How to dim CFLs

Compact fluorescent lamps (CFLs) are replacing incandescent light bulbs at a rapid rate due to their tremendous energy savings and longer lifetime. Additional energy savings can be achieved by dimming, but the electronic ballast required to control the lamp has a higher cost and is difficult to design. This article explains how a CFL works, how to dim them, and describes a solution for 3-way dimming applications.

Fluorescent, incandescent

Incandescence is the conversion of heat to light, which requires the filament inside an incandescent lamp to burn at a high temperature (350° F or 176° C). This conversion is simple but the disadvantages are that only 5 per cent of the total energy consumed by the lamp is used to generate light (95 per cent is wasted as heat!) and the lifetime is limited to about 2,000 hours.

Fluorescence is the conversion of ultraviolet (UV) light to visible light. Electrons flow through the fluorescent lamp and collide with mercury atoms, causing photons of UV light to be released. The UV light is then converted into visible light as it passes through the phosphor coating on the inside of the glass tube. This two-stage conversion process is much more efficient than incandescent lamp process, resulting in 25 per cent of the total energy consumed used to generate light, lower lamp temperatures (40 degrees Celcius) and a longer lifetime (10,000 hours). The lamp load itself is resistive, but the electronic ballast that is connected between the AC line voltage and the lamp for controlling the lamp current is a capacitive load. The complete CFL (see figure below) includes the Edison screw-base and plastic housing, the electronic ballast, and the fluorescent lamp formed into a compact spiral shape.

CFL components and assembly Click on image to enlarge.

CFL operation

The electronic ballast circuit block diagram (see figure below) includes the AC line input voltage (typically 120 VAC/60 Hz), an EMI filter to block circuit-generated switching noise, a rectifier and smoothing capacitor, a control IC and half-bridge inverter for DC to AC conversion, and the resonant tank circuit to ignite and run the lamp. The additional circuit block required for dimming is also shown; it includes a feedback circuit for controlling the lamp current.

CFL electronic ballast block diagram Click on image to enlarge.