Selecting LED COBs and Matching Drivers

When you’re picking your components and planning out your system, it’s very important to match your LED COBs with a proper driver. The goal of this guide is to get you comfortable with the basics involved with planning a simple system. If you’re brand new to growing with LED COBs, I’d recommend reading our COB LED Basics for Beginners article.

After you’ve read through this post, take a look at the DIY Guides page for a list of more specific and advanced guides and resources.

Selecting COBs

Popular Models

When it comes to COBs for indoor growing, the majority of people are currently using 1 of 3 proven brands: Cree, Citizen, or Bridgelux. In my opinion, if you’re looking for low-current efficiency and aren’t overly concerned with cost, go with the Crees. If you’re looking for a good all-around COB that’s easier on the wallet, or intend to drive your COBs with more current, go with Bridgelux or Citizen. The new gen. 7 & SE Veros and Version 6 Citi’s are really closing the gap in terms of efficiency, and can be found considerably cheaper (here in Canada, anyway).

  • Currently, the most popular COBs from Cree are the CXB3070 and CXB3590 models, with the 3590s being the best Cree offers for this application. The 3070 is a good alternative, and, while not cheap, is less expensive than the 3590.

Cree CXB3590

  • The Bridgelux COBs used by most indoor gardeners are from the Vero Gen.7 or SE Series. There are 4 different sizes in the Vero series: Vero 10, Vero 13, Vero 18, and Vero 29; the number reflects the size of the Light Emitting Surface. Of these 4,  the 18 and 29 models are most popular. While these COBs are not as efficient as the Crees, they still put out a lot of light (at higher currents) and are significantly less expensive.

Vero COB

  • The Citizen COBs that most growers are using are the Version 6 CLU048  units. They come in a few different configurations like the CLU048-1212 (contains 12 diodes in parallel and 12 in series) or the CLU048-1818 (18 in parallel, 18 in series). There is also a larger model which runs at a considerably higher voltage but puts out a ton of light: the CLU058.

Important Specifications

Aside from cost, there are a handful of important specifications that you’ll need to consider when choosing your COB. I suggest you read through the points below, then when you have a better understanding of these few specs, read this post on how to easily compare COB LEDs using manufacturer-provided simulation spreadsheets.

Colour Temperature (also referred to as CCT or Correlated Colour Temperature)

Measured in Kelvin (K), colour temperature determines how warm (red) or cool (blue) the light emitted is. Lower values (e.g.- 2700K) are warmer and higher values (e.g.-5000K) are cooler. When using LEDs to grow plants, we’re mostly concerned with the “PAR” range of light, which stands for Photosynthetically Active Radiation. PAR represents the range of light that plants use for photosynthesis (wavelengths of 400 to 700 nanometers), and it’s in our best interest to use colours in this spectrum.

It’s up for debate whether it’s better to use different coloured lights for different stages of plant growth – some say that 5000K lights are best for the vegetative growth phase, while a warmer light (2700K-3000K) is better for the flowering phase. Others argue that a middle-of-the-road temperature like 3500K works for both phases. My first grow of tomatoes, peppers, and herbs was done using only 3500K lights, and they performed nicely all the way through. For simplicity’s sake, I’d recommend going with 3000K or 3500K for the whole grow.

Below is an example of the Spectral Power Distribution for different colour temperatures of the Cree CXB3590, obtained from the product datasheet. You can see that higher colour temperatures (4000K and 5000K) have considerably more output in the ~450nm range (blue) compared to the ~700nm range (red). The reverse is true for lower colour temperatures.

Matching LED COBs to Drivers

Forward Voltage

In the interest of keeping things simple, I recommend wiring your COBs in series (unless you have a whole whack of them running off of a small number of drivers, in which case, you might need to do a combination of series and parallel wiring). The voltage of the COB is important, because when wiring in series, voltage is added, while current remains the same. This is the opposite of parallel wiring, where voltage is the same through every COB, but current is added up and varies. For a more in-depth look at series and parallel wiring, check out this post.

