How to Choose the Right Constant Voltage Driver for Your COBs

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.

20 Comments

  1. Got a question about constant current region specs, say I had a HLG-185-48A and I’m trying to drive some 50v chips, it has an adjustment of 43-53v. But the constant current region says 24-48v does that mean that cranked up to 50v that it would be stuck in constant voltage all the time?

    • LEDGardener

      March 26, 2017 at 11:00 pm

      Nope, you could have it dialed up to the max of 53V and as soon as you exceed the current producing abilities of the driver, it’ll go into constant current mode.

      I’m fairly certain the only reason they use 48 as their upper end of the constant current region for that driver is because you can’t get your voltage higher than that with anything other than the “A” version of the driver, so it wouldn’t make sense to say the constant current region goes up to 53V if you have a “B” driver and are limited to a fixed 48V anyway.

  2. Hi,
    Very interesting article.
    However i think the vero 29c is the 68v version not the 36v. 29b 50v and 29d 36v.
    And a question
    Could i run vero 29b or v22 x4 on a hlg240h-48 that i seen cheap on ebay? Spec says it gives up to 5.2a @ 53v or would that push the driver too hard
    Colinc

    • LEDGardener

      March 30, 2017 at 9:23 pm

      Hey Colin, good catch! I got it right the first time I wrote it, then screwed all the rest up. I’ll make the change.

      I looked at the spec for that driver and I see it’s rated to do up to 51.2V but the test report shows that the particular unit they tested was able to do 52.7V with a max of 5.4 amps of current. If we assume your driver is also able to output 5.4A of current, that works out to about 1,350mA per COB (might be a little more or a little less on each COB). At that current, on a Vero-29B Gen 7, you can expect to see ~50.1V across each of them (10,740 lumens) which is fine for that driver. You’ll find that once you turn your voltage up to 50.1V, if you continue to turn the voltage knob, the voltage will not increase any further, as it’ll be in constant current mode at that point.

      For the V22, your best bet would be to go with the V22-C. At 1,350mA each, you’d see a voltage of 51.1V (10,515 lumens).

      The driver on ebay is definitely an “A” version, with built-in voltage and current control, correct?

  3. Hi LEDgardener,

    Thanks very much for your detailed, informed and prompt reply.

    I have been doing a little bit of research myself and i felt a little unsure of biting the bullet. I feel more confident now.

    The vero29 are about £20 and v22 £11 on Digikey. Looks like the v22 would do the job nicely.

    Any problem mixing 3000k and 4000k, 2 of each?

    Cheers

    ColinC

    • LEDGardener

      March 31, 2017 at 8:14 am

      Good stuff. There’s no problem with mixing color temperatures.

      One correction to my last comment – at a color temperature of 3500K and 80CRI (right between your 2 colors should be a good approximation) and a case temp of 55, you’d actually get about 11,035 lumens from the Vero 29B and 10,805 lumens from the V22-C.

      • Cheers again,
        No 3500k in v22 and 8 weeks lead time and min of 50 order for for 3500k vero29b at Digikey.
        £25 for Driver
        £44 for HS x4
        £44 for v22 x4
        £15 for Al frame, chains, wiring and hangers etc

        £128 for a 40,000+ lux growlight, not bad eh?

        Will that be ok for a 80x80x160cm tent?

        ColinC

        • LEDGardener

          March 31, 2017 at 2:43 pm

          Compared to what it’d cost you to buy a comparable pre-built light, heck yeah! Those 4 COBs at 1,400mA and higher will cover an 80×80 tent just fine.

  4. PS from ColinC

    Yes, it was the A version Hlg240h48a, i also seen a Clg150-48a on ebay, could you suggest what bridgelux cobs i could run with it, rated at 3.2a at up to 53v?

    ColinC

    • LEDGardener

      March 31, 2017 at 9:59 am

      That CLG test report showed a current of 4.4A. If you wanted to run 4 COBs you could run the same ones as above at ~1,100mA each. It’d be very efficient but you might want to run Veros harder than that to get more light out of them. If I were running 4x Vero29B, I’d probably give them 1750mA or so each.

      • Hi Led Gardener yet again.

        Could i run vero29b x2 in parallel on the Clg150h 48a at about 1.8a to 2.1a each on suitable heatsink then?

        ColinC

        • LEDGardener

          March 31, 2017 at 2:44 pm

          You bet. I think that would be a great match and you could even turn it down a bit if needed. Always nice to have overhead.

  5. Hi LED Gardener
    My led light is 30watt could I use 2 led Driver 15watt on series connection?

    • LEDGardener

      July 2, 2017 at 9:55 am

      I’ve never tried connecting power supplies in series – I spoke about it with an electrician friend of mine a long time ago and he had suggested that power supplies don’t like being run together like that but I can’t remember why. I’d keep it simple and find a supply that’s capable of doing it by itself. For that low of power, you could find an inexpensive one very easily.

  6. Hi there LED Gardner! Great articles, I just read all of them on Constant Voltage v.s. Constant Current, and drivers.

    I have a quick question about using a driver in a small cabinet light I am building. I bought 2 sun boards at 35.5v each, and want to power them at 2100ma. The meanwell driver option is ELG-150-C2100A. I would like to be able to use either one board or two boards depending on the need… but, the output voltage for that Constant Current driver is 36-72volts. So, my question is… If I use this driver with only one Sunboard, will it even turn on? (Because the board’s voltage is 35.5… slightly below 36v). Also, if it did work, would I be able to dim the one board, or would that be a challenge with that driver?

    Would I be better off getting a Constant Voltage driver, and running one or two boards parallel off that?

    I am so glad that your articles are here, and that a beginner such as myself can find such a quell of information on how to start! Thanks, and God Bless!

    • LEDGardener

      October 16, 2017 at 9:44 pm

      Hey Johnny! That driver will be fine for 1 or 2 of those boards. Even though it comes in a hair under the constant current range minimum voltage, that driver will still run it at 2100mA. 36V-72V is the constant current range for that driver which just means that you’re guaranteed to get the rated current of 2100mA between those voltages. However, you can usually get away with coming in a little below the minimum of that range and still get full power (although you can’t do the opposite and exceed the maximum without losing power quickly).

      • Wow, great! Do you think I would also be able to dim just the one board using that driver? Thanks again for the rapid response! Designing a light is a lot more fun than I thought it would be! lol.

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