How to choose an LED driver: constant current vs constant voltage

Short answer: There are two kinds of LED driver. A constant current (CC) driver supplies a fixed current (e.g. 350 mA, 700 mA) and its output voltage varies within a range to suit the LEDs; downlights, spotlights, panels and the LED boards inside luminaires work this way. A constant voltage (CV) driver supplies a fixed voltage (12 V, 24 V, 48 V) and the current depends on the load; LED strips, LED bars and magnetic track systems work this way. To choose the right driver, match the type first, then the current or voltage, then the wattage; the dimming method, location and inrush current complete the choice.

Constant current vs constant voltage

An LED is a current-driven device: the current through it decides how much light it gives and how hot it gets. LEDs are therefore used either with a driver that regulates current directly, or in modules that limit current themselves (with resistors or circuitry).

• Constant current (CC): the label shows a current such as 350 mA, 500 mA, 700 mA or 1050 mA and a voltage range such as 9–42 V. LEDs are usually wired in series, so each gets the same current.
• Constant voltage (CV): the label shows 12 V DC, 24 V DC or 48 V DC and a maximum wattage. Loads are wired in parallel; each section of an LED strip limits its own current.

The two are not interchangeable. An LED board connected straight to a constant voltage driver can draw excessive current and fail quickly; an LED strip on a constant current driver either won't light or will behave incorrectly.

Matching a constant current driver

1. Current: the driver's output current must equal the current on the LED board or luminaire label. Feeding a 700 mA board with 1050 mA shortens its life; 350 mA dims it.
2. Voltage range: the total forward voltage of the LEDs must fall inside the driver's output range. Ten LEDs in series at roughly 3 V each need about 30 V, so a 9–42 V driver suits them and a 3–12 V driver does not.
3. Power: current × voltage must not exceed the driver's maximum (0.7 A × 30 V ≈ 21 W calls for a 25 W driver or larger).

On drivers with adjustable current (DIP switches or programming), set the current before installation. When replacing a driver in an existing fitting, the old driver's label is the most reliable reference: choose one with the same current and range.

Matching a constant voltage driver

The voltage must be the same: a 24 V strip runs on a 24 V driver. Size the wattage at the total load plus about 20% headroom; running a driver at full load all the time raises its temperature and failure risk. Watts per metre, voltage drop on long runs and multiple feed points are covered in our LED strip power supply sizing guide, so we won't repeat them here.

Magnetic track systems also run on constant voltage: on the Lamptime magnetic track the driver converts mains to 48 V DC and is sized for the total wattage of all fittings on the track. Lamptime offers 150 W and 250 W drivers for this system; long, busy runs use more than one driver and power feed.

Dimming: DALI, 0–10 V, phase-cut and PWM

If you want to dim the light, the driver must be chosen for it; a dimmer on a non-dimmable driver causes flicker and failures.

• DALI: digital and addressable; each fitting can be dimmed individually and scenes can be set. Common in offices, hotels and shops. If colour temperature is also adjusted, a DALI DT8 driver is needed.
• 1–10 V / 0–10 V: analogue dimming over a separate two-core control line; simple and widespread.
• Phase-cut (triac, leading or trailing edge): works with an existing wall dimmer; the driver label must state that it is compatible with that dimmer type.
• PWM: used with LED strips via a controller placed between the driver and the strip.

Mixing dimmer and driver brands on one circuit is the most common cause of flicker. Many Lamptime luminaires and strip drivers are available with DALI; it appears as a control option on the product page.

Efficiency, power factor and lifetime

• Efficiency: how much of the power drawn from the mains reaches the LEDs; the rest becomes heat. A more efficient driver runs cooler.
• Power factor (PF): 0.9 or above is considered good. Many low-PF drivers together draw more current from the supply, which matters on larger projects.
• Flicker: a good driver has a smooth output that does not cause banding on camera or eye strain.
• Temperature: heat is what shortens a driver's life most. The label shows a tc point and an ambient limit; do not bury drivers in sealed, unventilated ceiling voids or hot lofts.

Location and IP rating

Indoors in a dry space an IP20 driver is enough. Where moisture is possible, such as bathrooms or under kitchen cabinets, and outdoors, use an IP65–IP67 driver or house the driver in a protected enclosure; cable entries need the same protection. See our IP ratings guide. Place the driver where it can be reached for maintenance.

Inrush current: how many drivers per breaker?

LED drivers draw a very short but high inrush current when switched on. With many drivers on one breaker, the breaker can trip at switch-on even though the total wattage is low. Driver manufacturers publish a table in the datasheet showing the maximum number of drivers per 10 A and 16 A B- and C-curve breaker; size circuits from that table. Remedies: split the circuits, use a C-curve breaker where permitted, or add an inrush current limiter. For breaker sizing see home circuit breakers and the consumer unit; for RCD nuisance tripping see RCD types, where we explain why type A is recommended.

Signs of a failing driver

• The light flickers, blinks or comes on several seconds after switching.
• It runs for a while, switches off and comes back once cool (thermal protection).
• The LEDs glow faintly when switched off (a switch breaking the neutral or induced voltage on long cables can also cause this).
• The driver buzzes, gets very hot or smells.
• It flickers when dimmed (dimmer and driver incompatible).

Measure the driver output first: the rated voltage on a CV driver, or the current under load on a CC driver. If there is no output or it fluctuates, replace the driver; if the output is correct, the problem is in the LED board or wiring. Measurements and mains connections should be made by a qualified electrician.

Checklist

1. Read the LED board or strip label: does it give mA (constant current) or V (constant voltage)?
2. Match the current or voltage exactly.
3. Calculate the wattage and leave headroom.
4. Choose the dimming method (none, DALI, 1–10 V, phase-cut).
5. Check IP rating and temperature limits for the location.
6. Check the number of drivers per breaker against the manufacturer's table.

In Bursa, ENGN helps choose drivers and luminaires for lighting projects and supplies Lamptime products to order.

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