Technically speaking, dimmers control light output by adjusting the forward current supplied to the LED chip. Leading edge dimmers, as the name implies, cut the leading edge of the sine wave, while trailing edge dimmers cut the trailing part of the sine wave. Removing sections of the sine wave reduces the amount of time the light receives power.
Mains-powered 240V/110V LED downlights (including those with transformers) are dimmed using a phase-control leading edge, trailing edge or universal dimmer. These types of dimmers are designed to work with electronic transformers.
Some electronic transformers are claimed to work with leading edge dimmers, but the dimmer still must be suitable for use with electronic transformers, otherwise premature failure of the dimmer, transformer or LED lamp may occur.
It is possible to dim 12V LED downlights using existing halogen transformers, however if the combination of transformer, dimmer and LED lamp are not matched properly significant flickering, audible hum, and higher output at low dimmer levels can occur.
There are a number of factors which affect how well a retrofit LED downlight will perform when dimmed. Dimmer types and different dimmer qualities, transformer compatibility, minimum loads of the dimmer (and transformer), and whether the LED is 12V AC or DC.
A common scenario is where a customer has had halogen lamps installed for some years, generally starting with magnetic transformers and a leading edge dimmer. Over time, lamps and transformers have blown and an electrician has removed and replaced these lamps and transformers with new electronic transformers.
When retrofitting LED downlights, often the dimmer does not match the various types of transformers used and so dimming performance is poor, if it functions at all.
The waveform shows the leading edge dimmer delivers a step response to the capacitive rectifier of the electronic transformer or poorly designed LED driver. The result is a large dI/dt spike which can be destructive to the dimmer, the input rectifier and input capacitor.
When a leading edge dimmer is used with a magnetic transformer, the primary inductance prevents a large dI/dT.
This dimmer configuration is optimal for use with an electronic transformer. Because the phase control is on the leading edge, there is no large dI/dt spike. However, this type of dimmer cannot be used with a magnetic transformer. (see below)
Using a trailing edge dimmer on a magnetic transformer will result in large voltage spikes that will damage the dimmer and connected lamp. Magnetic transformers have inherent inductance which stores energy during the AC cycle in the core flux. When the trailing edge dimmer stops mid-cycle, the current in the primary of the magnetic transformer will attempt to continue conduction, resulting in a large dV/dT.
Have your transformers checked to ensure that: