The product being tested on animals
The goal of this project was to produce a decent grow light for personal indoor gardening – mainly starting vegetables for subsequent transplanting outside some time in May and possibly extending the grow season in the autumn. While trying to determine necessary light output I realized that good numbers are impossible to find. By reading numerous indoor gardener’s forums I learned that people are having good results with light sources ranging from household-type compact fluorescent bulbs to high pressure sodium street lights. After analyzing pros and cons of all available light sources I decided to use high-power white LEDs. Here’s why:
- They are low-voltage devices therefore they are much safer to work with than HID/fluorescent light sources. Low-voltage (<60V) LED drivers are also inexpensive
- They are efficient as grow lights. Photosynthesis in plants occurs differently under different wavelengths of light; the “good” light is known as Photosynthetically active radiation (PAR). White LED emits most of its light in PAR – out of all light sources it produces most “PAR lumens” per unit of electricity used to produce these lumens. In addition, LED outputs all its light from one side therefore a light fixture doesn’t need a reflector.
- They can last long time. LED manufacturers specify 50000 hours at some pretty high emitter temperature (75-85C) – it’s 11.4 years if lights are on 12 hours a day. At the end of 500000 hours a LED maintains 70% of its light output. The main factor here is temperature of the LED – if it is kept lower than specified the LED will last longer (and produce more light, see below).
As I mentioned before, it was difficult to determine the necessary light output of the fixture I was about to build. For this reason, I started to look at available power supplies first. After browsing through offerings on eBay, Aliexpress and electronics components supplier’s sites I sized my supply at 300W – the power is good and the cost is not excessive. The representative supply is Meanwell model S-320-48. The price is ~US$70 for a genuine one and ~US$30 for a clone. Dealing with clones is involving – at the very least, you’ll need to a) inspect a power supply inside (involves non-destructive removal of warranty stickers, requires finesse), b) test for load and ripple ( requires some pretty specific equipment, like 300W dummy loads), c) burn-in for a couple of days under full load (difficult to do safely in residential setting unless you have an empty 2 car garage), and d) undo the deal if you’re unhappy with one of the previous steps. Buying/returning on the Internet takes time; I suggest not buying from China – transit times are too long. I had to return 2 supplies; the third one turned out to be a genuine Meanwell with a cut-off sticker – for the price of a clone.
A 300W power supply can drive 250 – 280W of LEDs. At present, high-power LEDs are available in 2 flavors – single emitter and multi-LED modules. The big advantage of a single emitter is low price per unit. However, they are harder to work with – to build a 270W light out of 3W LEDs you will need 90 emitters each requiring 2 mounting holes, 180 drilled and tapped holes in total. Also, they come soldered on so-called “star” heatsinks with uncertain thermal characteristics – thermal conductivity of tin is 3.5 times lower than aluminum therefore thermal resistance between an emitter and its heatsink highly depends on the thickness of solder layer under the LED which no one seems to even attempt to control. In addition to this some “LED mounters” use lead-based solder which has even lower thermal conductivity – 5 times lower than aluminum. Another big disadvantage of single emitter LEDs is that it is hard to get a genuine one. Most mounted LEDs sold to me as “Cree” or “Bridgelux” were fake – I have about 30-40 pieces of each name bought from different places. What is interesting is that all Luxeons I have (another 50 pieces) are genuine. Lastly, single emitter LEDs are about twice as expensive per watt – 45W Cree CXA011 from the brightest bin costs US$15.23 at Mouser whereas mounted “Generic 3W Epistar” at stevesleds.com sells for $1.85 (prices at the time of writing), and if we start comparing lumens per watt (assuming that we can reliably determine real lumen output from generic emitter) the difference will become even larger.
CXA2011 on a heatsink
Now that we know that multi-emitter modules are the way to go let’s take a look at what’s available. Chinese make decent modules (search eBay for ’30W white led bead’ to see these good looking products mounted on hefty aluminum bases measuring ~45mm across). I have used them in my past projects – they work well and they are easy to drive, typical forward voltage at 1A being ~34V. The problems with Chinese products are a) the quality is unknown – you can get decent product or a piece from rejects bin, and b) the real characteristics of the device are unknown. After looking further and comparing watts per dollar, lumens per watt and whatnot, I decided to go with Cree CXA2011 module ( Link to the datasheet ). The main reason to pick up this particular module was ease of mounting. The second reason was price. Lastly, the module is available from established US suppliers in manufacturer’s packaging therefore the probability of getting a fake is low. Picture on the right shows the module bolted to a heatsink by a pair of #4-40 screws. My grow light uses 6 of these. CXA2011 has several color temperatures and light output variations, the full part number of ones that I use is ‘CXA2011-0000-000P00J050F’.
