High-Brightness LED Grow-Light

Note: This project page has morphed into a design discussion.  (Constructive suggestions and collaboration are still welcome.  See the comments section at the end.)
**There are no recommended construction-plans here.**
Instead, I recommend shopping around for a good deal on standard White LED Light-Bulbs.  Please try using them as grow-lights, and let us all know how it goes.  My calculations suggest they should should be economical, energy efficient, and great for plants.

After many hours of research I've concluded that it doesn't make sense to build DIY grow-lights, and I can't honestly recommend it to anyone.  This is partly because it is technically difficult (and making it easier makes it more expensive), but more-importantly because commercial products are much less expensive and higher quality in the end.

You're better off buying an off-the-shelf product.  And surprisingly, I wouldn't even recommend buying a red & blue "LED grow light", because the best option seems to be conventional white LED lights!  Sure, grow-lights are approximately 25% more energy-efficient for growing plants, however, they are priced 2 to 8 times more per unit of power.  LED grow lights cost $4/Watt-electric typically, or nearly $2/W-e if made in China.  White LED bulbs cost $1/W retail, and sometimes $0.50/W if there is a green-subsidy instant-rebate. 
White LEDs emit more green-light than typical "grow-lights", which unfortunately isn't used very efficiently by plants; however, paying for the electricity to emit that inefficient green-light actually costs less than buying a grow-light that doesn't emit green-light!  Even if hypothetically you have sky-high highest electricity rates of $0.50/kWh, and zero-percent financing, when I try to amortize the cost of grow-lights, I figure that the LEDs burn-out before accruing enough saved electricity to make up for their high up-front cost.

Step 1: Light-Spectrum Requirements

So what kind of light do plants need?  What is best?
Well, in terms of spectrum, the chlorophyll pigments are the main drivers of photosynthesis.  These are most active under blue light at ~430 nm, and red light at ~660 nm.  According to US patent #6921182, plant health is improved by the presence of light around 612 nm.  From the graph below, this corresponds to the peak for "Phycocyanin".


Some of the overall response curves I found, such as for "PAR" are graphed.  Also the spectral output of some common White LEDs are graphed.  It turns out that the peak sensitivity for chlorophyll from blue light (at ~430 nm) lines up almost exactly with the peak output from a white LED, so this is a fortunate coincidence.

Step 2: Light-Intensity Requirements

Suppose we have a plant with 1 square meter of leaf area.  The graph shows that when the light-intensity is about 200 "micro-moles" (umol) of absorbed photons (particles of light) per second, this causes about 8 umol/s of CO2 molecules to be converted into plant matter.  This ratio of 8/200 gives a particle conversion-rate of 4%.  With 6 times more intense light (1200 umol/s), the plant growth rate is increased by only about a factor of 3, since (24 umol/s of CO2) is "assimilated".

So we can see that plants grow more with more light, but not linearly.  There are diminishing returns.  This suggests that for a given amount of light energy: 1) a longer "solar day" (such as from artificial light) improves growth and efficiency, and 2) that it's better to have diffuse light that illuminates the foliage evenly, rather than unevenly, for example where there is greater light-intensity on the top leaves, and lower-intensity on lower-leaves (such as from shading of undergrowth).

Step 3: Obsolete Project Reminents That Was to Build-from-scratch

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** What follows is an old discussion of a build-from-scratch concept, that I now consider obsolete.... left here as a reference only. **
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Step 4: Circuit Schematic

For the power supply, use one with a somewhat higher voltage.  I chose 12v because it seems to be the most common "brick" voltage.
This allows you to put more LEDs in series.  You can always guarantee that all LEDs in a given "string" (or branch of the circuit) are carrying the same current, which simplifies matters.  Also fewer strings means fewer resistors. This reduces parts count, etc, and the whole system is likely to be more efficient, produce less heat, not overheat your plants, and keep your electric bill down.

Start the circuit design by arbitrarily adding LEDs to a string.  Add up the voltage of all the LEDs in a given string, and adjust the number of LEDs per string to get as close as possible to the power supply voltage without exceeding it. To do this it helps to mix and match LED colors in the string, since each color has a different voltage. 

