Have you ever seen or used something like one of these? If I am reading correctly either one will allow the 24v driver to power the 36v COB's? Opinion?
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Those will work on paper, sort of. The thing with wiring drivers in series is that the voltage does go up. However the current from the bigger driver will smoke one of those little drivers. That's why the drivers I was bringing up wouldn't work, you would be running too much current through the smaller driver.
The cheapest option I can find is to get the HLG-320H-12 and wire it in series with the driver you have now. With this combo, the drivers are safe...but your cobs are not. If you turn both of those drivers all the way up, they will hit around 640W, and smoke your cobs.
But, in the future, you can use these same drivers and put in more cobs.
You're options are do that, or get the HLG-320H-36. If you are going to get a whole new driver, I'd go with something like an HLG-320H-C2100 driver that lets you wire those cobs in series.