# LED circuit help!?

Discussion in 'The Projects Forum' started by andyjoness, May 17, 2012.

1. ### andyjoness Thread Starter New Member

May 17, 2012
2
0
Hi, i am looking to make a custom grow lamp for my chilies. In all honesty i don't have a great deal of knowledge about electronics, but i want to power it off of a spare 12 volt power supply unit from an old pc of mine.

I'd like to use many high brightness led's, possibly ones like this http://uk.rs-online.com/web/p/products/0414597/

I think the best way to do it would be to use an array of 9 strips, each with 3 led's on, and a 2.2ohm resistor with a power rating above 3 watts.

My question is, how would i work out how many led's the power supply unit would be able to power? It's 450 watt if that helps at all.

Any help at all is appreciated, thanks a lot

2. ### crutschow Expert

Mar 14, 2008
13,502
3,376
You might do some research to see if those LEDs put out a good frequency spectrum for growing plants. Some may have holes in the output spectrum of a light wavelength that the plants particularly need for good growth. For example this site claims to have LED lights that are well suited for growing plants. Google "plant light spectrum" for more info.

3. ### andyjoness Thread Starter New Member

May 17, 2012
2
0
They put out wavelengths that peak at 625 nm, which is pretty much spot on for maximum photosynthesis.

4. ### tracecom AAC Fanatic!

Apr 16, 2010
3,879
1,396
The mathematical answer is that each strip of 3 LEDs and 1 2.2Ω resistor consumes 12 watts. So, 450 watts divided by 12 watts equals 37.5 strips. However, that would only work if the power supply really could furnish 450 watts, and it would mean that the power supply was working at full output continually, which probably isn't a good idea. A more conservative approach might be run the PS at about half load, which would allow 20 strips. And the resistors should be at least 4 watts; 5 would be better.

Oh, and that's running the LEDs at close to their maximum current, which also might not be the best for their life expectancy.

5. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
Typically you will get some Amps out, but then voltage will drop down.

6. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
LED Grow lights are a popular subject here. The last one was Led Grow Light , which I helped with somewhat. I claim no expertise on grow lights, but I know LEDs well.

A common theme with all the grow lights is the mix of colors, apparently many folks figure it is a mix that works best, if I read your choice as a single color that is probably not the best. If it is what you want to use however, I will help as best I can.

Really small value resistors for LED current control are not a good idea, if you are going to get that close to running everything to spec some simple electronics would be best. Again, up to you.

Actually, this is incorrect. Running them over specification will shorten their life, but at spec most LEDs are rated for 10's to 100's of thousands of hours, which is plenty of time. Even with a shortened life span they can last 10's of thousands of hours.Running them under spec may extend this, but there are other failure mechanisms that could enter into play. It is why LEDs are gaining such traction.

The main thing with power LEDs is heatsinking. You must heat sink these parts, and they will get hot. However, with a bit of planning it is no big deal.

If you do an advanced search on this site and use the search phrase "grow lights" you will see many examples of other peoples projects, I've helped with about 90% of them. I don't know why, but people tend to be suspicious of what you are growing unless you give details. Personally I don't care either way.

I have created a tutorial on LEDs in general, mostly low power stuff. It will help get you up to speed.

LEDs, 555s, Flashers, and Light Chasers

Focus on the 1st chapter and half the second.

Last edited: May 18, 2012
7. ### John P AAC Fanatic!

Oct 14, 2008
1,638
225
If you're mounting these yourself, have you considered heat sinking? Even though LEDs are efficient, the heat they generate is produced in a very small volume, so mounting the LED on a conductive surface is important. I recently bought some LEDs like this:
http://www.alibaba.com/product-gs/445307404/1w_LED_with_Star_PCB.html

You will note that the actual LED is surface-mounted to a circuit board that also acts as a heat spreader--not exactly a heat sink, as it's not adequate to dissipate all the heat, but it's easily mounted to a larger plate with a couple of screws. I put 4 of them in series on a piece of aluminum extrusion, and it does get noticeably warm. My LEDs are only 1 Watt each.

Here is my setup:
http://dl.dropbox.com/u/28291527/IMG_2059.JPG

The LEDs are running off a switching current control device which you can see at bottom left. Mounted to the extrusion is a linear current control which I built earlier on perf board, but I've disconnected it. The ammeter is showing .327 amps, which is exactly right for this component.

I bought the LEDs as a group of 50, and in that quantity they cost me US\$0.56 each.

Last edited: May 18, 2012