LED Grow Light Questions

[karhas]

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!

 

[Sceadwian]

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.

 

[karhas]

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!

 

[Diver300]

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.

 

[audioguru]

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.

 

[karhas]

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.

 

[duffy]

I want to make an LED grow light for indoor hydroponic herbs

Wo0T!

(not weed, basil and cilantro mostly).

Aww...

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.

 

[Sceadwian]

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!

 

[audioguru]

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.

 

[karhas]

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!

 

[KJ6EAD]

The ferrite cylinder on the output of your power supply acts as a low pass filter to remove high frequency switching noise from the DC. It's best to leave it. It's also better to leave the coaxial DC plug and get an appropriate jack to connect to your LED circuit. It's probably 2.1mm X 5.5mm x 11mm.

Those LEDs are designed to solder on a PCB and make both their electrical and thermal connection via the solder joints. The LED manufacturer should provide a recommended PCB footprint and other thermal design information but the ebay seller you got your LEDs from just offers base plate PCBs and heat sinks. Can you even get data sheets for your LEDs?

 

[karhas]

Would something like **broken link removed** these do the trick? The power adapter I bought came with a bunch of adapter tips, so I guess almost any would work... and no old computers around to salvage

 

[KJ6EAD]

Maybe, none of those have any dimensions, just a mention of what computer they fit. Several look like they might be the right type but the seller didn't even bother to give a clear view of the connectors.

 

[karhas]

The stores are all in China, the only data sheets I have are the ones in the picture. I've downloaded ExpressPCB, but it honestly blows my mind. I'm not really sure how to do anything with it. Frankly I'm not sure I understand what PCB is... wires contained within the board, basically?

 

[KJ6EAD]

Wikipedia can help you some but buying a bunch of unknown materials from Chinese ebay vendors is a recipe for disappointment unless you have the technical skills/knowledge to work around the unknowns.

 

[karhas]

well, that's why I'm here asking for help

 

[KJ6EAD]

There's help and then there's "give me a complete working design using a bunch of parts with no data". You're getting close to the latter.

 

[karhas]

I'm asking specific questions. You answered two of them quite clearly (thank you!), and I now understand what my next step is: learning that program and creating a schematic to then come back here and ask about. I just want experts looking over my shoulder, I don't expect you to design my circuit for me... so thank you for your help, and I will be back later with more questions.

 

[karhas]

So here's my stab at a schematic: View attachment 63229
All the math done by the wizard, but playing it safe. I'm lowering the input voltage to 18 because I only have 10 white, 10 blue, and 12 red LEDs to work with.

Does it matter that the red strings have such large resistors compared to the blue and white?

To create a LED component in ExpressPCB, do I just need two pads spaced correctly? The LED component included doesn't seem like the right shape.

Thanks again!

 

[audioguru]

Your schematic is too small for us to see anything.