Why LEDs and dimmers can clash
Old dimmers were built for incandescent bulbs, which draw a lot of power. LEDs draw very little, so an old dimmer can struggle to control them smoothly — the result is flicker, buzz, or a bulb that only dims over part of the range or won't switch off fully. Two things need to line up: the bulb must be labelled dimmable, and the dimmer should be a modern LED-compatible (trailing-edge) type.
The low-voltage catch
Low-voltage MR16 spotlights add another variable: they run through a transformer or driver that also has to be LED-rated. If yours flickers, an incompatible transformer is a common cause.
The simple fix
Reliable dimming needs a dimmable LED bulb and an LED-compatible (trailing-edge) dimmer — and for MR16 spotlights, an LED-rated driver. Swapping the bulb is a simple job you can do yourself, but changing the dimmer, transformer or driver is electrical work for a qualified electrician. For a bulb that already looks or behaves wrong, try Fix My Bad Bulb.