Constant-current and constant-voltage LED drivers

An LED is driven by current, not voltage. Its light output follows the current through it, and a small change in voltage can cause a large change in current. Every LED installation therefore needs something that controls current. The question is where that control sits: in the driver, or on the LED board. That is the difference between constant-current and constant-voltage drivers.

Constant-current drivers

A constant-current (CC) driver regulates its output current to a fixed value, for example 350 mA, 700 mA or 1,050 mA. The output voltage then settles wherever the LED load needs it, within the driver's voltage window. LED modules for downlights, panels, high-bays and streetlights are usually designed for constant-current drive.

Matching a CC driver

• Output current: must match the current the LED module is designed for. Typical driver currents run from about 0.35 A to 3.5 A depending on the module.
• Voltage window: the forward voltage of the LED string, at the operating temperature, must fall inside the driver's output voltage range. More LEDs in series need a higher voltage. Typical CC driver windows fall within about 9–56 V DC.
• Power: the product of current and string voltage, with some reserve.

Constant-current drive gives the most direct control of light output and LED temperature, and it is the usual choice for luminaires built around a defined LED module.

Constant-voltage drivers

A constant-voltage (CV) driver, or CV power supply for LED loads, regulates its output voltage, typically 12 V, 24 V or 48 V DC. The LED load must then control its own current, usually with resistors or small regulator circuits on the LED strip or module.

Where CV fits

• LED strips and linear products sold in cut-to-length form, where the number of LEDs varies with the installed length.
• Installations with several LED loads connected in parallel to one supply.
• Signage and decorative lighting where loads are added or removed.

Matching a CV supply

• Voltage: must match the strip or module rating exactly.
• Power: the sum of all connected loads, with reserve, commonly around 20%.
• Voltage drop: long runs of strip lose voltage along their length, so longer installations are fed from both ends or from several points.

Comparing the two

Dimming

Both types can be dimmed, but the method must match the control system:

• 0–10 V or 1–10 V analogue control, common in commercial lighting.
• DALI, a digital addressable protocol for building lighting control.
• Phase-cut (leading or trailing edge), for retrofit with wall dimmers.
• PWM, switching the output on and off rapidly, often used with CV strips.

For constant-current drivers, dimming can reduce the current directly or chop it at high frequency. The choice affects flicker at low levels, which matters in offices, studios and spaces with cameras.

Other points to check

1. Input range: typically 100–240 V AC, 50/60 Hz, with protection against surges on the supply.
2. Power factor and harmonics: important where many drivers share one supply.
3. Operating temperature: the driver's case temperature limit and the ambient inside the luminaire decide its life.
4. Enclosure: sealed drivers for outdoor or damp locations; built-in drivers for mounting inside a luminaire.
5. Safety and EMC: the applicable standards for the market, such as IS 15885 for LED control gear safety in India.

Common matching mistakes

• Connecting a constant-voltage strip to a constant-current driver, or a CC module to a CV supply without its own current control. Either can overdrive the LEDs.
• Choosing a CC driver whose voltage window does not cover the string voltage when the LEDs are cold, when forward voltage is highest.
• Sizing a CV supply for the strip as first installed, with no reserve for the extra length added later.

Choosing

If the luminaire is built around a specific LED module with a stated current, use a constant-current driver matched to that current and voltage. If the load is a strip or a set of parallel loads rated at a fixed voltage, use a constant-voltage supply with enough power reserve. If both apply in one installation, both types can be used, each on its own circuit.

Where to go next

• LED Drivers: constant-current and constant-voltage drivers.
• CC/CV & Custom Power Supplies: power supplies for OEM products.
• Constant-current and constant-voltage compared
• Discuss your requirement: share the LED module data and the control method.