# LEDs and lighting and ugggh.

#### Osprey101

Joined Nov 4, 2011
1
First time poster, long time solder-eater.

I'm trying to piece together a lighting system for growing plants in sterile tissue culture; LED illumination is ideal, as we don't need the intensity used in other settings (for growing mature plants, for example).

I've decided on going with Osram Golden Dragon Plus LEDs, using two different models- one peaking in the red, one peaking in the blue. Reds run 2.15 volts, and I'm going to run them in a series of 9; at 700 mA each, that works out to about 14 watts (according to the calculator at http://led.linear1.org/led.wiz).

The other string is blue; they run 3.2 volts, and I want 1/3 as many as reds; so, with 3 blue LEDs, running at 400 mA, consumption is 4 watts. The problem is, they're at a separate voltage (10 volts).

Last night, I thought I had a solution using the website above using 20 volts for both, but Firefox crashed and I lost everything. I can't figure out how to run both strings off of 20 volts without huge resistors- and lots of wasted power.

My questions:

1) Is there any sort of way to figure out how to optimize efficiency without plugging in these numbers manually- maybe a spreadsheet? Or can someone else tell me how to optimize the values for 20 volts on both strings?

2) If I can't use separate voltage on each string, can I use a 20 volt power supply, and then a zener diode (or perhaps something else?) to get 10 volts for the other string?

Any help would be appreciated.

#### ke5nnt

Joined Mar 1, 2009
384
That wizard is kinda bogus. It uses "Washington" math. The red string with a forward voltage of 2.15V each is going to eat up 19.35V. With a 20V source, that leaves no headroom. You'll be better off going with a string of 8 LEDs. A forward voltage of 2.15V at 700mA probably makes the LEDs 1.5W LEDs. Can't really change that. With 8 of them, you're gonna have ~12W dissipated through the string, no matter how you hook them up, unless of course you run them at a lower current.

Anyways, 8 diodes at 2.15V each is gonna take 17.2V. 700mA in a string, if you were going to use a resistor for this, you'd be looking at a value of about 4Ω which would dissipate 2W, so you'd want a resistor that was larger than a 2W resistor. I don't think I'd recommend this, of course, I'm no expert, but I've been around the LED block, especially here recently, and I think a constant current source would be far more suitable and reliable for this.

More than likely, you'll be looking at needing heat sinking on the diodes and some current source components as well. Somebody else can probably tell you a lot more about current sources than I can, since they are still new to me, and I don't want to steer you in the wrong direction. However, an internet search for "constant current source" or "LED driver" will turn up lots of info that you can look over in the meantime.

#### Wendy

Joined Mar 24, 2008
23,421
Since you have some experience there are ways around your dilemma, if you are willing to put some work into it.