[This article by Peter Breen on EDN provides a simple introduction to the problems of adapting LED driver circuits to they work as expected with legacy triac dimmers. -- Ed.]
By Peter Breen
For several years manufacturers have been introducing LED bulbs onto the market with the eventual goal of replacing incandescent and CFL lamps. The design of these lamps has evolved from very simple non-dimmable solutions, to sophisticated but expensive dimmable solutions, then finally to more cost effective dimmable solutions.
Many LED lamps are advertised as dimmable, however in reality the performance of many of them is unimpressive with varying results depending on the dimmer used and loading of the circuit. In some cases the LED lamp has been taken out of its box and installed in a room with a dimmer switch only to discover flickering and uneven adjustment of the light level.
These deficiencies result from the fact that the vast majority of dimmers installed in the US are based on the triac-based two-wire leading edge phase cut circuit developed in the 1960s intended for use with resistive incandescent lamps. The triac, a bi-directional semiconductor power switch, is triggered by a pulse generated by a variable timing circuit and maintains conduction while its conducted current remains above a certain level termed as the holding current. There are many varieties of the dimmer circuit using devices with different characteristics combined with variations in the control circuitry and filtering components.
Read the rest of the story at EDN.
— Keith Dawson 
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