# Driver / Power supply / Dimmer advice needed!



## led-it-be (Sep 6, 2015)

Hello cpf community!

My goal is to build the most efficient led fixture possible for horticulture lighting (don't worry, only lettuce & vegetables).

I was thinking about 18x XHP35 and 12x XPE photo red 
mounted on a T-slot heatsink.

Now my problem: How to connect and drive them?
One XHP35 needs aound 12v, the XPE photo red around 1.9 - 2.5v

I would like be able to control the current of all white leds and all red leds individually (plants in flowering phase need more red light).

The goal is to really underdrive them to get the max lm/w.
I was thinking about going as low as 20w for the whole fixture (if the plants survive it). A cool white F2 bin XHP35 should work at >200lm/w at such a low current (~150mA). But I would like to be able to turn the whole thing up to 150-200w if needed.

My question: Would it be wise to connect all XHP's in parrallel and the XPE's in a 6s2p config and connect them to a 2 channel constant current power supply, and put dimmers between power supply and leds?

And where can I find the most efficient power supplies for 20-200w (97% or higher?)

Is a driver needed if I have a constant current power supply plus dimmers? I can just put the dimmers between power supply and led, right?

I was looking at metrolight.com power supplies/drivers and they look quite nice.

Thanks to everyone for the help!


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## CoveAxe (Sep 6, 2015)

> Would it be wise to connect all XHP's in parrallel and the XPE's in a 6s2p config and connect them to a 2 channel constant current power supply, and put dimmers between power supply and leds?



You should never connect LEDs in parallel. LEDs are never exactly the same and when you drive them in parallel, some will pass more current than others. You will not have any consistency from one to another.



> And where can I find the most efficient power supplies for 20-200w (97% or higher?)



That's asking for an extreme efficiency for that small of a power supply. It's much harder to have high efficiency at lower powers. If they even exist, you're going to be paying out the nose for it just to save a few watts when the lights are on. If all you're using is 20W, an 80% efficient power supply will be more than adequate and it'll only cost like $20. 90% efficient power supplies at 200W should be easy to find, with 95% maybe a little more pricey.

Also, don't forget: your power supply will only hit the peak efficiency when you are using near the maximum rated current. If you get a 95% efficient 200W power supply but only use it at 20W, your efficiency will be very low, like 80%.



> Is a driver needed if I have a constant current power supply plus dimmers? I can just put the dimmers between power supply and led, right?



What you are asking here doesn't make any sense. You want a constant current, but you want to put a dimmer in between? That's not going to work.


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## led-it-be (Sep 6, 2015)

Thank you for the reply CoveAxe!
As you guessed right I don't have any experience in connecting and driving multiple leds.

If I want to connect all of them in series, I'd need a 5 channel driver providing around 48V each: 4 channels for 16 XHPs, and one more channel around 24V for the 12 XPEs, right?

The constant current + dimmer combo might not make sense (I thought leds need cc, not cv power source, and wanted to dimm with PWM) ... Then how to drive them all and dimm the white and red ones individually?

Are there any 20W or 200W drivers that you would recommend for my case?


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## CoveAxe (Sep 6, 2015)

> As you guessed right I don't have any experience in connecting and driving multiple leds.



That's fine. We all had to start somewhere.



> If I want to connect all of them in series, I'd need a 5 channel driver providing around 48V each: 4 channels for 16 XHPs, and one more channel around 24V for the 12 XPEs, right?



You could do it multiple different ways, you just can't have any LEDs in parallel with others. The way you described would work fine. You could put them in 24V, 50V, 100V, etc strings as long as the power supply can handle it. It would probably be best if all strings were set up to be identical. Then all your channels are the same and you don't need a complicated power supply.

If you want efficiency, it's always better to have higher voltage and lower current.

You are correct that you want a constant current power supply, but it needs the current to be adjustable, usually with a controller. This has to be a feature of the power supply itself. If it doesn't have it, you could still dim it with some other components, but you will sacrifice efficiency or simplicity. PWM or current adjustment is fine.



> Are there any 20W or 200W drivers that you would recommend for my case?



Not really. I just use regular wall warts to power my LED lighting system (my lights do their own current regulation) and don't care about efficiency very much. I don't have anything high power. superbrightleds.com has lots of power supplies in the 90% and higher range that would probably work. I would suggest not trying to cheap out on the power supply. My guess for something like this is that you're looking at $50-$100 for a decent one.

Just remember that the efficiency they quote is usually at their highest output power.

