# Electronic Circuits and Projects Forum

1. ## LED Grow Light Questions

Hey folks,
This is my first serious electronics project (done some soldering before), so bear with me. I want to make an LED grow light for indoor hydroponic herbs (not weed, basil and cilantro mostly). I've done some research, but I want to get some expert advice and help now.

I've got some LEDs on the way, 3 colors that should be 3.5v 290mA, 2.2v 350mA, and 3.2v 300mA. I averaged the voltages and amps to ask the wizard, which told me this:

Solution 0: 7 x 4 array uses 28 LEDs exactly
+----|>|----|>|----|>|----|>|----|>|----|>|----|>|---/\/\/----+ R = 3.3 ohms
+----|>|----|>|----|>|----|>|----|>|----|>|----|>|---/\/\/----+ R = 3.3 ohms
+----|>|----|>|----|>|----|>|----|>|----|>|----|>|---/\/\/----+ R = 3.3 ohms
+----|>|----|>|----|>|----|>|----|>|----|>|----|>|---/\/\/----+ R = 3.3 ohms

each 3.3 ohm resistor dissipates 337.92 mW
the wizard thinks 1W resistors are needed for your application
together, all resistors dissipate 1351.68 mW
together, the diodes dissipate 29568 mW
total power dissipated by the array is 30919.68 mW
the array draws current of 1280 mA from the source.

Will this work with a 24v 3.75A power source?
I would also like to add a fan or two, would that just be another series that needs to add up to 24v and be added to the total current?

Like I said, I'm brand new to this, but I WILL complete this project eventually, so any advice about any later stages of what I'm trying to do will be amazingly helpful.
Thanks!

2. You can't average the voltage and amps, that circuit will either produce really bad brightness variation between each different color or outright burn out the lowest voltage LED, those are the only two outcomes

You have to run each separate LED type (colour) on it's own series string with separate current limit resistors for each string that will be of differing values.

Where have you determined that those wavelength of light are optimal for your grown lamps? I'm personally curious as I've always wanted to do some hydroponics myself but the drudgery of research on lighting and chemistry has lead me away from it.

3. Thanks!
Hydro is relatively easy so far, http://www.windowfarms.org/ was my start. Chemistry is easy when everything you put in is pH neutral. As for the wavelengths... the blue/red thing makes sense. Plants appear green, indicating that they reflect green; thus, blue and red light are absorbed. I bought one lot of red, blue, and white 1 watt LEDs because they were cheap. Exact numbers are bandied about, but it varies from plant to plant and lab to lab, so none can be accurate, unless it's monoculture that has been well researched (they have exact numbers for weed, but that's not what I'm growing so I don't care). I bought a cheap red/blue from amazon that hasn't killed anything yet, but I'd like to build a BAMF light rig and then use the knowledge on other projects. The numbers I came up with were around 440, 470, and 660, by the way. I added white lights in case I'm wrong.

How do I figure out how many strings I can add parallel? Is current purely additive? And if I wanted to add a fan or two, could they be another series with total voltage of 24?
Thanks again!

4. As Sceadwian says, the current rating is the maximum for an LED. You should not exceed it. It is certainly best to have only one current rating in each string.

You would certainly be able to run all those LEDs from a 24v 3.75A power supply. That is 90 W, nearly 3 times what you need.

What you should be careful about is the tolerance on the voltages. I guess that the power supply is a laptop power supply, so it will be quite well regulated, but you should measure the voltage at no load. Also the LEDs may not be exactly the voltage stated. The voltage stated is often the maximum voltage. The actual running voltage could be quite a lot less. I suggest that you keep the voltage of all the LED strings less than about 20 V, so that there is at least 4 V across the resistor. It is also a good idea to aim 10 - 20 % low on the current. That way, if the power supply or the LED voltages are not quite what you expected, the current will be less than the maximum.

5. Seven 3.5V LEDs need 24.5V plus a few volts for the current-limiting resistor so the power supply must be at least 28.5V.
Those LEDs will not light up with only 24V.

Ther 2.2V LEDs and their current-limiting resistor will quickly burn out.

Fire the LED Wizard because he is incompetent.

6. Thanks! I will certainly leave myself some wiggle room. I'm still waiting on the LEDs to come in, so I guess my next move is to buy a multimeter and a breadboard and begin messing around? I will certainly check back in once I've got some parts and more questions.

7. Originally Posted by karhas
I want to make an LED grow light for indoor hydroponic herbs
Wo0T!

Originally Posted by karhas
(not weed, basil and cilantro mostly).
Aww...

Originally Posted by karhas
And if I wanted to add a fan or two, could they be another series with total voltage of 24?
Don't use two 12V fans in series, use two 24V fans in parallel. Fans have dynamic current loads that will not evenly divide the voltage between them.

8. If the fans are used in free air and symmetrical in physical design series won't make a difference. This however does not include brushless DC fans!

9. Originally Posted by karhas
I want to make an LED grow light for indoor hydroponic herbs (not weed)
Yeah sure.
Whenever my wife and I walk our dogs along the boardwalk at the lake we smell Mary Wanna, not herbs.
Maybe from the Arabs with their huge water pipes. Look in Google Images at them.

10. Florida is a bit less open about its consumption than Canada. Thanks for the help! Here's my next round of questions:
1) What is the doohickey on the power cord about 3 inches from the tip? Will I be cutting that off or no?
2) How do I attach the LEDs to the heatsink? I have a large, finned aluminium heat sink that I want to mount these to, but do I just use thermal glue and go for it? Do I need to have insulation?
3) How do I figure out what kind of wire to use?
Thanks again for all the help!

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