The main concern is that you don't exceed the LED's current rating.
Ohm's law says that the resistor value "R" in units of "ohms" that you should use is given by this equation:
R = (Vs - V_LEDs) / I_LED

Where Vs is the power supply voltage,
V_LEDs is the sum of the LED voltages in the string, and
I_LED is the current in amps that the LEDs are rated for

If V_LED is just slightly less than Vs then you will only need a very low value resistor, like 1 ohm or less, assuming a 1 amp string. You shouldn't need to drop more than a volt or two across the resistor. If you're dropping over 2.2 volts, why not just add another LED instead?

Just calculate the proper resistor value for each string, and after you've built the circuit, you can measure the current through each string with an ammeter to make sure the current does not exceed the LED's spec, especially when it is operating at its highest temperature.  You can also calculate the current by measuring the voltage across the resistor and dividing by the resistance.  (again, ohm's law).

The figure below shows a simple schematic example.  12 volt power supplies are very common and you should be able to find an extra one lying around that you can re-purpose for this, or you can acquire one from your favorite surplus distributor, for example allelectronics.com.  In this example it should have a capacity of 2 Amps or more, or whatever is the sum of the current in each string. 

The resistor values may need to be adjusted to limit the current in each branch to 1 amp, particularly given the LED's negative temperature coefficient, which may be as high as -4 mV/deg.C (check the "datasheet" pdf for the device).  I'm also looking into using copper pcb-trace resistors (or "wirewound resistors"), mainly because copper has a positive temperature coefficient of resistance (about +0.4%/deg.C) which will help regulate current through the LEDs.  So far the approach looks promising.  Also, in theory this type of resistor is free (or under $1), simple (amenable to DIY), and high-power, which is ideal for our design goals.  In theory, if the LED and copper resistor are solidly connected to the same heatsink, and so are at essentially the same "case temperature" (although not necessarily the same "junction temperature", but I will gloss over that for now), then we can calculate what minimum value of resistance we need so that the overall temperature coefficient is above zero, so there is less risk of "thermal-runaway" that could damage or destroy one or more LEDs.  For example, for a 1 Amp LED, we would want at least 1 volt drop across the resistor, so that by Ohm's law, the +0.4%/deg.C resistor would yield +4 mV/deg.C, thus canceling the LED's -4 mV/deg.C.   Also by Ohm's law to get 1 volt drop at 1 Amp, requires a 1 ohm resistor.   This part can be bought for under a dollar, or can be made , for example with 2.5' of #36 wire, or 4' or #34 wire, etc.  (Table_of wire sizes)

Step 5: LED Selection, Bill of Materials

Use high-brightness LEDs that can handle at least 700mA to 1 Amp of current or more, because they put out a lot more light for the money.

The most common Red LED is made of GaAs and has a wavelength of 625 nm. This isn't bad, but it isn't ideal for plants. A "super-Red" or "deep-red" LED emitting a wavelength of 660 nm is better (FYI the LED material will have "Al' or "P" in addition to the "GaAs") . These are harder to find, and may cost a bit more, but in theory they are worth the trouble to obtain. Order from a site like digikey.com or mouser.com or even superbrightleds.com where they actually list the wavelength and other useful spec's.

For my LED selection, I used a Digikey (or Mouser) "parametric" search to narrow-down the list to the 2 or 3 ranges of wavelengths that are considered suitable for growing plants, and then used a spreadsheet to narrow this further to those having the greatest radiant output per dollar, although this is not necessary, especially as LED costs have dropped considerably.

One issue I found is that some LED devices will appear to be a better value, however if they are tiny, or "fine-pitch" devices (pins very close together), or non-standard component packages, then they will not be easy to heat-sink properly for maximum current without using a (aluminum) metal-core circuit board (MCPCB).  These can be tricky to source, and are too expensive for the hobbyiest's DIY one-offs.  It can also be rather difficult to properly solder the LED to such a circuit board, especially using only a soldering iron.  Preheating on a hot plate or small oven can help, but even still.  For this reason, it is probably easier to buy LEDs that are pre-assembled on a circuit board where possible. 