So in general: you want to run as high of a voltage that you feel comfortable with and design the strings and power supply so that the power supply is running at something like 90% of rated power. If you want to run your strings at lower current for better efficiency, that is fine, but you want to scale your power supply appropriately. Running low power on a high-power power supply will cost you a great deal of efficiency. If your power supply dims with PWM, this is much less of a problem so you will have more leeway, but it is still something to avoid.


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## SemiMan (Sep 7, 2015)

There are many things that will impact driver efficiency:

- Percentage of maximum power
- Percentage of maximum current (when dimmed)
- Percentage of maximum output voltage
- AC line voltage (for a wide line range supply)

At a system level, efficiency, especially for your system, has many facets:

- Spectral efficiency (can mix not only 650nm red to white, but also 450nm blue for better efficiency)
- Placement efficiency. If you are not lighting the whole canopy, your efficiency may be low. You may be better off with a "wall" of light from low power LEDs, though 650nm low power may be hard to find. Good 5630 LEDs are 170 lpw at reasonable drive currents


For your dimming requirements, just pick driver(s) with a 0-10V dimming line. You can hook up a potentiometer and dim then directly. Most will do 10% -100%. Some will do 5% or even 1%.

Most drivers <=60V output will have fully isolated outputs (safest). Higher voltage outputs may or may not have isolated outputs.

Generally most drivers have good efficiency from 50-100% of power, tolerable from 25-50% and all best are off below 25%. 25-50% being some combination of voltage, current to get there.

Stick to strings >40V and you will not take too much of a hit from efficiency at low output voltages.

97% is pretty much a pipe dream. Sure some drivers claim it, but odds are you will not be getting it in application and you lose so much everywhere else, i.e. optics, diffusers, LED binning, optimum plant placement, etc. that the difference between 91 and 95% at the driver level can be hard to justify.

Many commercial drivers will convert a PWM input to a linear output, so there is no efficiency gain. Even if they PWM the output, that does not mean you don't have an efficiency loss at low power either.


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## led-it-be (Sep 8, 2015)

Thanks again for the responses, they were very helpful!

I looked around a bit and think that I'll go with a board that takes 5x Meanwell LDD, for example:
ebay.com/itm/1piece-5UP-Ldd-h-meanwell-Driver-PCB-/261659233641

Or makersled.com makes a nice one too (makersdriver 5up). I think I'll use the 350mA version of the Meanwell driver, which takes up to 56V input, and is "up to" 97% efficient

This way I can limit voltage to a reasonable level and dimm each driver individually.

If I buy this 54V power supply as well, then I can connect 4x XHP 12V in series to 4 of the drivers and use the fifth one for the red leds (with a 48V power supply I'm limited to 45V of leds because the LDD is a step down converter). It should be around 89% efficient @ 20% load according to meanwell, which is pretty good at this low load.

@SemiMan: Do you think I'll gain more efficiency by using blue leds? I know they have high radiant flux per watt, but the XHP35 6500K also produces a lot of blue light at 450nm and is very efficient.

I'll use a small grow tent, 50cmx50cm, so should be able to light the whole canopy.
Thinking about using 90° or 60° reflectors/optics, but maybe I could do without anything and hang the fixture lower (XHP has a viewing angle of 115°)?


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## CoveAxe (Sep 8, 2015)

So I just noticed from your previous comment that are using white LEDs. Yeah, you want to use blue. You're losing a lot of efficiency from using white.


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## SemiMan (Sep 9, 2015)

CoveAxe said:


> So I just noticed from your previous comment that are using white LEDs. Yeah, you want to use blue. You're losing a lot of efficiency from using white.



It's not as simple as just using Red/Blue is coming out of the research though that has always been true.

Depending on the plant and what you are trying to achieve, a mix of red/blue and wide spectrum may give the best growing profile.

Another issue that exists is that red/blue do not penetrate very deep into leaves and you an actually saturate the photosynthesis at the surface. Green can penetrate deeper into the leave and increase the total level of photosynthesis.

Near UV and near Infrared also generate chemical processes, plant dependent.

Best you can do is play!


Good overall solution by the way! .... I think you will be happy with the results.


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## CoveAxe (Sep 9, 2015)

My university actually does a lot of research on using LEDs for growing plants like corn/wheat/lettuce/tomatoes/etc. They've been a pioneering institution doing this for at least 10 years (and partnered with Philips lighting, NASA, and the USDA for at least part of this) and they do not use green LEDs in their greenhouses. My guess is that they've found even with increased photosynthesis from green, it wasn't as cost effective as just using red and blue alone. They have lots of papers about this I could dig up if you want. They have also looked at far-red/infrared. I can't see any press release about it that would normally accompany a new finding, so I'm guessing that it also was not cost effective for crops.

But regardless, I still wouldn't recommend using white. You are still putting out power to spectra that aren't really going to help with plant development. At best, there should have separate green emitters.


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## led-it-be (Sep 10, 2015)

SemiMan said:


> Many commercial drivers will convert a PWM input to a linear output, so there is no efficiency gain. Even if they PWM the output, that does not mean you don't have an efficiency loss at low power either.



Do I get it right that if I drive an led with a 350mA cc driver, and dim with PWM to 100mA, that the led produces light with exactly the same efficiency as at 350mA?
Would make sense because the dimmed led is still driven with 100% current, but with a low duty cycle.


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## alpg88 (Sep 10, 2015)

grow lights are deep red and royal blue. 2 reds 1 blue ratio.


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## SemiMan (Sep 10, 2015)

CoveAxe said:


> My university actually does a lot of research on using LEDs for growing plants like corn/wheat/lettuce/tomatoes/etc. They've been a pioneering institution doing this for at least 10 years (and partnered with Philips lighting, NASA, and the USDA for at least part of this) and they do not use green LEDs in their greenhouses. My guess is that they've found even with increased photosynthesis from green, it wasn't as cost effective as just using red and blue alone. They have lots of papers about this I could dig up if you want. They have also looked at far-red/infrared. I can't see any press release about it that would normally accompany a new finding, so I'm guessing that it also was not cost effective for crops.
> 
> But regardless, I still wouldn't recommend using white. You are still putting out power to spectra that aren't really going to help with plant development. At best, there should have separate green emitters.



Great thing about knowledge is that it continues to grow. Even the best universities do not have a lock on all knowledge.

The benefit of green is a recent discovery (well best understood post 2009-2010). Keep in mind, this aids in speed of plant growth (and perhaps plant health), and is beneficial when the surface of the leaf is maximizing photosynthesis (from red/blue). Green penetrates deeper, generating more total photosynthesis. Green without Blue/Red (I believe more related to blue) is not beneficial and may even have negative effects. 

Red/Blue is based around driving photosynthesis (not exclusively), but that is not the only processes in a plant. Infrared has been shown to encourage the development of plants that are initially straight, and perhaps speed early growth. UV also has regulatory properties, can impact seed growth, and is involved in chemical processes that protect the plant from disease, etc.

Red/Blue (right combination) is ideal for generating "growth" but dry plant mass appears to be optimized under different lighting conditions. I was looking for a recent paper I read, but could not find it. This is a bit dated, 2011, but still relevant: http://adsabs.harvard.edu/abs/2011AGUFM.B31E0371C. 

From that paper, "Overall, white LEDs provided a more uniform spectral distribution, reduced stem elongation and leaf area, and maintained or increased dry mass as compared to RB and RGB LEDs." .... Need to dig deeper in how that related back to energy efficiency.

Semiman


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## SemiMan (Sep 10, 2015)

led-it-be said:


> Do I get it right that if I drive an led with a 350mA cc driver, and dim with PWM to 100mA, that the led produces light with exactly the same efficiency as at 350mA?
> Would make sense because the dimmed led is still driven with 100% current, but with a low duty cycle.



Most AC LED drivers on the market likely convert that PWM signal into a linear level. In terms of LED efficiency, that is a good thing as the LED is more efficient at 100mA versus 350mA.

PWM or Linear, the processes that drive efficiency of an AC\DC LED driver are both higher and lower frequency than the PWM signal. You will almost definitely have lower efficiency at 100mA versus 350mA, but how must lower will be dependent on the driver and there is no rule of thumb.


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## brickbat (Sep 10, 2015)

CoveAxe said:


> You should never connect LEDs in parallel. LEDs are never exactly the same and when you drive them in parallel, some will pass more current than others. You will not have any consistency from one to another....



I think this is a little over-stated. It's certainly possible to connect LEDs in parallel and have them share current within acceptable limits. Take a look inside one of the millions of 60, 75 or 100W equivalent Cree LED lamps for proof. You are right in that the LEDs don't carry exactly the same current, but they don't need to either...

Series-Parallel combinations (which is what I see in the Cree example) are a good way to mitigate, to a limited degree, the uneven current distribution.


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## CoveAxe (Sep 10, 2015)

> The benefit of green is a recent discovery (well best understood post 2009-2010).



