Constant-Current vs Constant-Voltage LED Drivers

Every LED fixture needs a driver, and drivers come in two main output types: constant current and constant voltage. Matching the driver type to the load matters. This guide explains what each type does, where each is typically used, and how to check which one a fixture or module calls for. It leans on what three manufacturers say about LED drivers (a 2014 Lutron white paper, a Mean Well product-page note and a Signify product page), and it does not cover wiring procedures, which belong to the manufacturer's instructions and a qualified electrician.

What an LED driver does

Lutron's technical white paper on controlling LEDs puts it this way: "LED chips are inherently low-voltage devices that need additional electronic components to convert the line-voltage 120/220/277V power to the low voltage needed for the LEDs." That conversion is the driver's job. Lutron notes that driver outputs come in two kinds, citing examples of "constant voltage (usually 12V or 24V) or constant current (e.g. 350mA, 700mA or 1050mA)." (Lutron's paper is dated May 2014, so treat those values as examples, not a current standard.)

Constant voltage: a fixed voltage, current set by the load

Lutron says that "constant voltage drivers provide a fixed constant voltage to LED modules connected in parallel." The driver holds the voltage, and the load decides how much current it draws. Mean Well's example is a 5 V supply whose "output current varies from 0~5A based on the load condition."

Mean Well, a driver manufacturer, says forward current is what matters when driving an LED, and describes the drawback of a constant voltage source this way: "The drawback of using constant voltage source is that LEDs will not produce the same brightness, resulting in uneven reliability, decrease in luminance and lifetime." In the same passage it calls constant current "more preferable when it comes to driving high power LEDs." Mean Well also says its LPV, LPH-18 and LPL-18 series, which it considers constant voltage supplies, can be used for LED applications with an "LED constant current driver IC" or a "ballast resistor."

Mean Well's note points to a constant voltage supply feeding a load that has its own current control. Signify's Advance page for its Xitanium drivers, which support both constant voltage and constant current modes, lists constant voltage uses including "strip and bar lights or under-cabinet lighting, ambient lighting and low-bay and high-bay industrial lighting." Its page also describes the drivers as providing general illumination for outdoor applications, including LED signs and canopy lights. Which modules need which supply voltage is set by the module maker, not by a general rule.

Constant current: a fixed current, voltage follows the load

Lutron says that "constant current drivers provide a constant current, such as 700mA, to a specific LED module." Mean Well defines it as a supply whose "output current is maintained at a fixed value by means of feedback control."

The reason to regulate current is the nature of the LED. Mean Well explains that "due to its nonlinear V-I characteristic, high Vf manufacturing tolerance, and Vf sensitivity to temperature change, the most common types of voltage source used are limited to either constant voltage or constant current." Of the constant current approach it says its current "will remain at a constant level regardless of the shift in LED Vf," where Vf is the LED's forward voltage. Mean Well adds that when a constant current supply drives LEDs directly, the designer must make sure the forward voltages of the series-connected LEDs add up to enough to keep the supply in its constant current region. In Lutron's terms, a constant current driver is matched to a specific LED module.

Side by side

	Constant voltage	Constant current
Driver holds fixed	Voltage	Current
Load decides	Current drawn	Voltage required
Typical match (as the sources describe it)	Per Mean Well: supplies used with a current-limiting IC or ballast resistor; per Signify: strip, under-cabinet and similar lighting	Per Lutron: a specific LED module designed for that current
Rating you will see	Output volts (Lutron's examples: 12V or 24V)	Output milliamps (Lutron's examples: 350mA, 700mA, 1050mA)

They are not interchangeable

Lutron is blunt about this: "Constant current and constant voltage drivers are NOT interchangeable, and it is the electrical requirements and the design of the LED load that determines which driver is appropriate." Lutron adds that "it is the fixture manufacturer that chooses the driver type and configuration to match the electrical requirements of the LED module they chose to use in the fixture."

For replacement or retrofit work, that points to a simple rule: start from what the fixture or module manufacturer specifies, not from what looks like it fits.

Dual-mode and "C.V. + C.C." labels

The line is not always clean, and the two manufacturers describe different things. Mean Well writes that "MEAN WELL's constant current LED power supplies also possess the feature of 'C.V. + C.C.'. That means before reaching the 'constant current region', they can be operated within the 'constant voltage region' just like traditional power supplies." In that constant voltage region, Mean Well says, "system design must incorporate suitable 'LED drive IC' or 'series connected with a ballast resistor'." Signify separately says its Xitanium drivers "with both constant voltage (CV) and constant current (CC) mode are compatible with respective loads and allow the user to utilize the same driver for CV and CC applications."

Neither statement changes the match-the-load rule. The driver's own documentation should tell you which mode suits which load.

How to check which type you need

1. Read the load's documentation. The LED module, board or strip maker states what it is designed to run on, either a voltage or a current.
2. Read the driver's output rating. Lutron's examples are volts for constant voltage and milliamps for constant current. If the label says both, read the manufacturer's data sheet for the modes. Our spec sheet guide covers where to find these.
3. Match the type, then the value. The driver type has to match the load first; the output value then has to be within what the manufacturer allows.
4. Keep dimming separate. Whether a driver dims, and how, is a different question. See our 0-10V dimming guide.
5. Defer installation to the manufacturer. Wiring, connections and any changes to an installed system should follow the manufacturer's instructions and be done by a qualified electrician.

Talk to Rock about your LED driver match

Rock Lighting & Electric supplies lighting to contractors, specifiers and buyers. For fixture selection across commercial projects, see our commercial LED light fixtures guide. Create an account, request a quote, or contact us with the fixture or module you are matching.

Frequently asked questions

What is the difference between a constant current and a constant voltage LED driver? A constant voltage driver holds a fixed output voltage; Lutron says it supplies "LED modules connected in parallel." A constant current driver holds a fixed current; Lutron says it supplies "a specific LED module." Lutron states the two are not interchangeable.

Can I swap a constant voltage driver for a constant current one? Not on the basis of fit alone. Lutron says: "Constant current and constant voltage drivers are NOT interchangeable, and it is the electrical requirements and the design of the LED load that determines which driver is appropriate." Follow the fixture or module manufacturer's specification, and have a qualified electrician do the work.

Which type does an LED strip use? Check the strip manufacturer's documentation. Signify lists "strip and bar lights" among the constant voltage uses for its Xitanium drivers, and Mean Well says its LPV, LPH-18 and LPL-18 series can be used for LED applications with an "LED constant current driver IC" or a "ballast resistor." That does not mean every strip is constant voltage.

Why is constant current used for LEDs? Mean Well explains that LEDs have a nonlinear V-I characteristic, a high manufacturing tolerance in forward voltage (Vf) and Vf sensitivity to temperature. It says constant current is "more preferable when it comes to driving high power LEDs," and that a constant current source's current "will remain at a constant level regardless of the shift in LED Vf."

Who decides which driver type a fixture needs? Lutron says "it is the fixture manufacturer that chooses the driver type and configuration to match the electrical requirements of the LED module they chose to use in the fixture." For replacement drivers, use the manufacturer's specification.

Can one driver cover both types? It depends on the product. Mean Well says its constant current supplies also have a "C.V. + C.C." feature, with a constant voltage region before the constant current region, and that in that region the system design must incorporate an "LED drive IC" or a ballast resistor. Signify says its Xitanium drivers "with both constant voltage (CV) and constant current (CC) mode" allow "the same driver for CV and CC applications." Read the driver's data sheet for which mode suits which load.

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