LEDs don't behave like ordinary light bulbs electrically. Their brightness depends on current, not voltage, and their forward voltage shifts with temperature and manufacturing tolerance. Drive an LED with a fixed voltage and the current running through it can climb as the LED heats up, which raises the current further, which raises the heat further. That feedback loop is called thermal runaway, and it's why LEDs are driven by current, not voltage, in almost every real fixture.
How a constant current driver actually works. Instead of holding voltage steady and letting current wander, a constant current driver holds current steady and lets voltage adjust to whatever the LED load needs. The driver constantly monitors the output current and adjusts its own output voltage to keep that current fixed, regardless of how the LED's forward voltage drifts with temperature or age. The result is stable brightness and a predictable LED lifespan instead of a slow climb toward failure.
Sizing a driver to a fixture. Two numbers matter: the driver's rated output current and its rated wattage. The LED array has its own rated forward current, and the driver's output current needs to match it, not exceed it. Too high, and the LEDs run hotter and degrade faster than their rated lifespan. Too low, and you lose brightness the fixture was designed to deliver. On wattage, the driver's rated output should sit at or slightly above the total wattage of the connected LED load, with some headroom rather than running the driver at its absolute ceiling continuously.
Many constant current drivers, including Rez-ti's own downlight and spotlight driver, offer multiple selectable current steps rather than a single fixed output (in that case, four steps: 150mA, 180mA, 250mA, 280mA, selected via an onboard DIP switch). That's what makes one driver usable across a range of fixture wattages instead of needing a separate SKU for every load.
Dimming a constant current driver. Dimming happens by rapidly switching the output on and off faster than the eye can perceive (PWM dimming) or by directly adjusting the current level within a safe range, rather than by lowering voltage the way an old incandescent dimmer worked. On a Matter-connected driver, this is controlled through the Level Control cluster from the app or automation, typically across a 1 to 100 percent range, rather than a physical dimmer switch. This is also why a constant current driver dims more evenly at low brightness than a voltage-based dimmer retrofitted onto LEDs, there's no flicker from voltage sag near the bottom of the range.
Wiring basics. A tunable white (CCT) constant current driver typically runs three output wires rather than two: a positive rail, and two independently addressable channels, usually labeled something like LED+, LED-Y (warm) and LED-W (cool). Blending the two channels' current independently is what lets a fixture shift color temperature, not just brightness. Input side is usually universal 100 to 240V AC, and a driver rated SELV (Safety Extra-Low Voltage) means the output side is safe to touch under normal conditions, which matters for anyone doing the physical install.
FAQ
What's the difference between constant current and constant voltage drivers?
A constant voltage driver holds output voltage fixed and expects the fixture itself (or an onboard resistor) to limit current, common in LED strips built with an internal resistor per segment. A constant current driver holds current fixed directly, which is standard for single high-power LED arrays like downlights and spotlights, where there's no internal current-limiting built into the LED itself.
Can I use a higher-current driver than the LED is rated for, if I just want it brighter?
No. Exceeding the LED's rated forward current shortens its lifespan significantly and risks thermal runaway. If more brightness is needed, that's a fixture selection question, not a driver overdrive question.
Why does the driver have multiple selectable current settings instead of one fixed output?
It lets one driver SKU cover a range of fixture wattages instead of stocking a separate driver for every load, which is standard practice for both OEM buyers spec'ing a fixture line and repair/retrofit use cases.