Continuing with our Cree CXB3590 example, you’ll note on the datasheet shown below that there are 2 versions available:  a 36V version and a 72V version. Let’s say you opt for the more common version which has a typical forward voltage of 36V. If you decided to run 2 of these 36V CXB 3590s together on 1 driver, you would need to buy a driver that outputs your desired current (I’ll get to this next) at 72 Volts (36V Cob 1 + 36V Cob 2 = 72V total).

A caveat to the typical forward voltage is that at different levels of current and heat, voltage will change slightly. For example, viewing the graph below, you can see that when you run a 36V CXB3590 at 1,400 milliamps (a common drive current), voltage will not be 36V, but somewhere between 34V and 35V. When the same COB is run at the maximum allowable current of 3,600mA, voltage is up above 37.5V. These small changes may not seem important but it can make a difference when you’re looking to squeeze multiple COBs onto a driver that operates in a particular voltage range and need the total voltage at each COB to be a few volts more or less than typical.

Cree COB data sheet

CXB3590 Voltage Current graph

Forward Current

The maximum forward current rating lets you know the largest amount of current you can apply to the COB. Referencing the chart above again, the maximum forward current for the 36V version of the CXB3590 is 3,600 milliamps.

Common drive currents used for COBs like this are 700mA, 1,050mA, 1,400mA, 2,100mA, and 2,800mA.

As you increase the current, you receive diminishing returns in terms of light output – more and more power is dissipated in the form of heat, rather than photons. For this reason, it’s always better to buy more lights and underdrive each of them if you can afford it. Though it’ll be significantly more expensive upfront, your system will be more efficient and will save energy in the long run.

As mentioned, when wiring in series, the forward voltage drop of each COB is added and current through the entire string remains the same. For example, if you take 4 CXB3590s and wire them in series to a 2,400mA driver, you have a total voltage drop of 144V (36+36+36+36), but every single one of these COBs will get the full 2,400mA of current.

Efficacy

Efficacy refers to the number of lumens the COB produces per watt of power. As discussed in this post on methods of measuring light, lumens are not the ideal unit of measure for horticultural applications (PPF or PPFD are), but they still serve as a solid way to compare one COB versus another. Efficacy is not to be confused with efficiency, which refers to the percentage of electrical power that an LED is able to convert to photons vs. the percentage of that power that’s wasted as heat. Efficacy won’t be listed in the data sheet, but can be easily obtained. Check out this guide on comparing LEDs to learn how to determine a COB’s efficacy.

Selecting a Matching Driver

Now that we’ve gone over the pertinent info for the COBs themselves, choosing a driver is simple. Be warned, when dealing with multiple large COBs, the LED drivers required to drive them are very powerful, and produce enough voltage and current to seriously harm you (or worse). BE CAREFUL.

Popular Models

Right now, the most popular LED driver for this application is the Meanwell HLG-C series. There are a number of different versions of the HLG-C series, but all that differs between them is their power output rating, with the lowest being the HLG-60H-C at 70 watts and the highest being the HLG-320H-C at 320 watts. There are a few other LED driver series from Mean Well that will do the job too, like the LPC or NPF-D series, which are made of plastic rather than metal and are for applications where less power is required.

For the true DIY enthusiast, you can also put together a driver using a power supply and DC-DC boost converter, but we’ll be sticking to the basics here.

Important Specifications

Constant Voltage or Constant Current?

The type of driver you’ll be looking for to use in conjunction with your series circuit is a constant current driver. Constant current drivers will list a range of voltage that your total system voltage drop must be within, in order for the driver to output the desired current. See my post on constant current vs. constant voltage drivers for a more in-depth look at the differences.

Choosing the Right Size of Driver for your COBs

If you want to do this the easy way, use our handy-dandy HLG-C selection tool! Enter your COB model, quantity, and drive current, and the spreadsheet will calculate your total forward voltage and highlight all of the Mean Well HLG-C drivers that are compatible with the parameters you entered. If the total forward voltage (Vf Total) of your system falls within the rated constant current range of a driver (between V_min and V_max), that driver will turn green, indicating it is a match. If the Vf total is out of range by 2 volts or less, borderline drivers will be highlighted in orange, indicating that they will probably work, but may depend on temperature and dimming. Now, if you want to learn how to properly size a driver yourself, continue reading below.