Step 6: Printed Circuit Board (PCB)

The image shows an example of the PCB layout for electrical connections and heat-sink requirements for an LED array.  Here the LED is a Cree XLamp component package.  There is an advantage to the configuration at left, where the heat sink can be a single continuous piece.  Considering for a moment the crazy possibility to "build-from-scratch", conceivably this single heat-sink could even be a piece of aluminum sheet metal, supposing there were a good way to mechanically assemble and secure the pieces.  I don't have a good way of doing this in my basement, so for now the industry-standard MCPCB is preferred, because it handles the wires and heat-transfer in one solid mechanical assembly.

If you buy LEDs that are pre-assembled on circuit boards, then no additional circuit board and surface-mount soldering are necessary.  This is much easier for DIY.

...

In an older design, the PCB layout was started using freeware from expressPCB
To keep the parts cost down, it can help to "panelize" the design.  In that case, 12 grow-lamps could be made from a single $51 order (plus shipping, and depanelization), thus holding the cost of the circuit board to around $5 per grow-lamp. 

Step 7: Heat-sink for LEDs

Here, high-temperature adhesive or heat-sink compound can be added to thermally-connect the MCPCB (or LED's heat-spreader) to a surplus CPU cooler, having a finned heat-sink.  Optionally, an attached fan can be used if you have a lot of high power LEDs and you're trying to use a small heat sink, or if you want to keep the LEDs and/or surrounding air as cool as possible.  Be sure to keep the LED operating temperature within spec, since their light output and the device life can be reduced significantly.  You may want to search the internet for information on sizing heat sinks for your application.

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The following information I now consider obsolete for this project, since it is simpler and more reliable to buy an LED that is pre-mounted on a metal-core circuit board.
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(OBSOLETE)

Now as a disclaimer, this concept is really "ghetto".  But if you're on a desert island, and don't have a MCPCB, you might consider this.
Else, eBay eschews this..

For the LED heat-sink, a short piece of #12 AWG solid copper wire, (as found in "Romex" cable of the type used for wiring an electrical outlets), could be used to make a solid copper through-hole via.  Cut a very short piece of wire and shape this until its length is the thickness of the circuit board, about 0.062". One of these pieces can be inserted into a large via in the PCB under each LED. This will conduct heat from the LED to the heatsink on the other side of the circuit board. This will keep the LEDs much cooler than a solder-filled via, and so will provide higher light output, higher efficacy, and longer lamp life.

The copper vias will probably need to be reflow-soldered at the same time as the LEDs, so that all voids are filled with solder.  (If this method of assembly is not possible for some reason then this thermal design probably won't work very well, and alternative designs will have to be considered; see below**).

The bottom-side (or "solder-side") of the PCB will have a copper pad around the copper via to act as a heat spreader.  As a simple rule-of-thumb, the heat-spreader will be efficient out to a length of about 100 times its thickness.  Expresspcb.com plates to a thickness of 0.0017", so roughly speaking, the heat spreader will be effective out to a radius of about 0.17"

** If the above design doesn't work then one option for an alternative design would be to put the heat-spreader and heat-sink on the same side of the circuit board as the LED (the "component side"). One idea would be to combine the heat-sink with the reflector by making it out of aluminum.

Step 8: Luminare / Lamp Fixture / Lamp Optics

The low-cost LEDs that are commonly available tend to emit light over a very wide angle, for instance a 120 degree angle.  In order to use this light it will help to have a reflecting lamp fixture, or reflecting walls near the plant.  There are many possible options for this.  My favorite option for this is the aluminum reflector mentioned in the previous section. 

Alternatively, you may already have a reflecting lamp fixture you can reuse for this project.  You may have some shiny aluminum flashing, or aluminized-mylar, or a light-colored material of some sort with which to make a reflector.  Make sure it is non-flammable, and use plenty of electrical insulation and waterproofing where necessary to keep the electronics dry. 

