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Name: Chris
Status: Other
Grade: 12+
Location: MA 
Country: United States
Date: January 2009


Question:
How do they make florescent lights dimmable?



Replies:
Hi Chris,

Actually, ordinary fluorescent lights can in fact be dimmed slightly with a standard light dimmer... perhaps down to 60% or so of their full brightness. But to dim them further, requires a very special dimmer. The problem is that as you dim a fluorescent light, the filaments at each end get cooler and cooler, and eventually no longer emit electrons into the ionized mercury vapor inside the lamp. Special dimmers actually drive some current through these filaments to keep them hot enough to allow electrons to be emitted, which in turn ionize the mercury vapor in the lamp to create light. These dimmers are relatively complex and expensive.

Regards,

Bob Wilson


For a dimming fluorescent lamp, the difference is in the dimmer control, not the bulb.

Incandescent bulbs are powered directly from 120 volt electricity from the wall. The frequency is 60 Hz. An incandescent bulb is dimmed by reducing the power to the bulb, which makes the hot filament produce less light.

In contrast, a fluorescent bulb requires the use of a "ballast" to make it work. A ballast is required because 1) fluorescent bulbs need a special circuit to get started, and 2) the electric properties of a fluorescent bulb are unusual and (in the simplest method) an inductor and capacitor in the circuit is needed to maintain the correct amount of current and voltage to the plasma, but not too much or too little. The bulb voltage is not 120 volts, and sometimes the frequency supplied to the bulb is not 60 Hz, but may be 20,000 Hz or more.

An old-fashioned ballast is simply an inductor and capacitor. There are electronic ballasts which contain electronic components and are more efficient and produce a flicker-free light. Finally there are dimming electronic ballasts (power controllers actually) which cost even more and can operate a fluorescent tube from 100% power all the way down to 1% power.

A ballast (ordinary, electronic, or dimming electronic) operates properly only for the size bulb it is made for.

As a previous post notes, some varieties of fluorescent tube contain starting filaments which may need to be kept hot to keep the ionization going, even at low power levels.

A fluorescent lamp operates because a self-sustaining plasma is generated in the tube. How exactly does a dimming ballast work? It is difficult to find out. The manufacturers are reluctant to put that information in public. However it is done, the dimming method involves reducing the power to the tube, while still maintaining discharge. It is my understanding that pulse width modulation may used to dim the light, which suggests that the discharge is cycled on and off, and is on only a small fraction of the time.

As a side note, there are compact fluorescent bulbs that are advertised as "dimmable." These work poorly using conventional dimmers. The bulbs do not dim much before going out. On the other hand, tubular fluorescent bulbs that are driven by properly designed electronic dimmers are able to control the range of light from barely visible to full brightness.

Robert Erck



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