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Note: ** This is a work in progress. The design is not entirely complete, nor has it been built yet. Constructive suggestions and collaboration are welcome **
For more information on LED Grow-light design, see US patent #6921182 .
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Signing UpStep 1Circuit 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, although a 24 volt supply is even better if you can find it. This allows you to put more LEDs in series. You can always guarantee that all LEDs in a given string are carrying the same current, so fewer strings means fewer resistors. This reduces parts count, improves circuit board real estate efficiency, also 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 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, 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 in ohms that you should use is:
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. In this example it should have a capacity of 2 Amps or more. 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 of -4 mV/deg.C. I'm also looking into using copper 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 temperature, then we can calculate what minimum value of resistance we need so that the overall temperature coefficient is above zero, so there is no thermal-runaway issue. 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)
Start the circuit design by arbitrarily adding LEDs to a string. 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, 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 in ohms that you should use is:
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. In this example it should have a capacity of 2 Amps or more. 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 of -4 mV/deg.C. I'm also looking into using copper 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 temperature, then we can calculate what minimum value of resistance we need so that the overall temperature coefficient is above zero, so there is no thermal-runaway issue. 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)
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Hence, knowing the electrical power is better than having no information at all, but knowing the luminous output is better still. However, even this can be misleading, especially for the blue LEDs where the human eye is not nearly as sensitive as to red LEDs. Better still is knowing the "radiant" data. Sometimes for blue LEDs the datasheet will spec the radiant output in WATTS. I think this is the best data, because it tells you exactly what the optical power is, *and* it is trivial to calculate the efficiency, simply by dividing by the electrical input power in WATTS.
I guess what arnookie might be trying to say is that a plant leaf can only make use of light up to a certain intensity. Once that threshold is passed, where all of the plant's photochemical processes are saturated (or "maxed out"), then any additional light has no further benefit to the plant. I am speculating on this point, which I shouldn't do, but it seems reasonable to assume that somewhere there will exist a point of diminishing returns.
Also, it would seem that one leaf could easily shade another leaf, and it seems reasonable to assume that there would be losses here, so it is probably preferable to "diffuse" the light, so that it is spread as evenly as possible over all the leaves. This is noteworthy, because LEDs are *not* diffuse sources in the way that florescent lamps are. Fortunately, the laws of optics allow a point-source to be diffused (just not the other way around, you can't focus a diffuse light to a point).
There are many ways to diffuse light. One can reflect the light off of a white wall for example, or shine the light through any material that is translucent but not transparent.
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.
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