Most of their papers are from 2012 and later (and they are still writing more), so pretty sure this has been looked into by them. I'm admittedly not in that department, but I know one of their main focuses is trying to find the best energy efficiency with regard to growing. It's not necessarily about finding the optimal light spectra for the plant, but finding the right combination of spectra that will give the most biomass for the least kWh. This is the metric that's usually the most important if you are doing indoor farming or farming in space.



> and is beneficial when the surface of the leaf is maximizing photosynthesis (from red/blue). Green penetrates deeper, generating more total photosynthesis. Green without Blue/Red (I believe more related to blue) is not beneficial and may even have negative effects.



Yes, but none of that says anything about how cost effective it is. If, for example, you use 25% more energy to add green light, but only get 10% more biomass, then it's a net losing process compared to not using it at all.

UV/IR seems to be more useful in early-stage development, but much less so once biomass starts to accumulate.


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## SemiMan (Sep 10, 2015)

brickbat said:


> I think this is a little over-stated. It's certainly possible to connect LEDs in parallel and have them share current within acceptable limits. Take a look inside one of the millions of 60, 75 or 100W equivalent Cree LED lamps for proof. You are right in that the LEDs don't carry exactly the same current, but they don't need to either...
> 
> Series-Parallel combinations (which is what I see in the Cree example) are a good way to mitigate, to a limited degree, the uneven current distribution.




Almost every single edge lit LED TV, monitor, etc. is using a parallel-series combination for the LEDs as well. Generally you are building from a reel and reels are normally binned in those cases though. LEDs also have inherent resistance that helps with current sharing. If you measure the Vf ahead of time, then is acceptable, just keep them close to the same temperature.


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## led-it-be (Sep 21, 2015)

I came to the conclusion that it's easier and more efficient to use a COB.

Look at cree's product characterization tool here: http://pct.cree.com/dt/index.html
You'll see that a cxb3070 CB bin produces 2943lm at 31.1W. That is 224.4 lumen per watt!! this is insane!

Only problem: It's very hard to find top bins on the market...


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## CoveAxe (Sep 21, 2015)

> You'll see that a cxb3070 CB bin produces 2943lm at 31.1W. That is 224.4 lumen per watt!!



No, that's ~95 lm/W. 225 lm/W is no where near commercially available anywhere.

White is also probably the last spectrum you want for horticulture if you care about efficiency. You will be emitting a lot of spectra that do not help with plant growth. You normally want just red and blue. You can find papers that can give you the optimal combination for each plant if you look around. Here's a start.


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## SemiMan (Sep 21, 2015)

CoveAxe said:


> No, that's ~95 lm/W. 225 lm/W is no where near commercially available anywhere.
> 
> White is also probably the last spectrum you want for horticulture if you care about efficiency. You will be emitting a lot of spectra that do not help with plant growth. You normally want just red and blue. You can find papers that can give you the optimal combination for each plant if you look around. Here's a start.



Lots of studies .... all showing something a bit different, some showing not that much difference and even advantages with white when looking at dry weight and nutritional value plant dependent. Certainly not a 50% advantage. That would be so LED dependent as to be almost impossible to compare.


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## CoveAxe (Sep 21, 2015)

I imagine some of it is obviously plant dependent, but OP asked for lights to grow lettuce specifically, and that's what I provided them. If you have published studies behind white being better for lettuce, I'd be more than happy to look at them.


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## SemiMan (Sep 21, 2015)

Can't put here because behind a paywall but one I linked above relates to that and does look at many veggies. Just skimmed the one you sent ... Seemed narrow. Here is another on lettuce http://www.sciencedirect.com/science/article/pii/S0304423812004797

Showing more depth on various components under RB and RBW.

The one you linked comparing to MH and HPS in 2014 seems strange. When did they start writing it?


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## led-it-be (Sep 21, 2015)

CoveAxe said:


> No, that's ~95 lm/W. 225 lm/W is no where near commercially available anywhere.



typo, I meant 2943lumen at *13*.1 W. Please have a look at the cree tool.
The idea would be to drive ~ 5 of them in series in order to get max efficiency and enough light for the lettuce.

And yes, i found a place where I could buy cxb3590 DB bin (the second highest). That would produce at least over 200 lm/W at relatively low currents.

edit: and I've seen guys who bought top bins in june, so they were available and hopefully will be in stock again somewhere.

I get what you are saying about white light, and in terms of light absorption rate of chlorophyll it would make sense to use blue/red light, but then again the weed growers on various forum say that the COBs blow everything out of the water.

You need quite a lot of blue/red leds to get much light, compared to only a few cobs.


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## SemiMan (Sep 21, 2015)

Don't look at absorption ... It's action spectrum that matters.

Deep red / blue are the most efficient just need to supplant with white.


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