Smart dimming circuit learns and adapts to each triac dimmer

One of the biggest challenges for new, energy-efficient CFL and LED-based lights that will replace incandescents is working with existing triac-based dimmer switches. Triac dimmers vary widely in both quality and switching characteristics. There are virtually no standards that they have to meet – electricians have selected and installed the dimmers, usually based on price, and in general they have worked fine with incandescent, which present a simple resistive load running directly off of the line ac power. However, LED and CFL luminaires are most definitely not purely resistive loads and require quite a bit of processing to dim off of a phase-cut dimmer switch. Most LED and CFL-driver circuits rely on coming up with a dimming circuit that can support a wide variety of triac dimmer switch types, but you’ll notice that in a lot of the “dimmable” lights you’ll buy that they are not unconditionally guaranteed to work with all triacs. Some lights list the triacs they work with, but most just say bring the light back if it doesn’t work with the installation’s dimmer switches.

Renesas was demoing a new approach to working with triacs at the LightSpeed booth at last week’s LightFair in Las Vegas: The circuit uses a Renesas microcontroller to learn each individual triac’s characteristics so that each light adapts to its triac. I asked LightSpeed director Cary Eskow to explain how the dimming circuit works.

“The small circuit uses an MCU to determine each “3rd party” triac dimmer’s minimum sustainable hold current. It then recalibrates itself accordingly, so the HBLED is smoothly dimmed, w/o flicker, etc., from min to max. 

“In the Renesas-LightSpeed demo unit, we had four off-the-shelf triac dimmers, purchased locally in the Bay area and widely ranging in price. A 3×3 array of Osram Oslon HBLEDs, mounted on Astarte board, were then placed in a Nuventix MR16 cooler to represent a typical load.

“Natively, the dimmers all had various hold current thresholds. Our demo allowed the user to select one of the four using a rotary switch. The selected dimmer’s “before” behavior could be observed using the Oslon HBLEDs, then by pressing a small button wired to the smart dimmer and moving the dimmer through its max and min positions, in a few milliseconds the MCU would determine the characteristics, compute a table of phase angle to PWM values, and store that table in EERAM. From that point forward, the selected dimmer’s action would be smooth and flicker-free.”