Selection & sizing

Constant voltage vs constant current: how to choose

Tell from the luminaire datasheet in one step whether you need a CV or CC driver — output behaviour, wiring, the parameters that matter, what happens if you mix them up, and three common mistakes.

Last updated：2026-10-31

On this page

The one-line test
Comparison table
Sizing a CV driver
Sizing a CC driver
Three common mistakes
Field troubleshooting

Getting this wrong is not a small brightness difference — a CV supply on a CC luminaire burns it out, and a CC driver on a strip gives unpredictable light. The datasheet tells you which one you need in a single line.

The one-line test

Open the luminaire datasheet and look at what it asks to be fed:

It states 12V / 24V / 48V — a fixed voltage → you need a constant-voltage (CV) driver
It states 350mA / 700mA / 1050mA — a fixed current, usually followed by a voltage range → you need a constant-current (CC) driver

The logic runs the other way too: a voltage-only rating means the luminaire already limits its own current (the resistors on a strip, the driver IC on a module), so you just supply a stable voltage. A current rating means the driver has to do the limiting.

Comparison table

	Constant voltage (CV)	Constant current (CC)
What is regulated	Fixed voltage (12V/24V/48V); current follows the load	Fixed current (350mA–2000mA typical); voltage adapts within a window
How loads connect	In parallel; total current is the sum	In series; one current through the whole string
What to match	① Voltage must match exactly ② Wattage ≥ load × 1.2	① Current within ±5% ② String voltage inside the driver's window
Typical use	Strips, linear lighting, light boxes, modules, cabinet lights	Spotlights, downlights, track lights, street lights, high bays, horticultural
Typical failure	Too little wattage → thermal shutdown and restart cycling; long runs → dim far end	Current too high → accelerated lumen decay; window missed → no start or flicker
Multiple branches	Yes, up to the wattage limit	Generally no — current splits unevenly

Sizing a CV driver

On CV the voltage has to match the luminaire exactly (a 24V strip cannot run on 12V or 48V). That leaves one question: is the wattage enough.

Total load P = watts per metre × metres (14.4 W/m × 10 m = 144 W)
Driver wattage ≥ P × 1.2 (144 W → ≥ 173 W, so take the 200 W class)
Running a supply at continuous full load raises internal temperature and shortens life — the 20% is not waste

Sizing a CC driver

A CC driver nameplate always carries two numbers, for example 700mA / 30–42V:

Current must match the luminaire, typically within ±5%. LEDs specified at 700 mA run at 700 mA; 1050 mA accelerates lumen decay
Voltage window must contain the actual string voltage. For N LEDs in series, voltage ≈ N × forward voltage (about 3 V for white): 12 LEDs ≈ 36 V, inside 30–42 V; 16 LEDs ≈ 48 V, outside this driver's capability
Wattage check: P = current × actual voltage (0.7 A × 36 V ≈ 25 W), and the driver rating must exceed it

Three common mistakes

"A bigger power supply makes it brighter." No. On CV, brightness is set by the voltage; the wattage is only the ceiling on what it can deliver. A larger supply runs cooler and lasts longer, not brighter.
"I can run two strings off one CC driver." Not reliably. The current splits according to each string's I-V curve, so one runs bright and one dim — and if one string opens, the other takes the full current.
"A resistor turns a CV supply into a CC driver." It lights up, but every watt in the resistor becomes heat, efficiency drops, and the current tracks input voltage variation. It is a latent fault on site.

Field troubleshooting

Symptom	Most likely cause	One-step check
Far end of a strip visibly dimmer	Voltage drop on a long low-voltage run	Measure at the far end under full load; more than 5% below nominal confirms it
Runs, then goes dark and comes back	Overload or poor cooling tripping protection	Check case temperature; retest with a higher wattage class
CC luminaire dark, driver output at its top voltage	Open string, or string voltage outside the window	Measure open-circuit, then test the string section by section
Uneven brightness across one batch	Branch lengths differ too much on a CV system	Compare the far-end voltage of each branch

Do not fight voltage drop with thicker copper — once the answer passes 4 mm² it is cheaper to move from 12V to 24V or to feed the run from the middle. Run the numbers in the sizing calculator.

Not sure which one? Send us the voltage/current on the datasheet, the quantity and the dimming method — we will match it against 47 in-stock models. Contact →