Question about constant current LED driver and lower vf

Hey guys,

I’m in process of replacing my cobs with qbs and I’d like to reuse my driver’s.

I have HLG-120H-C1400a drivers.

I use these drivers in tents that are 2x2 foot.

I’d like to use 2 qb 120s in series on my driver. They will be 22 inches across which is cutting it close but should be very even coverage.

I’ll only drive them at 100-120 watts.

So now my issue… The vf on the 120s is only 24 volts.

The constant current region of my driver is 54-108v.

So I would be about 6 volts short of the minimum because 2 boards in series will be a voltage drop of only 48.

I emailed hlg and they said my driver will work, but didn’t give an explanation why… I tried to all but they didn’t clarify much.

I’m thinking if I add a 5 ohm resistor in series that will drop the voltage another 7 volts and I’ll be in range and I could then use my old driver. However… That seems a little crazy.

Not sure how much heat a 5 ohm resistor will add at 1.4 amps…

Any ideas here?

I’d also be interested in strip builds but those seem to require constant voltage drivers, which I do not have. Thanks for reading!


If HLG says your driver would work, I would go with it. Remember the fV they suggest is an optimal voltage. They should be able to handle a little higher fV to reach your minimum voltage of driver.

You can run strips off your driver so long as the can handle the current. For example, three of the SolStrip X2 (24V) can be run in series off that driver.


You will be fine. The driver will limit max current based on the pot setting and adjust voltage automatically. You are a little outside the range, but these drivers have a good wide safe range.

Dont use a resister.The drivers are expecting the load resistance to fall within a certain range and that resistr will push it well outside that range. Plus, it will run very hot. Heat loss is I squared time R, so 1.4A ^2 x 5 ohms = 9.8 watts.


@Larry3215 @ReikoX

Thanks for the info and advice guys, I appreciate it!

Still mulling over everything… Not sure if I’ll pull the trigger on the boards or continue looking at strips…

Psyched that I can reuse my driver though :+1: