Constant voltage drivers are becoming more popular lately. They’re safer, flexible, and often cheaper than their constant current counterparts, but it can be a little more difficult to pick the right CV driver for your system than it would be for a CC build. Don’t sweat though, it’s really not very hard. All you need is an understanding of how parallel wiring works and to know how to manipulate forward voltage and current numbers using data sheets or product simulators.
Below are the steps to take to find a suitable constant voltage driver. I’m going to assume you’ve already picked your COBs since this is usually what builds are based upon, but this should still be useful to you even if you haven’t.
1- Decide What Current You Want to Run Your COBs at
The first step is picking a current to run each of your COBs at. Most LED gardeners strive for highly efficient setups and will run their COBs at or below their “typical” ratings. For example, even though the data sheet for the Cree CXB3590 shows a typical forward voltage of 36 volts which translates to ~2,400mA of current, the majority of CXB owners run at 1,400mA or below. Some go all the way down to 700mA in some highly efficient builds that utilize a large number of under-driven COBs. The lower the power you feed your COB, the more efficient it will be – that is, it’ll turn more of those electrons into photons rather than heat.
Currently, the most common drive currents for most COBs with a typical forward voltage of 36V are 1,050mA and 1,400mA.
2- Determine What Your COB’s Forward Voltage Will Be at the Current You’ve Chosen
Every COB has a unique current-voltage relationship. When you put a constant 36 volts across one COB, it might pull 2,400mA, but when you put the same 36V across another model of COB, it might pull 300mA, or 3,000mA. With this in mind, now that we have chosen our current in the step above, we need to figure out what the voltage across the COB will be at that current. The easiest way to do this is to use the manufacturer’s simulation tool, if one is available. If not, you can usually extrapolate this data from the COB’s data sheet.
Let’s look at 2 examples and use a current of 1,400mA for both of them to make comparing them easier.
Example 1 – Bridgelux Vero 29 Gen 7 “D” Array
Using Bridgelux’s product simulator, I entered a 1,400mA drive current. Below, it spit out the expected voltage for this COB at this particular current, which is 35.7V
Example 2 – Citizen V5 CLU048-1818
Entering the same values in the Citizen calculator, it looks like we can expect a voltage of 51.8V across a CLU048-1818 when we have 1,400mA flowing through it.
3- Calculate Your Total Current Draw
Now that we know the approximate voltage we’ll need our driver to produce, we need to figure out what our total current draw will be. In constant voltage applications, we’ll be wiring our COBs in parallel and whatever voltage our driver is outputting will be the same across every COB that’s paralleled onto the circuit. However, the current drawn by every COB will be added up, and our driver needs to be able to supply as much or more current than the total draw of all our COBs.
To figure out your total current draw, multiply the number of COBs you’re going to use by the drive current you chose in step 1. If we return to the Bridgelux/Citi examples above, they will both have the same total draw if you use the same quantity of each. Let’s say we’re building a light with 4 COBs.
4x Bridgelux Vero 29-D @ 1,400mA = 5,600mA
4x Citizen CLU048-1818 @ 1,400mA = 5,600mA
So, our total current draw is going to be 5.6A regardless of which of these COBs we go with. The only difference between these 2 are the voltages required to create this current – the Vero 29 D needs 35.7V and the CLU048 requires 51.7V.
4- Calculate Your Total Power
The easiest way to narrow down your driver is by figuring out what your total power requirement is. Many driver model numbers have their power included in it (e.g. – a Mean Well HLG-240H driver is rated for 240 watts), so once you know how much wattage you need, you can sort through driver model numbers quickly to find your part.
To calculate power, multiply your voltage from step 2 by your total current draw from step 3.
Example 1: 4x Bridgelux Vero 29-D
If we’re running 4x Veros from the examples above, our total power requirement will be 35.7V * 5.6A = 199.9 Watts
Example 2: 4x Citizen CLU048-1818
For the Citis, our total power requirement will be 51.7V * 5.6A = 289.5 Watts. You can see that the CLU048 will need a lot more power to run, but it will also put out a couple thousand more lumens than the Vero will.
5- Find CV Drivers Capable of Outputting Your Power From Step 4 and Select the Right Voltage
When you’re picking a constant voltage driver, it should be able to output your required power at the very least. There are some exceptions to this rule, like when people deliberately overload CV+CC drivers in order to force them into constant current mode, but generally you’ll want your driver to be rated for the same power or more than your circuit requires.
The nice thing about CV is that you could choose a driver that’s 10 times more powerful than you need, and your circuit will only draw what it requires. This makes these drivers very flexible for adding more COBs in the future. If your circuit requires 36V and 2,000mA of current, you could purchase a 36V driver that’s capable of providing 10,000mA of current, and it’ll run your little system no problem with room to add on.
Example 1: 4x Bridgelux Vero 29-D
We’ve determined we’ll need a driver that can do 200W at the very least to power this circuit. The HLG-185H won’t work, but the HLG-240H will. Have a look at the data sheet to find the proper model. Our voltage for the Veros circuit is 35.7V so we’ll want the HLG-240H-36. You can see on the data sheet below, the HLG-240H-36 is rated to provide up to 6.7 amps of current, which is more than than the 5.6A we need. Good stuff.
I would recommend getting the “A” versions of these drivers, as it allows you to fine-tune your voltage and current levels with built-in potentiometers on the driver. If you don’t get the “A” version, you won’t be able to directly adjust the voltage (“B” types come with a dimmer lead which reduces current).
An important spec to check is the “Voltage Adj. Range”, which specifies how much you can tweak the output voltage of the driver. The HLG-240H-36 is capable of outputting from 33.5-38.5V. If you get the “A” version, you can set the driver to exactly 35.7V as we calculated in the steps above. If you opt for the B model, your COBs will get 36V exactly, which translates to about 1,500mA of current – close enough.
Example 2: 4x Citizen CLU048-1818
These suckers draw a good amount of power so we’re going to need to use an HLG-320H. Since our circuit voltage is 51.7V, we’ll go with the HLG-320H-54. Again, I would get the “A” version of the driver, because we don’t want it to put out the full 54V it’s rated for. At 54V, each COB would be trying to pull about 2,400mA rather than the 1,400 we’re aiming for! With this particular driver, we’d actually be fine, because it would hit the max current of 5.95A and the driver would switch to CC mode and limit the current at that point, but if you had a bigger driver that was able to produce more current, it would dump another 4 amps into these COBs at 54V.
Well, congrats if you managed to make it through to the bottom of this post. I think that about sums up the basics of picking the right constant voltage driver for your system! As always, feel free to leave a question or comment below.