Comments

[ComposerX]

Those lamps you showed at the top, the regular, cree, LAMPS, lights, how many would you need for 300 watts and is it worth it, what spectruc for veg and flower, am reading white is the way now and CREE has a new COB with 100 diodes at 303 LUMENS per watts. I believe each chip is 100 watts, if so thats an HPS killer right there = any advice appreciated, I will build if someone will guide, I can start telling you the part, amps, Ma, meters, et-cetera, its a beta test, 2 for me and hoping this is the one, this one I’m doing now but would like all LED, Also, what are these:

• grow light 1D.sch

Its a file obviously but my mac would not open it!

Thanks

[RalphM1]

I realize this is a super old post, but I can't help myself from correcting something: phycocyanin and phycoerythrin are not found in plants (as far as I know). They are used by cyanobacteria (a type of photosynthetic bacteria) and some types of algae. The chart includes them because they are major photosynthetic pigments and plants aren't the only organisms that perform photosynthesis. Plants probably have other pigments that can absorb light in this range; I don't know. Cyanobacteria are cooler anyway, so who cares.

[ThePerfectGrow]

there are some great LED grow light reviews on this site http://ledgrowlights.theperfectgrow.com/

[arnookie]

If you are thinking of building a diy grow light please take note of this.
Each and every led needs to be atleast 1watt each. Anything below that will be useless so don't use old leds out of toys or old boards, they simply will have too little Total lumens versus lumens per watt. You need atleast 10mm LEDs with 1watt per LED to supply enough light to the plant anything lower will not work. A good combination is a pannel made from 75% 1watt red high brightness leds, 20% 1watt blue high brightness leds and 5% 1watt amber high brightness leds. somewhere in the region of 660nm for red and 460nm for blue
There is also no effective difference in penetrative power for horticultural purposes between a 1W LED and a 3W LED. So anything over 1watt is just wasted. This means brightness has very little to do with the benefit you will get once you use 1wat leds. Don't confuse this with a pannel made from say 20 LEDs rated a 10watt as to one with 10 LEDs rated at 10watt. As the 20 watt pannel will use the useless 0.5watt leds verses the 10watt pannel that uses 10x10watt 1watt LEDs that are ideal. This has been tested and proven that 1watt single LEDs have great benefit to plants and anything less is just a waste of time and has no benefit at all to plants. The same applies with going brighter than 1watt has no benefit either.
Hope that may help some of you. Especially if you are growing indoors.
Also LEDs are more efficient than any other form of grow lighting available.
The commercially available LED growlights outperform all other growlamps from HID lamps to including high pressure sodium (HPS) and metal halide (MH) lamps.
So prepare to see other grow lamps become obsolete as LED growlight take over.

[arnookie]

Just to clear a point up. If you make a 100watt pannel with 100x1watt leds you will benefit. But if you use 200 0.5watt leds to make a 100watt pannel it will not benefit the plant at all. The same applies if you use say for eg 10x10watt leds to make a 100watt pannel then you are only really getting 10watts overall because the extra 9watt per LED is just wasted as the plant will not benefit. SO using 100x1watt LEDs will benefit the plants by 100watts. 10x10watt LEDs would not because 9Watt of each led is just wasted giving the plant just 10watt of usefull light. Sorry but thats just the way plants absorb light.

[awesomelumens]

are you talking about led outputs?

[steven4872]

I recently built a LED grow lamp using LED strip lights (a 8mm wide flexible PCB with LEDs and resistors on one side and adhesive on the other) It has about 468 LED s each with an output of less than one watt. total power consumption is 40watts with about 5 watts being consumed by the 12V power supply. The light panel is 6 inches by 24 inches and is in fact a glass shelf mounted on the wall about 20 inches off the floor.