I wanted to keep my LEDs as close to ambient temperature as possible. This means I needed forced-air heatsink. The one on the picture is Rosewill RCX-Z80-AL AMD CPU cooler designed for 70W load. It is available from many places for ~US$7.00. It is many times cheaper than “specially made for LED” heatsinks and it works very well. I learned from the reviews that the included fan is good for two years.
An advantage of having a grow light with fans is that air movement in a grow room is beneficial for the plants and 6 fans produce pretty decent wind – strong but not excessive. The disadvantages of having a fan is a) extra current consumption and extra power supply, b) noise, and c) extra maintenance. However, since convectional cooling is not possible anyway I would need to learn how to deal with them.
After mounting a module using Arctic Silver 5 thermal interface material and running it at 1A (the maxumum allowed by a manufacturer) I measured 1C temperature difference between the case of the module and the heatsink surface right next to it. Anywhere else the heatsink measured 22C which was ambient temperature at the moment. It was probably possible to have 2 LEDs per heatsink and still get good dissipation but at the end I decided to just have one.
Meanwell LDD-100H LED drivers
Up to this point I did all my tests using bench power supply. I learned that at 1A the forward voltage of the module is close to 48V while the datasheet showed slightly less than 46V. It also means that the power consumption is 48W instead of specified 45. I don’t know how this extra power increases light output so I will continue using datasheet numbers for light. I also know that LED forward voltage increases slightly with temperature therefore I need a LED driver which will go over 48V at 1A. The one I picked is Meanwell LDD-1000H. Picture on the left shows 3 drivers mounted on a protoboard. The driver needs at least extra 3V on the input so I tuned up the power supply output to 55V.
When all major components are specified it is possible to calculate the price. Here is a list:
- Power supply, 48V, 312W, Meanwell S-320-48 or similar. $30 for “similar”
- Cree CXA2011-0000-000P00J050F 45W LED modules, 6x$15.23 = $91.38
- Meanwell LDD-1000H 1A 52Vout max. LED driver. 6x$11.52 = $69.12
- Rosewill RCX-Z80-AL AMD CPU cooler. 6x$6.99 = $41.94
Couple other notes about pricing. First, it is possible to negotiate prices on eBay. Many sellers provide “Make offer” button which you need to use wisely – do not try to offer $5 for $30 power supply since your offer will likely be declined without further discussion. It is much better to offer $20 initially and then add a little during each round. It is also possible to discuss (and lower) shipping charges.
Second, it is easy to scale down this light – just use smaller number of LEDs. You will need to account for other pieces too – a single combination of LED, driver and heatsink costs ~US$35. In my opinion, the smallest number which makes sense is 3.
The structure
The construction of the light is very simple. The most tedious task is drilling/tapping holes in heatsinks and two 22″ 1/4 aluminum angle pieces. After the main structure (pictured on the right) is put together LEDs with wires soldered on are mounted on the heatsinks. After that it is necessary to cover the LEDs with a piece of transparent plastic to protect the lens – I used 0.118″ polycarbonate. The drivers are mounted on the sides of the structure, 3 on each side. LED wires are soldered to the drivers and zip-tied to the angle pieces. Finally, fans are mounted on heatsinks and wired together. They are powered from their own 12V 1A wall wart at the moment; I’m planning to add a step-down converter to power them from the same 48V supply which still has some power to spare.
Bottom side
Next two pictures show construction details. First picture shows LEDs mounted on heatsinks, wire routing and transparent protective LED covers – again, very important pieces! Second picture offers view from the top showing fans as well as LED drivers mounted on the sides.
I don’t own a photometer so I need to estimate light output from numbers in the datasheet. First thing to do is to determine the working temperature of the module. The datasheet states effective thermal resistance junction to case being 0.4C/W, which means that at 48W the die will be ~20C warmer than the case. I am assuming that all energy in the LED is converted to heat, which is very conservative. Early illumination-grade LEDs converted about 10% energy to light, modern ones are close to 20%. For 20% the temperature rise will be 15C. Now, the measured difference between case and heatsink (given earlier) is 1C – let’s just forget about it since it doesn’t make any difference. Another measured temperature is ambient in my small grow room where this light has been deployed for several days – 30C. Adding 20 and 30 together gives 50C – this is the temperature of the LED die during operation.
We can continue going through numbers in the datasheet or we can take a shortcut by looking at the following presentation, which gives output of 3480 lumens per module at 45W and 85C. This means my light puts out 23040 lumens (page 7). Since my emitters run colder I’m getting more light. How much more? If we refer to the datasheet again (page 5) and analyze flux vs temperature relation we will see that at 50C the light output rises by roughly 10%, which means my light outputs more than 25000 lumens – 25344, to be exact. The difference is equal to 0.6 light output of a LED module; it can be seen that heatsink fans pay for themselves – they consume 12W but add (or save) 27W!
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Oleg.
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