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Mains dimming

Most LED fittings cannot be dimmed with a conventional mains (or “phase”) dimmer.

That’s because LEDs need a driver, and the great majority of drivers are not designed to operate with their supply being switched on and off for a variable period, twice in every mains cycle (which is how a mains dimmer works). The two diagrams below illustrate how leading edge and trailing edge mains dimming work, and most drivers will not function with either type of dimmer. A small number of our products are mains-dimmable (such as our Garland, Lowland, Oslo & Bergen downlights), but if mains dimming is not specifically mentioned in relation to one of our products then the fitting is not mains-dimmable.




Many LED fittings can be dimmed if a dimming driver and the appropriate dimming control signal is used.

/media/technical/library/hf-dimming.jpgMany fittings can be dimmed if the correct driver is used. To do this the driver is permanently connected to mains power but also receives a dimming control signal, instructing it to increase or lower the light output.

On receipt of a dimming control signal a driver will dim the fittings by use of pulse width modulation, amplitude modulation or sometimes a combination of them both.

Pulse width modulation (PWM) entails turning the LEDs on and off. The % “on” time is the duty cycle. The higher the duty cycle the brighter the light appears to be to the human eye. PWM has many advantages as a dimming method (efficiency, simplicity of implementation and low cost), but it has disadvantages too, one of which is the risk of discernible flicker and stroboscopic effects.

Amplitude modulation (AM) involves the gradual reduction of the drive current to bring about a reduction in light output. This method of dimming avoids the risk of visible flicker but it lacks the simplicity and efficiency of PWM.

Some driver manufacturers use a combination of PWM and AM to achieve the best possible performance.