My small collection of orchids are growing just fine. In one pot I accidentally got two weeds started growing (one a Snapdragon and the other a vine hummingbirds like (both came from by carden) The Snapdragon has flowered and the vine is also growing well but has not yet flowered but is growing.

the issue isn't power. Instead it is light penetration If you have poor penetration you will only have leafs on the ends of all the stems and the interior of the plant will be all stems and no leafs. While having more power helps penetration, it is not the best solution to the. problem..

For the best light penetration you want light from multiple sources and reflective surfaces . The leds I have don't have any lenses so they emit at a wide angle. Light that doesn't travel straight down to the plant gets reflected off of reflective Mylar plastic sheeting around the growing area. any leaves near the top don't block light coming in from the sides. I have excellent light penetration with no shadows.

One advantage of low power LEDs is thermal management becomes easier. My light panel doesn't have a fan or heat sink. Most LEDs have a maximum temperature rating of about 130C. My light panel is running at 30C and produces no unwanted noise.

[mr_man]

Thanks Steven... I think light "penetration" (or light diffusion), and power are important. You say you're using about 35-40 watts electric for a 1 sq.ft. array. How much growing area is this illuminating? From my quick market research on it, I read that people recommend 25-50 watts electric (LEDs) per sq.ft. of growing area, for intensive growing comparable to full-sun.

When LED grow-light products cost around $4 / Watt electrical, this equates to a significant investment of $100-200/sq.ft of growing area.

The sun is not a diffuse source, however, since the earth turns, sunlight effectively is rather diffuse over the course of a day. Using reflectors, as you say, is a good way to make the light more diffuse, as well as being more efficient overall by directing more of the light to the plant that would otherwise be wasted.


Pre-fabricated arrays like you mention are a good option that I should have researched more, but my experience with LEDs happened to be with single devices, and I hadn't gotten around to researching arrays. Fabricating arrays at home likely doesn't make sense to do DIY, especially for most people, even though I've been stubborn on this point, and maybe this is *part* of the reason why this instructable has languished (to put it nicely). The availability of these arrays with high power output and lower cost seems relatively new, or at least it is rapidly advancing.

By the way, when you mention a max temperature of 130 deg.C, that is the internal die/substrate temperature. The LED's max case temperature is *a lot* lower than that! Like 60 deg.C. Anyway, for the greatest light output, and good efficiency (efficacy), and consistent light output (lumen maintenance) over the life of the LEDs, and long life, it's often better to run somewhat below the max temperature. At least reference the thermal operating characteristic chart/graph in the LED device datasheet.

You might think that you don't have a "heat sink", but technically you do...it's just that in this case the large 1 sq.ft. circuit board is the heat sink. It almost certainly has an (industry standard) aluminum core, assuming it has good light output (good efficacy or efficiency) and long life. If you don't need a fan or a heat sink, you might consider that to be good, and if it works, okay, but it doesn't necessarily mean that you've completely sidestepped the thermal issue, and of course it doesn't mean that physics no longer applies. I'm *not* saying that all LED luminares require more fans and heatsinks, but I am saying that generally LEDs benefit from improved cooling.

I agree that the use of "medium" power LEDs on a large array *could* be a fairly well optimized design, in the sense that it *might* have found a "sweet-spot" with good compromises (or "trade-offs") in cost, light-output, thermal management, and power-density (or diffusivity).

Fan noise depends enormously on how fast it's run (at what voltage). You won't hear a fan turning at a moderate rate, but it can still move a worthwhile amount of air.

[steven4872]

The array is 1sqft and with the curtain I have a 1sqft growing area. Witout the curtain it will cover about 9sqft but growth rate would be slower.

"By the way, when you mention a max temperature of 130 deg.C, that is the internal die/substrate temperature. The LED's max case temperature is *a lot* lower than that! Like 60 deg.C."

That is the maximum LED junction temperature. It will very a little from manufacture to manufacture and case styles but most I looked at averaged around 130C. My grow lamp is operating at only 40C, The maxim case temperature is not generally listed but it is going to be a lot higher than 60C. The case has to be structurally stable when it is soldered to to the PCB. lead free solder melts at about 200C and the soldering iron will be even higher. If the case gets soft while soldering the delicate wire attached to the actual led would break if tj\he metal solder tabs moved.

