Constant Current vs Constant Voltage LED Drivers: How to Choose

Q: What happens if the LED string voltage falls outside the driver output window?

A constant current driver can only hold its rated current while the load voltage stays inside its stated output window. Below the minimum the driver cannot reach rated current and the LEDs run dim; above the maximum the driver reaches its voltage limit and the current can no longer be regulated, so output becomes uncontrolled. Check the window against the string voltage at the temperature extremes the luminaire will see, not only at room temperature.

Q: Is a constant current driver more efficient than a constant voltage one?

Driver efficiency depends on the topology, component quality and operating point, not on the regulation mode by itself. The larger efficiency differences in a luminaire usually come from how much voltage headroom is burned in current-limiting components and from optical efficiency, so driver efficiency should be compared from measured data sheets at the actual load rather than from the regulation principle.

Q: Do I need a constant current driver for dimming?

Both types can be dimmed, but the interface must match. Constant current drivers commonly offer 0-10 V, DALI or PWM dimming inputs that scale the regulated current, while constant voltage drivers often use PWM on the output bus or a dimmable mains interface. The dimming interface is part of the driver's certified construction: ANSI/UL 8750 Supplement SF specifically addresses drivers with integral wired controls such as 0-10 V and DALI.

XineLam · Zhongshan City Ruixian Electronics Factory · Published 17 September 2026

Almost every LED failure that is blamed on the LEDs turns out to be a driver mismatch. The two ways of regulating a driver output — constant current and constant voltage — are not competing quality levels; they suit different loads. Choosing wrongly produces dim output, flicker, thermal runaway or outright failure, and the labels that distinguish them are printed on every driver. This guide explains what each type regulates, how to read the label, and how to check the arithmetic before a luminaire is built.

Illustration of a rectangular LED grow light panel mounted on a rigid suspension frame with a plain control-gear enclosure bolted to its top surface above a lettuce and basil canopy — **Figure 1.** Illustration: an LED panel luminaire on a suspension frame, with the control gear mounted on the top surface of the housing. The driver is a structural part of the luminaire, not an accessory.

Why an LED needs a driver at all

A light-emitting diode is a diode. Its current rises steeply and non-linearly with the voltage across it: for a white LED, a forward voltage of roughly 2.7 to 3.4 V at rated current is typical, and a shift of a few tens of millivolts around that point changes the current by a large factor. Two consequences follow.

Mains voltage cannot be applied directly. A driver converts the AC supply into a regulated DC output suited to the load.
Something must set the current. If a fixed voltage is applied and the LED heats up, its forward voltage falls for the same current, which draws more current, which raises the temperature further. Left uncontrolled, this is thermal runaway. Something in the circuit has to break that loop.

There are two established ways to break it: regulate the current, or regulate the voltage and make the load limit its own current.

The two regulation strategies

Table 1. Constant current and constant voltage LED drivers compared.
Dimension	Constant current (CC)	Constant voltage (CV)
Regulated quantity	Output current, held at a set value	Output voltage, held at a set value
Typical label format	“700 mA” plus an output voltage window, e.g. 20–40 V DC	“24 V DC” plus a maximum output current, e.g. 2.5 A
What sets the LED current	The driver	The load’s own resistors or current regulators
Output voltage behaviour	Floats within the stated window as the load voltage changes with temperature	Held constant; current varies with load
Typical load	LED strings, arrays and modules wired in series	Constant-voltage strips, tapes and modules with onboard current limiting
Matching rule	Current must equal the module rating; the string voltage must sit inside the window at all operating temperatures	Bus voltage must equal the module rating; total current draw must stay within the driver maximum
Main failure mode if misapplied	String voltage outside the window: dim output or loss of regulation	Unlimited current into a load that does not limit its own: overheating and failure
Example specification	700 mA, 20–40 V DC, 21 W at 30 V	24 V DC, 2.5 A maximum, 60 W

Reading a driver label

A constant current label always carries two numbers, and both matter. Take a driver marked 700 mA, 20–40 V DC. The 700 mA is the regulated current; the 20–40 V is the range over which the driver can hold it. Common regulated currents across the industry include 350 mA, 500 mA, 700 mA, 1050 mA and 1400 mA. A constant voltage label carries the regulated voltage and a maximum current, for example 24 V DC, 2.5 A, which is a 60 W driver.

The arithmetic for a constant current design is straightforward. If the string is built from white LEDs at roughly 3 V each:

7 LEDs × 3 V ≈ 21 V | 13 LEDs × 3 V ≈ 39 V

A seven to thirteen LED string therefore sits inside a 20–40 V window at room temperature, and the driver will hold 700 mA through it. At 30 V and 700 mA the load is about 21 W.

Do not size from room-temperature numbers alone. Forward voltage falls as the junction warms, so the string voltage at operating temperature is lower than at the bench. If the cold string voltage is already close to the bottom of the window, a warm luminaire may fall below the driver’s minimum and run dim. Leave margin at both ends, and check the driver data sheet for the minimum load it can regulate.

Photograph of a white LED grow light panel fixture with an array of diodes and a control-gear enclosure with a rotary dimming control and cable entry mounted on one edge — **Figure 2.** Photograph: XineLam. A panel luminaire with the control gear mounted on the fixture edge, including the manual dimming control. The dimming interface is part of the driver, and the safety requirements for wired control inputs apply to it.

Constant power drivers: the third option

A third category regulates output power rather than current or voltage, adjusting the two against each other. These are chosen where a manufacturer needs one part number to cover several module variants of similar wattage but different voltage and current combinations. They do not remove the need to check the operating window: the load still has to fall inside the range the driver can regulate, and the same rule about minimum load applies.

Dimming interfaces

Dimming is where driver choice and control design meet, and where an otherwise sound selection goes wrong. The interface must exist on the driver, not be added afterwards by an external switch on the mains side, unless the product was designed and certified for that method.

