I get an email catalog from an electronics surplus and they have this for sale
http://www.goldmine-elec-products.com/prodinfo.asp?number=G18748
do you think it'd be any good for plants??
Holy crap!
Until now I thought, that the industrialized countries export their electronic waste to all the underdeveloped third world countries. But meanwhile China seems to be exporting electronic waste to the USA.
OMG, dozens of defective LEDs on a new board fresh from the manufacturer, that must be a total quality manufacturer who builds it!
Your question is easy to answer: This is a very good learning board to learn the lesson, why you should not consider low-power LEDs for plant lighting. The very good thing is, that the 600 wired and working LEDs on the board are cheaper than the same type of LEDs when purchased in 6 packs of 100 LEDs and that the wiring had already be done by some Chinese worker.
The construction is easy: The board is build from standard low-power LEDs which are made for a current of 20mA and a nominal voltage in the range of 3.2...3.4 volt. Put 7 in a serial row you get a forward voltage of 3.4*7= 23.8 volt. That's the voltage of 24v you need for the DC adapter. And you can see: 7 in a row, so if one single LED in the row becomes defective, the whole row of 7 LEDs will stay dark.
The rows are connected as a parallel curcuit, so the board cannot have exactly 650 LEDs, as 650 cannot be devided by 7 without rest. Most likely the board has 651 LEDs as 651/7 = 93 rows of 7 LEDs each. With a nominal current of 20 mA = 0.02 ampere the total board draws 93*0.02 A = 1.86 ampere. They state their board uses only 1.6A, so this may be caused by two things:
- several LEDs are defective, less LEDs glowing need less electrical current
- the LEDs used are of lower quality than standard
To be on the safe side, I'd better use a 24 DC-adapter with that board that is able
- to provide a stabilized voltage of 24 V
- to provide at least 2A (2000mA) oder better more like 2500 mA
You can use that for plant lighting, although the white LEDs are a bit blueish like 7500 K or so. No problem. But you can then find out, what problems will come with such a construction: More and more LEDs will become defective. And the LEDs will get darker and darker the longer you use them.
With white low-power LEDs operated at 20mA I found out: Such serial-parallel LED panels will become much darker after 2000 hours of usage (one year of lighting with 13 hours per day). Initially you can use them für plants with low light demands if you put them directly over the plants, but after one year the low-power LEDs are no good any more. You can try Cephalotus and seedlings. Which means seedlings after they have germinated, because with several species the germination rate is bad under such bluish white LEDs.
For plant lighting on the long run you better use a construction with high-quality (like CREE), high-power LEDs (350 mA or more per LED), those keep most of their light/lumen for much longer time (with good cooling) than low-power noname LEDs.