"You might think that you don't have a "heat sink", but technically you do...it's just that in this case the large 1 sq.ft. circuit board is the heat sink."

Replace the word board with flexable ribbon. This is a what I am taling about http://www.superbrightleds.com/moreinfo/top-emitti...

It is a flexable plastic ribon with copper foil conductors and adhesive. It is only 0.5mm thick (excluding leds and resistors). There is no aluminum core. Some of the ribbons I purchased use transparent plastic and I can see through the areas with no copper foil wiring. With 468 leds and 35watts power consumption, each led it only dissipating about 10mW of heat that is easily transferred to the surrounding air . Most resistors are rated at 1/4 watt max power dissipation and they don't need heat sinks..

The problem I had with LED ribbon was finding companies that specified the wavelength of light and finding the colors I wanted. It was fairly easy to find 440nm and 460nm blue but finding 640nm and 660nm red was difficult to imposible. I never found a source of 660nm red. Everyone carries 626nm and I only found one company selling 660nm. I also found the 440nm, 460nm, and 626nm ribbons at clearance sail prices but I had to pay full price for the 660nm, Overall I spent about $160 on the LED ribbons allone. Not the cheepest way but it did ellininate a lot or wiring and assembly work.

[mr_man]

Right, the junction temp.

Usually air-convection is the bottle-neck for heat-transfer. There's a very good reason why radiators and air conditioners have many finely spaced aluminum fins.

you could still sort-of attach a flex circuit to a heat sink. Flex can be a pain though. It's not very robust or reliable. Only use it if you need its flexibility.

660 nm LEDs are hard to find. I mentioned mouser.com had some. Even the surplus dealers are expensive...especially when you consider all the work to make the component into a viable end-product. That's why I suggested using off-the-shelf white LED bulbs.

http://www.allelectronics.com/make-a-store/item/LED-250/3W-RED-LED-WITH-STAR-HEAT-SINK/1.html

It's not easy to make phosphors work well. Here are some links. The simpler ones, and ones without rare-earth elements may be cheaper. Often they're not that efficient, nor long-lasting....

http://en.wikipedia.org/wiki/Phosphor#Standard_pho...

http://www.google.com/#q=phosphor+site:sigmaaldric...

[antennas]

That would appear to be the most flawed logic I have ever read.

[mr_man]

@antennas, I don't get what @arnookie is saying either. All I can guess is that he's referring to the problem of excess luminuous intensity at the top leaf and shading of lower foliage. As I said above, it would seem that a top leaf could easily shade a lower leaf. This is not optimal. If the illuminated leaf is saturated with all the light it can handle, then conceptually I imagine that additional light emanating from the same lamp would not benefit the plant. I don't know if this is a valid theory or not. I'm not a botanist or horticulturist or biologist. However if this theory is correct then it would be preferable to use indirect illumination, and "diffuse" the light, so that it is spread as evenly as possible over all the leaves. LEDs are highly directional sources, and are *not* diffuse sources in the way that florescent lamps are. Fortunately, this is not at all difficult to do and there are many ways to diffuse light. The light can be reflected off of a bright-colored or reflective surface, or shown through frosted-glass or any material that is translucent (passes light) but not transparent (don't pass an image).

[antennas]

Leafy plants aside 10W of light is 10W of light regardless if it comes from 100 0.1 W lights or 1 10w light. That's light not the power used...

[saib30]

Dear Arnookie,
Do you have a reference to a scientific experiment that proves what you are saying about 1W LEDs? Because for me, it does not sound correct, as some of the 1W LEDs are actually multi core LED with lets say 2 core of 0.5W!! check the following:
http://www.ledwv.com/en/images/LED%2080W.jpg

Second, what does matter for plants growth is the Luminous and wave length, and LED Luminous efficiency does deffer between manufacturers, then not all 1W LED will behave the same. Check the following:
http://en.wikipedia.org/wiki/Grow_light#Luminous_efficiency_of_various_light_sources


Regards,
Saib

[mr_man]

I did some numerical integration in a spreadsheet, and I calculate that even a white LED is about 70% to 80% as good as a single monochromatic LED on the peak for photosynthesis. A "cool white" lamp will have more blue, which is good for vegetative growth, and "warm white" will have more red, which is better for flowering (according to all the weed growers on the 'net).