0–10 V — an analogue control voltage on a separate pair of wires; widely used in commercial installations.
DALI — a digital addressable bus that allows individual or grouped control.
PWM — a pulse-width signal, often used at module level.
Phase cut (TRIAC) — dimming at the mains side, workable with drivers designed for it and prone to problems with those that are not.
Resistance or potentiometer dimming — a simple external control, common on fixtures with a local knob.

Any wired control input changes how the driver must be constructed. The ANSI/UL 8750 standard for LED equipment addresses drivers with integral wired controls — 0–10 V and DALI among them — in Supplement SF, whose clause SF3.1 requires the control circuit to be separated, or isolated, from the other circuits of the driver. An exception agreed in April 2020 permits an alternative design where the output power circuit is Class 2 and the control circuit is either evaluated as a Class 2 circuit or intended for connection to an external Class 2 supply. UL extended the effective date of that requirement set by six months, to 2 November 2020.

Standards and qualification programmes

Two families of requirements sit behind the label. For the control gear itself, IEC 61347-2-13 covers electronic control gear for LED modules; the standard describes control gear designed to provide either constant voltage or constant current, at safety extra-low voltage or at higher voltages. In North America, UL 8750 is the safety standard for LED equipment used in lighting products and covers modules, arrays, controllers, drivers and power supplies, including electrical insulation, grounding, overcurrent protection, temperature management, fire resistance and mechanical integrity. It is applied together with the luminaire standard UL 1598 when a complete fixture is certified.

Performance requirements are separate from safety requirements. The DesignLights Consortium’s Horticultural Technical Requirements V4.0, effective 18 April 2025, qualifies complete LED fixtures or modules and uses ANSI/IES LM-79-19, LM-80 and LM-84 testing, including in-situ measurement tests. Its headline change was raising the photosynthetic photon efficacy threshold to a minimum of 2.5 µmol per joule, an 8.7 per cent increase over V3.0 and more than 45 per cent above the most efficacious non-LED option, the 1000 W double-ended high pressure sodium luminaire. Driver efficiency is part of the input power that determines whether a luminaire clears that threshold at all.

Common mistakes

Matching voltage to a constant current driver. The window is a range, not a setpoint; matching the nominal figure alone ignores both ends of it.
Running a bare LED string on a constant voltage supply. Without onboard current limiting the current is set by the diode curve, and that curve moves with temperature.
Ignoring the minimum load. Some drivers cannot regulate below a stated load, which shows up as flicker or dimming at low settings.
Sizing for the cold string only. Forward voltage falls with temperature; the warm string may leave the window.
Using an external mains dimmer on a non-dimmable driver. If the driver is not designed for phase cut, the result is flicker, audible noise or premature failure.
Overlooking inrush. Large installations with many drivers need the inrush current considered at the circuit-breaker level.
Treating a certificate as a performance claim. A safety listing says the driver is safe, not that the luminaire hits an efficacy or light-output target.

Frequently asked questions

Can I drive an LED module with a constant voltage driver?

Only if the module limits its own current. Constant voltage drivers suit LED strips, tapes and modules that carry onboard current-limiting resistors or their own regulators, and assemblies designed for a fixed bus voltage. A bare LED string with no current limiting will draw whatever current the applied voltage produces, which is why constant current drive is the normal choice for it.

What happens if the LED string voltage falls outside the driver output window?

A constant current driver holds its rated current only while the load voltage stays inside its stated window. Below the minimum it cannot reach rated current and the LEDs run dim; above the maximum it hits its voltage limit and current can no longer be regulated. Check the window against the string voltage at the temperature extremes the luminaire will actually see, not only at room temperature.

Is a constant current driver more efficient than a constant voltage one?

Driver efficiency depends on topology, component quality and operating point rather than on the regulation mode itself. The larger losses in a luminaire usually come from voltage headroom burned in current-limiting components and from optical efficiency, so compare measured data-sheet figures at the actual load instead of reasoning from the regulation principle.

Do I need a constant current driver for dimming?

Both types can be dimmed, provided the interface matches the driver. Constant current drivers commonly accept 0–10 V, DALI or PWM inputs that scale the regulated current, while constant voltage drivers more often use PWM on the output bus or a dimmable mains interface. The interface is part of the driver’s certified construction: ANSI/UL 8750 Supplement SF addresses drivers with integral wired controls such as 0–10 V and DALI.

References

IECEE, IEC 61347-2-13:2014 — Lamp control gear, Part 2-13: Particular requirements for d.c. or a.c. supplied electronic control gear for LED modules. https://www.iecee.org/certification/iec-standards/iec-61347-2-132014
Intertek, UL 8750 — LED Equipment Safety Standard (scope, covered components and tested areas). https://www.intertek.com/lighting/standards/ul-8750/
UL Solutions, Updates to LED Equipment for Use in Lighting Products — UL 8750: What you need to know (Supplement SF, clause SF3.1, Class 2 exception, effective date revision). https://www.ul.com/news/updates-led-equipment-use-lighting-products-ul-8750-what-you-need-know
DesignLights Consortium, Horticultural Technical Requirements V4.0 (effective 18 April 2025; PPE threshold of 2.5 µmol per joule; testing per ANSI/IES LM-79-19, LM-80, LM-84 and in-situ measurement tests). https://designlights.org/our-work/horticultural-lighting/technical-requirements/hort-v4-0/

About the publisher

Zhongshan City Ruixian Electronics Factory (XineLam) has 17 years of experience designing and manufacturing LED lighting, and holds 300+ patents in China and internationally. The company builds LED luminaires, drivers and grow light modules. This article is written as technical reference material for specifiers, integrators and growers, not as a product brochure.