COB LED Lights Simplified

Build your own COB LED light – the absolute basics

DIY COB light – 310 W optimal for a 9 ft² space

Cool, so you are interested in building your own light? Here is what you need to know! We dive a bit into LED tech and how to design a simple circuit, that will enable you to choose the parts for your grow LED to your individual needs. You will also learn about the importance of reading datasheets.

Disclaimer: Please please please DO NOT try to connect any selfbuilt circuits to your electrical circuit at home without being 100 % sure that what you build is OKAY. It is easy to kill yourself in the process if you do anything wrong.

Why even consider building a LED grow light yourself?!

Good grow LED tech is expensive. With 2-4 $ per Watt you can end up paying thousands of dollars on the best lights.
The advantage of this is mostly, that you get a finished plug&play module with things like warranty and product testing. In the case of DIY LED Kits sold by Horticulture Lighting Group, this is not the case. The sets you buy there are great, but its way cheaper to build them on your own.

Comapnies like Timber Growlights or Pro Emit are selling complete builds out of these items. Yes, they are companies, and therefore need to make a profit. But with a little insight and help, you will be able to but together a high quality LED grow light yourself for about 1$ per Watt or even less.

Why LEDs are superior

This is the future.

Wiliam Texier, “Hydroponics for everyone”, Chapter about LEDs

LED tech is advancing at an enormous speed. Companies producing LED light fixtures have to deal with this and need to apply very fast, to not fall behind. Almost every other month there is a new chip out there crushing all numbers that were there before.
LED light is semiconductor technology. Without going any way into solid state physics you can try to imagine that electrical energy gets used to push electrons into another state where they can recombine to a energetic more favorable position. The excess energy  of this recombination is then emitted as light. This is a very efficient process. In 2006 the first LED by Nichia with a light yield of 150 lm/W, equal to the output of HPS lamps. 

Disclaimer: Lumen per Watt is actually not a good way to compare LED and HPS lights. LEDs and HPS have different spectra and Lumen favors lights with more green light, because it weights the spectra with the so called luminosity function. The high output in the yellow/red range of a HPS light is not favored as much as a full spectrum white light from a 3500K LED.

Since then alot has happened. Current tech pushes the the light yield way beyond the 200 lm/W levels for Quantum Board chips (LM301b or LM561c). Current record holder is Cree with 303 lm/W with a 5150K (Cold white) LED chip. (cool tech, but not suitable for growing)

The spectrum of a warm white LED at 3000K or 3500K is also way better to reproduce the McCree intensity distribution to induce photosynthesis than HPS lights. 
HPS lights also tend to heat up alot. Modern Quantum Boards as well as COB lights can be cooled passively with heatsinks. That saves even more power, since air conditioning could potentially cost alot of money in an operation if the room gets too hot.

spectral properties of different lights
A grow light should reproduce the McCree curve as  accurately as possible. HPS light has a spectrum that is fairly limited to yellow and red wavelengths (upper right), while LED light is able to distribute energy to all the wavelengths needed (lower three spectra).

In comparison to HPS, CMH or LEC light, high quality LED systems are still pretty expensive. In the long run, a high quality system will be much superior in that regard, compared to the rest.

The Current Circuit

Current flows when they its enabled to do so by the existence of voltage. LEDs need a distinct workpoint, a combination of direct voltage and current to work efficiently.

The power plug in your house gives usually gives you between 110 V (U.S.) and 230 V (EU) and up to 12 A to 16 A of current before the fuse will have a serious word with you. Short circuits are just a unhinged connection between two poles creating a big ass current peak. The fuse will jump out and the light goes of.

So the first thing you will need is a driver.

Transforming electricity with a driver

To transform the alternating voltage to a usable direct voltage you need (over simplifed)three things: A transformator , capacitor and a rectifier. All of these things are included in a driver. Since you manipulate a form of energy this driver will never be 100% efficient. Good drivers will work with an efficiency of more than 90% though.

The Transformator will change the value of the incoming voltage, while the rectifier is used to reverse the part of the waveform that has the wrong signum. The capacitor is used to smooth out the curve and to fine tune the process. There is much more than this happening in a LED driver, but this is the basic principle. 

Schematic of a rectifier
Simplified circuit: The pink waveform is already rectified. The capacitor C smoothes out the form further to create the rippled curve above. If you use more than one capacitor, you can ease this out even further. 

Important: NEVER connect LEDs to the power plug of your home without a driver. Could end lethal.

The right combination of Drivers and COB modules

Okay, now that we can safely convert our electrical input, we can now proceed and choose the right combination of light modules and drivers.
First you should determine the amount of power you will need in the end. 

Since you will build an awesome high quality COB light you will need about 35 Watt of power per squarefoot.

Example one: A 3’x3′ tent light

For this setup we need approximately 315 Watt of high quality COB light. (3 x 3=9, 9 x 35 = 315)

The BXRC-35E10K0-D-73-SE is the perfect canditate for that. It will run with a current of 2.1 A at a voltage of 36.6 V. Power is determined by multiplying these two values, resulting in 76.86 W for each COB. So running four of them will yield 307.84 W, which is close enough to the desired 315 W we need.
The COB delivers hell of a light!
Since we know, that lumen are not really useful to determine light quality, its an okay ballpark number to compare lights with similiar spectra (3500 K).
This light runs at 147lm/W @ 2.1A and 36.6V

COB efficiency

One quick notice: A COB runs less effecient the more current you push through it. You will get a better lm/W value if you run just 1.4 A. This will result in less power and less absolute photon output, but a cooler COB and higher efficiency. So if you plan to build more efficient, you want to consider to use more COBs at less current.

Now that we got our lights, we need a driver, that can run them properly. For this purpose we need to plan the configuration of the lights we are using. Lets assume we connect all 4 COBs in series. In 9th grade in school you learn about Kirchhoff’s circuirt laws, that tells you: The current in a series connection is constant. On the other hand the voltages will have to be added together.
Conclusion: We need a driver with an output of 2.1 A and 146.4 V.

series connection of 4 cobs
Series connection of 4 COBs

The most common drivers on the market right now are from MeanWell.
The model HLG-320H-C2100B will deliver a constant current of 2100 mA = 2.1A and has an output voltage ranging from 76 V to 152 V which is quite perfect for our cause, because we calculated with 146.4 V!
The driver costs about $100 plus $80 for the COBs. Not a bad starting point for a 310 W light!
You will need some more items before you plug in your light and let it shine on your ladies. (Build guide coming soon)

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5 thoughts on “COB LED Lights Simplified”

  1. Thanks for doing all the hard work for us, much appreciated. I’m new to the indoor scene so every little bit helps. Ok, now can you add more COBs to the driver mentioned ? iv’e a4x4 tent and to get the required 35watts per sq ft would need 7 COBs. (bit less than 35w) 8 would give 38.43.w or is there a driver more suited ?

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