Even if the LED only gives you an extra 10 lumens per watt compared to a CFL (60 vs. 50) , the savings in electricity can still in some cases justify the cost of a replacement bulb. I picked up a 60 watt-replacement (13.5 Watts electric) , 850 lumens, for about $7 or $8 at a wholesale-club/ big-box store. (Yes it was in a neo-liberal state that automatically subsidized half of it. We also have much higher electric rates too @ 20 cents/kWh). Anyway, this lamp cost $0.50/Watt with discount, or about $1/watt normally. Sometimes the LED device/components alone can cost that much! so to get the whole package for that price is amazing.

[steven4872]

Keep in mind that Chlorophyll (the main food generating pigment in plants) has 4 absorption bands, 440nm and 460nm blue and in red 640 to 660nm. A 2700K 90 CRI (color rendering index) white LED lamp will cover all of the red spectrum. However no white led will cover much of the blue spectrum. The phosphor used in LEDs works best when energized by 460 nm to 470nm blue light. I know of no LED white lamp that cover both blue absorption bands of chlorophyll.

I have tried white LEDs par 20 lamps (8watts each) one 4000k (no CRI rating on the package)and the other 3000K 85CRI . I wasn't satisfied with the result (minimal growth and the lamps were running quite hot (75C).

I am beginning to think the optimal LED grow lamp design would be multiple blue wavelength LEDs and red phosphor. I have found companies that make LED phosphor and and from the specks I have seen one type of red phosphor would cover most of the red spectrum. However the phosphors appear to be quite expensive (based on only one site that listed prices for sample sizes. And you would still have to make some sort of paint to apply to the LED's

[mr_man]

It's not clear to me where the exact peaks are for which pigments and exactly what effect each of them have. I don't see a chlorophyll peak at 460nm. The info I have (in the graph above) is that there is only a carotenoid peak at 460nm. Also these 2 links.

http://en.wikipedia.org/wiki/File:Par_action_spect...

https://en.wikipedia.org/wiki/Chlorophyll#Spectrop...

There is a "chlorophyll-a" peak at ~440 and "chlorophyll-b" peak at ~490.

One's opinion on this may depend up whether more importance is given to the absorption spectrum for all pigments or the photosynthesis rate.

It's true that "chlorophyll-b" does have a very nice absorption peak at ~490nm, that is at least 3-4 times higher than at 660nm. However, the proof is in the pudding, and if you believe the published data, it says the photosynthesis rate isn't significantly different between the two. I can speculate that perhaps this is due to carotenoids absorbing some of that 490 light, and/or "chlorophyll-a" also being a bit more active at 660 than at 490, but that's a guess, I don't really know.

The individual pigments and non-phosphor LEDs have fairly narrow spectral widths. At first glance this looks like an opportunity, and it's what got me interested in this topic. However, now it seems to me that there are enough pigments with overlapping spectra that it gives the PAR curve a relatively wide spectrum. It appears that any light in the ranges of about 390-510 and 640-690 should be relatively efficient. It's tempting to want to line-up LED peak wavelengths with the PAR peak wavelengths. But where efficiency is concerned that isn't the true objective . The true objective is to make the *area* under the spectral curves overlap as much as possible, for the light-source and PAR. the efficiency is the ratio of the area that is overlapping. (In math, the wavelength integral of the product of the 2 spectra)

So because of this, now it seems that the narrow spectral width of non-phosphor LEDs aren't necessarily the breakthrough in spectral-efficiency that I thought they might be, especially when costs are taken into account. The breakthrough has more to do with their radiant-power efficiency.

