led help

im looking to make an led growlight. yeah yeah its not for what you think. its for my college plant bio class. i want to make a strong growlight for my semester project. im looking to make something like the 90whatt ufo like http://www.amazon.com/LED-Wholesalers-2506RBOW-Quad-Band-Hydroponic/dp/B002TZ6I14/ref=sr_1_4?ie=UTF8&s=home-garden&qid=1281669139&sr=8-4. the problem is i dont understand leds. ive seen the high power growlight on this site but im looking to make a large panel with regular leds. someone help. please.

mr_man6 years ago
I'm an electrical engineer experienced with lighting and high-brightness LED applications.

For good results, specific wavelengths of light are needed, using high-brightness LEDs drawing 1 watt or more, with proper heat-sinks.

Basically the LEDs should operate with a constant current. However most power supplies are a constant voltage. The simplest solution to this is to use a series resistor. ... (more info below...)

Here's what I would recommend for building a LED grow-light.

1. 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.

2. 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.

A note on heat... Make sure the thermal pad on the LED is *soldered* to a copper heat spreader that you have first tinned with solder. For a surface-mount part, you will have to use a hot-plate or frying pan, or hot-air gun, or oven to reflow the solder. The Cree website has pdf documents on how to do this. It is often easier to just buy high-brightness LEDs that come pre-mounted on a hexagonal "star"-shaped circuit board with mounting holes and everything. These typically have an aluminum core, which makes an excellent heat spreader, so then you only have to put heat-sink compound on the back of it and screw it down to a heat-sink, or you can just epoxy the whole thing down semi-permanently.

2. Use a higher voltage power supply, like 12 or better yet 24 volts. This allows you to put more LEDs in series. Since so you can always guarantee that all LEDs in the string are carrying the same current, less resistors are needed. The whole thing will be more efficient, and produce less heat, and keep your electric bill down.

4. The voltage regulator probably isn't necessary, even if your power supply is "unregulated". You can probably get away with using smaller resistors if you just add up the voltages 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. To do this it helps to mix and match LED colors in the string, just make sure that you don't exceed the LED's current rating. Ohm's law says that the resistor value in ohms that you should use is:
R = (Vs - V_LED) / I_LED

Where Vs is the power supply voltage,
V_LED 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, 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.

I hope this helps. When I get organized enough to build a light-source and take photos, I'll try to put together an 'ible.
This is a good reference:
US Patent 6921182

The biggest advantages of LED grow lights are that they will reduce by several fold the electric power consumption, and reduce the waste heat produced. The size will be similar to a CFL setup. The biggest disadvantage is that it will cost at least 5 times more money for the lamp, and this is only if you build it yourself (free labor) with a good set of free plans, and a preassembled parts kit.

I have a nearly completed design that I'd like to make available.
I could probably make a parts kits available on ebay for about $80. Or I could just provide the circuit board for $5 plus shipping. This design would put out nearly 8 watts of radiant light, and use about 26 watts of electrical power. I estimate it would roughly compare to a 54 watt CFL. This is a much bigger investment than an equivalent florescent lamp costing only $10 or $20. However, the savings in electricity would pay for the LED lamp in just 2 or 3 years, depending on your electric rate and how many hours/day the lamp is on. The higher these are, the faster it will pay for itself.

Do you think I should make an 'ible on this project?
Would anybody be interested in building one of these?
mas9779 (author)  mr_man6 years ago
yeah man i know several people interested and i know where to find more.
mr_man mas97796 years ago
Sounds good. What I have so far, I put here: