Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!rex!ames!dlb!zygot!john From: john@zygot.ati.com (John Higdon) Newsgroups: sci.electronics Subject: Re: Economy of turning off fluorescent lights Message-ID: <32236@zygot.ati.com> Date: 22 May 91 20:21:44 GMT References: <1991May19.164508.65064@pttrnl.nl> Reply-To: john@zygot.ati.com (John Higdon) Distribution: sci Organization: Green Hills and Cows Lines: 51 In article <1991May19.164508.65064@pttrnl.nl> walvdrk_r@pttrnl.nl writes: >But ... the tubes have a very long lifetime. Only the heated wires at both ends >of the tube haven't. These wires are only heated during the start-up of the >tube and that's when they suffer more than during the normal "burning" >operation of the tube. However, I don't know how many start-ups a standard tube >is supposed to last, but suppose it is 5000 times. There are three basic starting types each representing a distinct trauma to the tube (or starter). The oldest type uses a bi-metal starter which is in series with the filaments of the tube. When power is applied, an inert gas (such as neon) conducts in the starter, heating the bi-metal strip. When the strip heats, it completes a circuit through the tube filaments which then heat up. Meanwhile the strip is cooling since its closure stopped the conduction through the neon gas. When it opens once again, voltage is once again available at the ends of the tube and since the filaments are hot, the tube conducts and lights. Sometimes several tries are necessary and this is why lamps of this type flicker when starting. What usually fails first in this system is the little capacitor across the bi-metal strip. This cap suppresses noise and helps with the inductive kick from the ballast to start the lamp. When this cap shorts, the lamp sits there and glows orange at the ends. A nicer method of starting flourescent lamps is "rapid start". A special ballast is used which applies current to the lamp's filaments at the same time it applies a high voltage across the tube's length. The high voltage causes the tube to conduct almost instantly, and as the filaments heat the tube draws more and more current, lowering the voltage across it. The tube smoothly comes up to operating current within a second or so. This is the least destructive way to start a flourescent lamp, but since current is applied to the filaments constantly, it is not the most efficient. It is very popular, however. The third way is called "instant start". This is the almost-universal method used in the longest tube lengths. No current is sent through the filaments. In fact, these tubes only have one pin at each end. The ballast sends a very high voltage through the tube, causing it to conduct instantly (as a cold-cathode lamp). Very quickly, the filaments heat from the current flowing through them as cathode/anode. (They switch roles sixty times a second.) When they heat, the tube switches from a cold cathode lamp to a hot cathode lamp, and the ballast allows the voltages and currents to change accordingly. There is some filament wear during the cold-cathode phase of operation, as well as during the transition to hot-cathode state. Starter-type lamps should be turned on and off as little as possible. Rapid start lamps can be turned off whenever desired. Instant start lamps fall somewhere in between. -- John Higdon | P. O. Box 7648 | +1 408 723 1395 john@zygot.ati.com | San Jose, CA 95150 | M o o !