I’ve been busy implementing what I’m calling the Leaf Spa.  I hope to write an overview at some point.  In this post I’m focusing on the LED lighting I’ll be using.  I realize after writing this post that not having an overview makes it extremely difficult to follow what I’m writing.  However, I wanted to write out my build decisions as I was making them so that I would have less of a chance to skimp on explanations of decisions made.

I waver on the grow light setup I will implement for the Leaf Spa.  The wavering is between a setup of discreet red and cool white Cree XP-E2’s versus using COBs.  I’ve decided on using COBs.  This decision was based on taking less time to implement, be less expensive, and let me explore using COBs to grow plants.  I haven’t used COBs before.

Thanks to Those That Went Before

growmau5 – I learned a lot about DIY LED lighting from his DIY LED Basics videos.  THANK YOU.

Bill Herd for his excellent video “Calculating Heat in Electronic Circuits: Do I need a Heat Sink?

Area

The area of the grow space is 30” x 14”.

Here are 3-D drawings of the LED “shelf” that I am implementing:

LEDShelfLookingAtCOBs                                         LEDShelfShowingHeatSink

I did the drawing in Fusion 360.

The Grow Chamber section of the Leaf Spa will allow the LED shelf to be moved up in 5” increments as the plants grow taller.  This way, I can maximize the PAR hitting the leaves.  

GrowChamberShowingPins

Each post has holes drilled at 5” increments above the initial LED shelf setting of 9”.  The 9” is because of this is the default for the IKEA shelf.  I chose 5” based on previous PAR measurements I have made with my other LED lightings.  There will be a pin/metal dowel inserted within the post and shelf to hold the shelf in place.

Choice of COB

I decided on using 3 Vero 18 Gen 7’s @ 3500K running a current of 1.05A (BXRC-35E4000-D-73 – data sheet linkDigikey link).  I wanted to try out a lens, so I went on Alibaba and found this lens.  I ordered 3 for $3.

Choice of Driver

The Vero18 data sheet states the typical forward voltage = 29 when the current = 1.05A.  Given the Leaf Spa will have 3 Vero18’s, the amount of power needed to run the COBs in series = 3 * (1.05 * 29) = 91.35W.

The Meanwell ELG-100-C1050A (Mouser link) should work well.

Choice of Heat Sink

I decided to use a passive extruded aluminum heat sink from heatsinkusa.com.  In order to know what to order, I first needed to know the surface area of aluminum needed in order to dissipate the heat generated by the COBs.  Using the advice given in Bridgelux’s application note:

  • As noted at the beginning of the app note: “when voltage is applied across the junction of an LED, current flows through the junction generating both light and heat.”  The app note goes on to note that warm white (which 3500K is considered to be) has 75% of the power going toward heat.  Sad….only 35% light..and I keep hearing how efficient LEDs are. 
  • A little later on, the app note states: “As a general rule of thumb, for a well-ventilated heat sink, there should be 10 in2 of heat sink in contact with the cooling air for every 1 W of thermal power dissipated.”  Note: gromau5’s terrific video on choosing your heat sink, he states passive heat sink should have 17 in of heat sink.  So what to use?  Since I don’t know any better, I have chosen to use the average between 17 and 10 – 13.5 in of heat sink to dissipate 1 W.

Calculations

Calculating the thermal heat being dissipated based on Bridgelux’s app note:
Heat generated by the three Vero18’s  = 3 * (29 V * 1.05A * .75) = 68.51W
 
Using the rule of thumb (and 13.5 in of heat sink to dissipate 1 W):
The heat sink surface area needed to dissipate  68.51*13.5 = 924.89 in 2.
 
Moving on to heatsinkusa.com’s heat sink profiles, I chose to go with the 7.984” wide heatsink.  Based on my limited knowledge, it seemed to be a good compromise between price and surface area.  The heat sink profile states the outer perimeter is 46.290. 46.29 is the surface area (in 2) for one inch.  The minimum 7.984” heat sink length = 924.81 / 46.29 =  19.98″”…rounding up to 20”.

What To Buy

Based on the calculations, I’ll buy 20” x   7.984” wide heatsink.  

Mounting and Wiring of COBs

I plan to follow the mounting advice growmau5 gives in this video. Molex provides Pico-EZmate cabling that works for the Vero COBs (digikey link).
 

 

 

That’s it for now. Please find many things to smile about.

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