A 9 to 48 V constant-current driver does not hold regulation across the whole span. String voltage peaks cold and bottoms out hot, and crossing either end fails differently. Cold and hot string voltage formulas, series count limits, and three worked examples.
A constant-current driver labelled 9 to 48 V does not regulate across that entire span. It holds the set current only inside the range where its output stage can work, and the two ends of that range fail in completely different ways. Sizing the string is arithmetic: the cold state sets the upper limit, the hot state sets the lower one, and power has to pass a third check.
The one-sentence rule: the window clamps your string count, and cold sets the ceiling
Upper limit: string voltage is highest when cold (Vf falls as temperature rises), so cold string voltage must stay at or below 0.9 x the window maximum
Lower limit: string voltage is lowest when hot, so hot string voltage must sit at or above 1.1 x the window minimum
Power: being inside the window is not enough; output voltage x total current must stay at or below 0.8 x rated power
The three examples below all use a constant-current unit of the H19 class: 9 to 48 V output, 10 to 52 W, eight current steps set by DIP switch.
Crossing either end looks completely different
Direction
What triggers it
What you see on site
How to confirm
Hitting the ceiling (overvoltage)
High cold Vf plus too many LEDs in series
Flashes once at switch-on and dies, restarts repeatedly; clusters in winter or early morning
Power up from cold and measure the output; a transient spike near the window ceiling confirms it
Dropping under the floor
Too few LEDs in series
Brightness drifts with temperature, drops out at low dimming, faint noise from the driver
Measure steady-state output after 30 minutes; below the window floor confirms it
Inside the window but over power
Voltage x current exceeds rating
Output current is folded back, brightness is short of target
Clamp the current; measured value clearly below the DIP setting
The first row is the one most often misread as a failed driver. The driver is protecting itself; the wrong parameter is the string count.
Three worked examples (white 2835, nominal Vf 3.0 V at 25 degrees C, bin spread 0.1 V per LED)
Example 1: ten in series, works
Hot (junction about 85 degrees C): Vf about 3.0 minus 0.002 x 60 = 2.88 V per LED, string 28.8 V
Cold (starting at 0 degrees C): Vf about 3.0 plus 0.002 x 25 plus 0.1 bin = 3.15 V per LED, string 31.5 V
31.5 V sits inside the 9 to 48 V span; at the 300 mA step that is 31.5 x 0.3 = 9.5 W, about 18 percent load
Example 2: sixteen in series to save a driver, out of bounds
Cold string voltage = 16 x 3.15 = 50.4 V, above 48 V
Working backwards: N_max = 48 x 0.9 / 3.15 = 13.7, so 13 LEDs maximum
Dividing 48 by the nominal 3.0 gives 16, which is exactly the trap - the three LEDs you must give back are eaten by the cold state and the bin spread
Example 3: twelve per string, three strings in parallel, works but needs ballasting
Cold string 12 x 3.15 = 37.8 V, hot 12 x 2.88 = 34.6 V, both inside the window
300 mA per string, three strings = 900 mA, so dial the 900 mA step
Power 34.6 x 0.9 = 31.1 W, sixty percent of the 52 W rating
The risk is the parallel connection: Vf relates to current exponentially, so a 0.1 V difference between two strings can split the current by 20 to 40 percent - one string runs visibly brighter and ages first
Fix it with a 1 to 2 ohm ballast resistor per string, or require one Vf bin from the board supplier
Ten and twelve LEDs fit the same driver; sixteen only fail once cold - the ceiling must be set from cold Vf
Formulas to apply directly
Vf_cold = Vf_25 + 0.002 x (25 - T_cold) + dVf_bin cold forward voltage per LED
Vf_hot = Vf_25 - 0.002 x (T_j - 25) hot forward voltage per LED
N_max = floor(0.9 x V_win_max / Vf_cold) series ceiling, set cold
N_min = ceil (1.1 x V_win_min / Vf_hot) series floor, set hot
n_par = floor(0.8 x I_rated / I_string) parallel strings allowed
P_ok = V_hot x (n_par x I_string) < 0.8 x P_rated power check
Vf_25 nominal forward voltage at 25 degrees C
0.002 temperature coefficient, about -2 mV per degree C (order of magnitude, use the LED datasheet)
dVf_bin bin spread, use 0.1 V; zero if one bin is specified
Do not reverse the order: ceiling from cold, then floor from hot, then power. Size from the hot state and winter will always find you.
Five frequent mistakes
Using nominal Vf: 48 / 3.0 gives 16, but cold and bin spread leave you 13
Changing LED supplier without recounting: bin centres can differ by 0.1 to 0.2 V per LED, which is 1 to 2 V over ten
Paralleling strings bare: without measuring each string, one ages first and the luminaire goes patchy
Raising the current step without checking power: 48 V at the 1.05 A step is 50.4 W, close to the 52 W limit, and fold-back kicks in
Treating the window as a constant-voltage rail: on a constant-current driver the output voltage is a result, not a setting
Acceptance checklist
Measure the output transient at cold switch-on: peak at or below 0.9 x the ceiling (43 V on a 48 V unit)
Measure steady state after 30 minutes: at or above 1.1 x the floor (10 V on a 9 V unit)
Clamp each string: spread between parallel strings 10 percent or less, add ballast resistors if not
Check total current against the DIP setting: clearly low means the unit is folding back
Compute output power: steady-state voltage x total current at or below 0.8 x rated
Dim down to 10 percent: no dropout and no flicker means the low end is still inside the window
Constant-current unit H19 (9 to 48 V, 10 to 52 W, eight switchable current steps) suits downlights and spotlights where the LED count is fixed. If the load is self-limited, such as strip or modules, use a constant-voltage unit instead - H01, K01 or S01 - and the reasoning is in constant voltage versus constant current. Current and power conversion runs in the LED driver sizing calculator, and why the 0.8 factor shapes case temperature and life is in how much power headroom.
The 0.002 V per degree C coefficient and the 0.1 V bin spread are typical orders of magnitude; take the real values from the LED datasheet. Any change to the electrical installation must follow local rules and the latest requirements of the certifying body.
Send us the LED Vf, LEDs per string, number of parallel strings and target current. We will check whether the string voltage sits inside the window, whether you need ballast resistors, and which current step to dial in. Contact →