LED Flickering When Dimmed: Causes and Solutions

If an LED luminaire flickers when dimmed, the dimmer and driver usually do not match: a phase-cut dimmer on a driver built for 0–10 V or DALI, a control signal the driver interprets differently, or a dimming level below the stable range. Other causes include interference on long control cables, ageing drivers and control loops that amplify each other. The fix almost always starts with the driver datasheet and suitable control.

Flicker, shimmer, hunting: what you see

Not every lighting problem is the same. An accurate description helps find the cause:

• Visible flicker: irregular brightness jumps, often at certain dimming levels.
• Shimmer: fast, regular brightness fluctuation that is barely visible directly but shows up during movement or as stripes through a phone camera (stroboscopic effect).
• Hunting: brightness slowly drifts up and down, typical of a badly tuned daylight control.
• Blinking or on-off cycling: the driver shuts down and restarts, for example due to overload, overtemperature or too low a signal.

The most common causes

Phase-cut dimmer on a driver without phase dimming

Leading-edge and trailing-edge dimmers cut away part of the mains voltage. That worked well with incandescent lamps, but LED drivers must be explicitly designed for phase dimming. Commercial drivers with a 0–10 V or DALI interface usually are not. If such a driver sits behind a phase dimmer, it flickers, hums or switches off. Remedy: remove the phase dimmer and dim via the intended control interface.

Control signal and driver do not match

There are differences between 1–10 V and 0–10 V too. A classic 1–10 V driver supplies the control current itself, which the controller pulls down; some 0–10 V systems expect an actively generated signal. If signal type, current or voltage range do not match, jumps or flicker occur at the low end. Applying a PWM signal to an analogue input or vice versa is also likely to cause unstable behaviour. Which interface your driver has is shown in Reading the LED driver label.

Dimming level below the stable range

Every driver has a minimum dimming level, usually given as a percentage in the datasheet. Below it, some drivers no longer regulate cleanly and the LEDs start to shimmer or flicker. If background lighting is set to a few per cent below this value, the problem appears exactly when nobody is in the room, for example in a corridor. The fix is simple: set background lighting and the minimum dimming level above the stable range.

Interference on the control cable

Analogue 0–10 V and 1–10 V cables transmit brightness as a voltage. If they are long and run parallel to mains cables, interference can couple in or voltage drop can distort the signal. This shows as slight flicker or different brightness in neighbouring luminaires. With DALI, an overloaded bus power supply or cable faults can cause dropouts.

Two control systems at once

If two controllers send commands to the same drivers, for example an old controller and a new sensor, they can overwrite each other. The light jumps back and forth between two values. Only clear responsibility helps here, see Retrofitting sensors to DALI luminaires.

Aged or overloaded drivers

Drivers age, especially at high ambient temperatures. If a luminaire also flickers at full brightness or after switching on, the driver is a candidate for replacement. The same applies if the driver is operated at the limit of its output range.

Hunting daylight control

If a daylight sensor measures part of the light from its own luminaire and the control is set too fast, brightness can oscillate. A better position, a suitable setpoint and smooth transitions help. Background in Daylight harvesting.

Taking a systematic approach

How proper sensor control avoids flicker

Many of these causes can be ruled out at the planning stage:

• Matching interface: sensors are matched to the driver, for 0–10 V drivers for example with sensors such as the IFS105SE or EFS106, for DALI drivers with sensors with a DALI connection.
• Short control paths: with luminaire-level lighting control the sensor sits directly in the luminaire. Long analogue control cables through the building disappear, and communication between luminaires is wireless.
• Dim-to-off instead of minimum: drivers with dim-to-off switch off cleanly instead of lingering at an unstable minimum. If the auxiliary output is missing, there are alternatives, see The 12 V auxiliary output on LED drivers.
• Sensible dimming levels: background lighting and dimming levels are set via app above the stable range and checked in the pilot installation.
• One control system: old dimmers and competing controllers are removed or clearly integrated.

This keeps the light steady while the savings still add up: as a guideline from our projects, sensors alone save more than 60 % of lighting electricity; with daylight and background lighting up to 75 % is possible.

How we can help

Our focus is retrofitting: we turn existing LED luminaires with dimmable 0-10 V or DALI drivers into smart lighting with sensors and app-based lighting management – without new luminaires and without new control cables. Details under Retrofit.

If your lighting already flickers today, we check drivers and control and take this into account when planning the sensors. Installation is usually carried out by us, or by your own electrical contractor if you prefer. Every project includes the AU-Manager, so occupancy detection, zoning and monitoring are included. Get in touch via our contact page.