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Simply calculate the "worst case current" of LEDs with minimum voltages then calculate with maximum voltages.
If the resistor does not have enough extra voltage and both LEDs are minimum voltage then they might burn out.
If both LEDs have max voltage then they might be too dim.
Ok confused again,
the resistor is used to reduce the current, but it also uses up 2Vs? What would happen if I wanted to power one 2V led from this 5V Power supply.
Your red LED might be 2.0V, or 1.7V or 2.3V. With a 5V supply then calculate the resistor for a 1.7V LED at 25mA. Its value is (5V - 1.7V)/25mA= 132 ohms. Use 150 ohms as the closest standard value.
With a 150 ohms resistor and a 1.7V LED the current is (5V - 1.7V)/150 ohms= 22mA.
If the LED is actually 2.0V then its current is 20mA.
If the LED is actually 2.3V then its current is 18mA.
There is not much brightness difference between 18mA and 22mA.
Now calculate the current for two LEDs in series. Their total voltage will be 3.4V to 4.6V. The resistor will not have enough voltage across it to keep the current in minimum LEDs close to the current in maximum LEDs so the maximum LEDs will look very dim.
Your red LED might be 2.0V, or 1.7V or 2.3V. With a 5V supply then calculate the resistor for a 1.7V LED at 25mA. Its value is (5V - 1.7V)/25mA= 132 ohms. Use 150 ohms as the closest standard value.
With a 150 ohms resistor and a 1.7V LED the current is (5V - 1.7V)/150 ohms= 22mA.
If the LED is actually 2.0V then its current is 20mA.
If the LED is actually 2.3V then its current is 18mA.
There is not much brightness difference between 18mA and 22mA.
Now calculate the current for two LEDs in series. Their total voltage will be 3.4V to 4.6V. The resistor will not have enough voltage across it to keep the current in minimum LEDs close to the current in maximum LEDs so the maximum LEDs will look very dim.
Calculate the gamble of having LEDs with maximum voltages and a resistor calculated for minimum LEDs and a low supply voltage.
It is a gamble because you don't know the voltage of your LEDs unless you measure each one. If you have plenty of voltage (2V to 3V) across the current-limiting resistor like I showed then it won't matter.
Your meter does not calculate anything. It measures the forward voltage of an LED if the LED has a current-limiting resistoir and a voltage source. Then you can calculate a suitable current-limiting resistor for it.
The current changes if the voltage of an LED is different since its voltage is a range of voltages listed in the datasheet as minimum voltage to maximum voltage.
E-Bay sells cheap Chinese LEDs that are bright because their case focusses the beam into a narrow angle.
For years I bought American LEDs from Digikey. The Hewlett-Packard ones were good then the name changed to Agilent and now the name changed again. You can select a certain colour bin and brightness bin. They have many angles up to 40 degrees.
You should be paying around $5-$10 or more for a single decently powered LED for a grow light. Those ones that are $.40 - .90 don't emit enough light unless you are using numbers in the hundreds.
The cost savings for an LED comes from not wasting electricity in producing photons of a wavelength you don't need. Thus, you might pay thrice as much for an LED light up front but the cost to run it is minimal and it will take 10 years or more to burn out.
$5-10 for just one LED, how many watts? Guess these are a better deal than I thought...
**broken link removed**
Been a little tempted, but wanted to find some high wattage LEDs of the right wavelength (not much luck, but found this site, prices weren't too bad on their other stuff).
Cheap Chinese LEDs appear bright because the beam is focussed into a very narrow beam. If the case provided a wide angle beam then the LEDs would be much dimmer than "normal priced" LEDs.
Why buy super cheap stuff? It might not last longer than a few days.
It doesn't say anything about what LEDs they used. All light bulbs (including LEDs) are measured by how much light they produce in either lumens (luminous flux) or radiometric flux. That link says the LEDs they are using are .5W but they don't say what that value represents. Is that the amount of radiometric flux or the amount of power they draw? I find it hard to believe those numbers because blue and red LEDs have different values for both radiometric flux and power draw.
I will use the numbers for some Luxeon Rebel LEDs as an example.
A Royal Blue Luxeon Rebel emits dominant light in the neighborhood of 455nm. At 700mA the typical voltage is 3.4V and the average bin produces approximately 525mW of radiometric flux. Thus this LED draws 2.38W of electrical power while emitting .525W of light.
A Red Luxeon Rebel emits dominant light in the neighborhood of 630nm. At 700mA the typical voltage is 3.6V and the average bin produces approximately 75 lumens. Since luminous flux is a weighted average for light that humans are most sensetive to, we must translate lumens to radiometric flux to really compare the two LEDs. To do that we divide radiance by the weight for that wavelength (google: luminous efficacy table) which is .265 for 630nm. Thus this LED draws 2.52W of electrical power while emitting .283W of light.
2.38W for blue is very different from 2.52W for red
.525W for blue is very different from .283W for red
For reference, the blue one is about $5 and the red almost $3. (good) LEDs aren't really that cheap to buy but they are cheap to operate. Anyways, I would not bet on that lamp but it might be good to experiment with.
Like most parts sold on E-bay, the cheap Chinese LED does not have a manufacturer's name nor a datasheet. Its "spec's" say nothing about its brightness.
You can buy cheap LED's and you can buy expensive ones, but if they aren't the correct wavelength, they are a waste of time and money no matter what they cost. The light Harvey mentions is virtually useless for plants, unless the "Specifically Pick Red Wavelength is > 630nm*" and "Specifically Pick Blue Wavelength is <460nm*" gibberish means you can specify LED's at 660nm and 420-430nm, which you can't. This is the problem with 99% of the LED "plant lights" sold - they are simply the wrong wavelengths. Sure, if you focus a 100w worth on a 3cm point, you can get something to grow...
Re-read the post from thepaan with the chlorophyll curves. The importance cannot be stressed enough. Plants prefer chlorophyll a and only use b when they aren't getting the a wavelengths. There are always exceptions, but this is generally true for all plants. If you're going to go through this much effort to work out the electronics, make sure you're not wasting your time - the purpose of the project is to grow plants.
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