I’m wanting to make a little indoor greenhouse using some of the 1 watt LED’s. I don’t want a want to figure out voltage drops and resistor values when manufacturing differences would make things tricky to do, especially since I’m not sure what voltage level to use, 12V, 19V like a laptop ps, or 24V which I have available. The LED’s would be in series as much as possible of course. But an LM317 with a 3.6 ohm resistor would work perfectly over a wide range of voltages and keep working properly if an LED shorted out bad.
Plus it’d be much cheaper to buy than the $15 constant current sold, and could have multiple versions for different LED’s.
Correct me if I am wrong but I thought the LM317s were not voltage boosters just regulators so you could use a feedback resistor to get a cheap constant current circuit, but would still only have the max voltage of the supply, and so any series LEDs would eat up the voltage after a few LEDs.
What would be easiest and cheapest is say a simple LED driver IC such as a Catalyst CAT4237TD, and then a few resistors caps and an inductor. Then you have a constant current, boost circuit with a few components. I just built this to power 6 white LEDs in series from 3V (TFT backlight) and it worked a treat. Just remember to watch your traces for the current loop, but here is an example of how simple the entire circuit is:
That chip can only handle up to 5.5V input, and implies only a 40 milliamp current drive, which is only about 1 watt for 8 LED’s. With a 12v rail, an LM317 should be able to handle 3 of the 1 watt LED’s per circuit. And for coming close to sunlight conditions, I don’t think 20 milliamp LED’s can come close without a lot. Take a high brightness LED and shine it on the ground outside on a slightly overcast day and you’ll see it’s not that bright. Look at your headlights in the middle of the day too and they aren’t too bright either.
Yes the IC is not for Lum high power LEDs I was just mentioning it as a suggestion as a voltage booster LED controller as I thought you would be trying to power LEDs from a lower voltage, in which case hardly any LEDs could be stringed together. If you only intend to series 2-3 LEDs in a row and have a 12V line then yes that is fine.
That chip’s $10 plus parts and a PCB, so it might cost $25 for everything. Using an LM317 might cost sparkfun about a dollar a circuit for parts.
And for three LED’s at 12V, the LM317’s still 75% efficient. According to the datasheet, the LT3478 might be less efficient in some circumstances.
It’s like the difference between using an Arduino for a light dimmer or a dual opamp(rc oscillator and a comparator). I’ll prefer the latter before the former.
True, but the example again is crazy powered, at 4.5A thats 54W at 12V, and you only need 1W (83mA at 12V), so you could go much cheaper (same price or cheaper than LM317) and still get enough for 10 LEDs in series if you take a look at the range. But its just an option if you prefer to just do a direct drive its simpler, quicker and easier for sure.
But it’s such a simple circuit, why even bother with a board? When I do this, I just solder a regulator and a resistor together pin to pin, then stick the whole thing in a big piece of shrink wrap and heat it. Two wires coming out and the circuit’s done.
I’ve been playing with micro greenhouses using some old lab glassware I got on surplus. How big are yours?
I was going to plan on a storage shelf I saw at walmart for about $70. I could put in a computer PSU on the bottom, along with a water tank and pump, probably an aquarium water pump controlled by a relay. Then I’d just need to deal with monitoring soil moisture and providing a day cycle. This way I can store it anywhere, grow miniature trees, or modify it simply for rows of seedlings, etc. With a water tank and monitoring I’d worry less about watering and make it simpler.
But I have thought about just doing the direct soldering technique, especially for the smd version.
Switchmode is the way to go with these high-powered LEDs. You can make your own, but I wouldn’t bother when you can buy stuff like this from Hong Kong with free shipping:
Sounds like you might be growing some “special” plants…
This isn’t something I have experience with, but it will be pretty expensive to put together enough LEDs to grow any reasonable amount of plants indoors. You will want to do some research on the most effective wavelengths to use. After than, have a good look at the LEDs offered by Deal Extreme. Get the units pre-mounted on the “star” PCBs, and make sure you mount these on a decent heatsink.
Yeah, but can you actually get any? I gave up on DealExtreme because I got tired of waiting 3 months to get parts!
MichaelN:
Switchmode is the way to go with these high-powered LEDs. You can make your own, but I wouldn’t bother when you can buy stuff like this from Hong Kong with free shipping:
Yep, I’ve ordered from them heaps of times; delivery normally takes a couple of weeks to Australia. Never had any problems. All those units I mentions should ship within a few days; actual delivery time would depend on the country you’re in etc. If you want faster delivery, pay the extra $$ for EMS…
I live an apartment and work second shift so using a bedroom windowsill won’t work. I also have a 19 month old son so the sliding glass door isn’t the best place either. But I was watching Trailer Park Boys and one of the schemes was growing marijuana in trash cans with hot grow lights and I was just watching it thinking about how stupid that was and could be done better.
I might get some smps drivers though since it might be simpler overall. But dealextreme’s extremely light on details. Is 3*1W supposed to be three 1W LED’s in parallel or series?
jrisom:
I might get some smps drivers though since it might be simpler overall. But dealextreme’s extremely light on details. Is 3*1W supposed to be three 1W LED’s in parallel or series?
You need to read the reviews and comments, since the title description isn’t always accurate:
http://www.dealextreme.com/details.dx/sku.13557 Despite the title, it looks like this can drive one or 2 LEDs in series at 650mA or so (ie, 2W per LED). You should be able to drive 2 identical 1W LEDs in parallel (or 2 strings of 2) although it’s a good idea to include small “balancing” resistors if you do this. If the LEDs are identical the balancing resistors aren’t really needed though. You may be able to drive even more in series, since most current-regulating “buck” converters can output almost the same voltage as the input (minus a bit of overhead).
http://www.dealextreme.com/details.dx/sku.13552 It looks like this is designed to only drive a single LED at 650-700mA (ie 2W nominal, despite the title saying 3W). As above, you could drive 2 X 1W LEDs in parallel, but it is a good idea to include low-value “balancing” resistors. If the LEDs are identical the balancing resistors aren’t really needed though.
I did forget to consider one thing for the LM317. By National’s datasheet, at about 350 milliamps, the LM317 will drop about 1.75 volts average at room temperature. Then the resistor has to drop 1.25 volts giving me a three volt drop before the LED’s. There would be so little room for tolerance that it wouldn’t work without an LDO variant of the LM317, at least not with three LED’s which would be the focus for efficiency.