800 lumens from a CFL consumes 14 watts, but an LED uses only 9.5 watts. The difference of 4.5 watts seems trivial, especially when the LED costs $8 and the CFL only $1. However, the electricity savings puts breakeven at about 1-2 years of continuous use (for electric costs of $0.20/kWh to $0.10/kWh respectively)

Anyway, if cost is a factor, then it makes sense to use the light that gives the most moles of photons @ PAR per unit currency.

I agree that a "warm white" lamp is relatively more efficient in the red, and a "cool white" phosphor is relatively more efficient in the blue, and neither is particularly efficient at both. To get both red and blue using only "white" lamps, I suppose you could use both "warm white" and "cool white", or compromise and use "neutral white". Or to change the ratio of blue/red, switch between one or the other as needed.

[mr_man]

UPDATE: I think the supplier stopped carrying the LED I specified, although there are newer, cheaper, better LEDs coming out all the time. Available for instance from mouser.com

Also my original design called for ordering a custom circuit board. While that would give a higher quality result, now I'm thinking that might be unnecessarily complicated, or expensive, or not in the DIY spirit.
It may be possible to simply solder the LED to a couple of strips of copper sheet metal, (e.g. roofing flashing), or even a couple of solid copper coins, such as a pre-1981 U.S. penny, of which there are plenty still in circulation. Total cost, 2 cents! If the coin is not flat enough, then it can be sanded down a bit. The copper serves as both the electrodes and the heat spreader, which is then thermally coupled to the larger heat sink. If the LED is the type where the heat sink pad in the middle bottom of the LED is electrically connected to one of the electrodes, then you just have to make sure that the external connections match the internal ones so that the circuit isn't shorted out.

[mr_man]

Regarding the electrical circuit design. The main difficulty is that the high-power LED ideally should be driven with constant-current, yet most power supplies are considered "constant voltage". However, it occurs to me that these power supplies are only constant voltage up to their maximum current rating. Any attempt to lower the load resistance and draw more current beyond this point will cause the voltage to drop so that no additional current flows, This is actually what a constant current source does. So to summarize, at low currents the power supply will act like a constant voltage source, but at high currents it will act like a constant current source. This is actually exactly what we want to drive a LED. We still have to take care to match up the power supply's rated voltage and current with the LED's rated voltage and current, but assuming that these parts are available and can be obtained, then if I'm correct, then we don't really have to worry about limiting the current with resistors or with the current limiting circuit that you see on some of the other LED instructables that has a current-sensing resistor and 2 transistors.

[steven4872]

All a LED cares about is the voltage across its terminal and that it is operative at the correct temperature. If you have the correct voltage the current will also be correct. The temperature however depends on how you dissipate the heat generated. Semiconductors, including LEDs, are susceptible to a failure mechanism called thermal run away.

If a led heats up is voltage requirement will change. that will lead to current increase and that will cause higher power dissipation. The higher power dissipation caused the temperature to increase will causing a even greater power dissipation. When this occurs the LED will quickly overheat and burn out..

You can mitigate the problem with a constant current source. Constant current sources limit the current so that the LED will always have the correct voltage across its terminals. However if the glue holding the LED to the heat sink failures the LED will heat up to its maximum rated temperature and fail. The down side to constant current sources is that all LEDs must have the same current rating and dimming by pulse width modulation is more difficult. If the leds have different current ratings some will be to bright and may fail while others will put out less light. the series parallel design in this project would work out just fine as long as the power is safely dissipated to prevent the LEDs from overheating.

Parallel configurations have the advantage that if a LED should fail for whatever reason, the others will continue to stay on.

So you can drive the LED safely with a constant voltage source and in fact most of the time that is what is used. What really matters is how the LED is mounted to its heat sink. The easiest way to mount a led is with thermal glue. But glues will over time fail and when that happens the LED could burn out. Securing a LED to a heat sink with screws is the pest way but sometimes the manufactures doesn't supply any mounting holes. Another option is to use lower wattage LEDs and have enough space between them so that the heat will be safely carried away by the wiring and dissipated.. Unfortunately that means more parts and more assembly time.