supply which according to the website for the buckblock, is based on a voltage drop of each parallel series.feel free to correct me if anything sounds wrong, or if you just have some input that might make the project better im all eyes :D

thanks again for everyone who is taking time to help, i appreciate it greatly
 

Thread Starter

JoeJoe88

Joined Dec 23, 2011
12
ive been thinking this 2100mA buckblock ,and reading on it
this is from the website
"Due to the nature of this buck regulator, the input voltage to the driver must be 50% higher than the total forward voltage drop of all series connected LEDs in order to power the LEDs at their full potential. For example, to power 3 parallel strings of 3 series connected LEDs per string with an average forward voltage drop of 3.15V per LED, you will need to use an input voltage of at least 18.9V. (3 * 3.15) / 0.50. A standard 24VDC power supply would be a good choice for this example."

how efficient could this thing really be if i have to pump 50% of the total forward voltage drop?
 

Adjuster

Joined Dec 26, 2010
2,148
As its maker states, it's a buck regulator. This is a class of switched-mode power converter, not a linear regulator. In a switching regulator, voltages can be reduced without the sort of power loss that results when a resistor or a linearly operating transistor is used to do this.

These things are relatively modern, buy you would not really call them novel these days. Is the idea of switched-mode operation new to you?
 

Wendy

Joined Mar 24, 2008
22,141
They use conversion, which is extremely efficient. I don't like the 50% greater voltage requirement, since it will not use half the power supplies wattage, but it has its advantages. Now for the power supply.

I did notice it has 2.1A output, you need a current that matches the LEDs. I didn't see any options for multiple chains of LEDs, so I suspect this one is fixed at 2.1A for one chain won't work very well. You need 0.7A.
 
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Thread Starter

JoeJoe88

Joined Dec 23, 2011
12
sadly, yes. a switch-mode operation would be new to me. im currently reading up on it, if you have a link that you think would help me understand that method better, id really appreciate it

edit:after reading what i could find on the internet, i really think SMPS method is the way to go
from what im reading it regulates both current and voltage at the same time.
how would i go about calculating the exact specs i would need

edit edit: looking deeper into this im not sure. it seems extremely complex and if something went wrong, it would be a lot hard to fix, idk which method would be best for me
 
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Thread Starter

JoeJoe88

Joined Dec 23, 2011
12
"While we currently do not offer any LEDs that can be powered at 2100mA, this driver is still very useful as it enables you to power three parallel connected strings of series connected LEDs at 700mA using a lower voltage then would be required if you were to use a standard BuckPuck driver."
this is what the maker claims
 

Adjuster

Joined Dec 26, 2010
2,148
"While we currently do not offer any LEDs that can be powered at 2100mA, this driver is still very useful as it enables you to power three parallel connected strings of series connected LEDs at 700mA using a lower voltage then would be required if you were to use a standard BuckPuck driver."
this is what the maker claims
The maker's datasheet http://www.luxdrive.com/download/?dltf&dmid=1468 shows three series strings in straight in parallel with each other, but I would be dubious about doing that, even with a bit of resistance in series with each string to help the share.

If any string fails open the rest get too much current, and so may burn out in turn. Of course, it's your decision how reliable you need this thing to be.

You would certainly want any strings used in parallel with each other to be of identical devices, or possibly three strings of an identical mix would do. Different colour combinations in paralleled strings would be a bad idea.
 

Thread Starter

JoeJoe88

Joined Dec 23, 2011
12
The maker's datasheet http://www.luxdrive.com/download/?dltf&dmid=1468 shows three series strings in straight in parallel with each other, but I would be dubious about doing that, even with a bit of resistance in series with each string to help the share.

If any string fails open the rest get too much current, and so may burn out in turn. Of course, it's your decision how reliable you need this thing to be.

You would certainly want any strings used in parallel with each other to be of identical devices, or possibly three strings of an identical mix would do. Different colour combinations in paralleled strings would be a bad idea.
you make a great point i never thought about
how often/likely is it that a string would fail to open?
 

thatoneguy

Joined Feb 19, 2009
6,359
How are you going to attach heat sinks to each LED? Will they all be mounted on an aluminum panel or other surface to dissipate heat?

Make sure you have a solution to keep the LEDs from overheating before you get too far in.
 

Adjuster

Joined Dec 26, 2010
2,148
you make a great point i never thought about
how often/likely is it that a string would fail to open?
Open-circuit failure may not be as dominant failure mode for LEDs as it is for tungsten lamps, but clearly it is not impossible. You might accept the risk that one failed LED would take a few more with it, but it might be as well not to make the groups too large.

Having said that, this light is going to be more complex (and costly) to make if you use with very small groups and hence lots of regulators.
 

Wendy

Joined Mar 24, 2008
22,141
"While we currently do not offer any LEDs that can be powered at 2100mA, this driver is still very useful as it enables you to power three parallel connected strings of series connected LEDs at 700mA using a lower voltage then would be required if you were to use a standard BuckPuck driver."
this is what the maker claims
Actually this is a terrible idea. If one chain fails you are now overdriving the remaining chains by 50%, or 1.05A. It is never a good idea to design for catastrophic failure, especially when the components are so expensive.

Better still to get the right parts for the job.
 

John P

Joined Oct 14, 2008
1,808
Apart from the possible failure of an LED, there's the simple chance that a wire could become disconnected, and that would have the same effect. But you could probably improve the chances of the LEDs' survival by putting a fuse in series with each string.
 

Wendy

Joined Mar 24, 2008
22,141
I had not thought of the fuse idea. It would work. You would surge the remaining LEDs, reducing their life a little, but they would be spared.

A typical LED has a life span measure in several hundred thousand hours. It is a major strength of the little parts, and they can last longer, this is an average. Every time you abuse the parts you remove some of their lifespan. Abuse them enough and they just die on you, usually given odd symptoms like color changes or dimming.

You will find an occasional thread we actually recommend going over their given ratings. I am talking about part that is low power and cheap, it is easy to disrespect a part when it costs 10¢. Personally when a part costs over a $1.00 I want the best performance I can get though.

Merry Christmas, Everyone!
 
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