Let’s say you were able to get a good deal on 6 CXB3590 COBs and decide you want to drive them at 1400mA, which is a little under half of their rated maximum current, in order to improve efficiency and reduce heat. Which of the HLG-C series would you have to buy to be able to power them?

The first step is to figure out what your total required voltage will be. According to the data sheet, the forward voltage of each CXB3590 is 36v (measured at 2400mA of current) so 6 of them would equal 216V , however, we are going to run them at 1400mA, so we must consult Cree’s graph once again to determine the Vf for our current. According to the graph, at 1400mA, the Vf of each COB would actually be closer to 34V, resulting in a total of ~204V.

To get an idea of how much power your driver needs to be able to put out in watts, multiply the voltage you calculated by the current you’ve chosen. In this case, you’re looking at 204V x 1.4A = 286W

Now, you need to pull up the data sheets for your potential driver. We know that we’ll need a driver capable of outputting at least 286W, so this narrows the search to only 1 driver in our case (HLG-320H-C), but let’s look at 2 data sheets as examples – one for the HLG-240H-C and one for the HLG-320H-C. Look to the column that shows the 1400mA model and find the “Constant Current Region” row. This specifies what voltage range is required in order to produce the constant current for each rated current in the series.

On the 240H-C, the constant current region requires a voltage range of 89-179V in order to output 1400mA. Our total voltage is 204V, so this will not work. On the 320H-C, the voltage range required for 1400mA of current is 114-229V. We are within this range, so this driver would do the trick. As you can see in the data sheet, if you decided to lower your current to 1050mA, the smaller 240H-C would run these COBs, but if you want to stick with 1400, you’ll need the big boy.

Matching LED COBs to Drivers Mean Well HLG-240H-C data sheetMatching LED COBs to Drivers Mean Well HLG-320H-C data sheet

Dimmers

Many drivers allow you to adjust the current output and effectively dim the lights. Some come with a built in potentiometer (a variable dial that changes the output when turned), some come with leads that you can solder a potentiometer onto, and others require more advanced methods such as pulse-width modulation, or feeding a DC signal of a specific voltage. The easiest of the above options would be to opt for the model with the built-in potentiometer. If this isn’t available, grab the “B” version and get your own 100K ohm potentiometer to solder on.

 

With any luck, this has shed some light on how to go about selecting and matching different LEDs and drivers – at least at a basic level. Researching new parts and configurations is one of my favourite parts of the build process and I hope you can share in that enjoyment now too.


2 Comments

  1. “If the Vf total is out of range by 2 volts or less, borderline drivers will be highlighted in orange, indicating that they will probably work, but may depend on temperature and dimming. ”

    If I use a driver that is in this borderline range, what will happen if the voltage is just not enough? Will any damage occur or will the LEDs just not turn on?

    • LEDGardener

      February 22, 2017 at 7:56 pm

      Hi Crystal,

      Since Mean Well underrates some specs on their drivers, you will be fine if you’re a few volts under. If you look at some of the test reports Mean Well produces for their drivers, you’ll see that the minimum voltage spec for constant current region is often much lower than they rate it for.

      For example, their HLG-240H-C1400 is rated for a constant current region of 89V-179V, but their test report shows that on the unit they tested, the constant current range extended all the way from 46V to 178V. I just used the numbers they publish for my spreadsheet to be safe. Here is a link to the test report if you’re interested: http://www.meanwell.com/webapp/product/search.aspx?prod=hlg-240h-c&pdf=SExHLTI0MEgtQzE0MDAtcnB0LnBkZg==&a=2

      I’m actually testing my HLG-185H-C1400B right now with just a single CXB3590 to see how it handles it. The rated constant current region for this driver is 71V to 143V but it’s having no issue producing its rated current with this single CXB on it at 34 odd volts.

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