Constant Voltage Thermal Runaway Test

After playing with my new HLG-100H-36A constant voltage driver for awhile, I conducted a little experiment this weekend to see if I could get either of my Vero 18 COBs to go into thermal runaway. Thermal runaway can happen in constant voltage systems where the current is allowed to vary, while the voltage is held steady. As the COB LEDs heat up, their properties change, and this causes them to draw more current, thereby heating them up further and drawing even more current. Eventually, the LED can destroy itself due to this cycle of drawing more current and heating.

Quite often, you’ll see recommendations to add a resistor in series with your LED if you’re using a constant voltage driver. I wanted to see if I could get away with skipping these resistors and simply running my COBs hooked up directly to the driver.

I set my voltage to 35V and then hooked up my COBs one at a time so I could easily monitor their current with my meter.

Setting a constant voltage of 35V.

VERO 18 #1

When I first turned COB #1 on, it was drawing about 1260mA of current.

After an hour, the current draw had risen about 40mA. It remained remarkably stable at around 1300mA over the course of the 4 hours I tested it.

After 4 hours of run time, the COB was still holding stable at 1,300mA of current draw. Nice!

VERO 18 #2

I switched to COB #2 to assess its stability as well. It started a little higher than the first COB at 1,364mA of draw during the first few seconds of turning it on.

After running for an hour, current draw had risen about 65mA.

Just like the first COB, current draw stabilized after quickly climbing to its peak of about ~1,430mA, and it maintained this level with no sign of increasing.


Prior to doing this test, I had expected the current to continue to creep up and up, the longer the light ran. I was a bit surprised to see that it leveled out so quickly and maintained such a consistent level after that point. I think there’s a chance that results could vary if the COBs were being run harder and there was more heat being produced, but at 35V and 1,300-1400mA of current, I was driving these Vero 18 COBs right at their recommended levels. I intend to test again at higher voltages to see if that makes a difference.

After seeing how these COBs worked on this driver, I’m much more optimistic about using constant voltage drivers. I think constant current models are still the way to go, but if I was able to get a really good deal on a CV driver instead, I’d probably snap it up and go that route instead. The added benefit of the system being low voltage makes it quite a bit safer too – it’s a lot harder to shock yourself with 36V than 100V+.


  1. Great info thank you for doing this.

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