Naked Science Forum
Non Life Sciences => Technology => Topic started by: Rob_van_Halen on 11/07/2009 19:30:02
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Rob_van_Halen asked the Naked Scientists:
Hi, my name is Rob Van Valen from New York City.
Occasionally when you walk you will see OLD fluorescent tube bulbs that are dimly lit and fluttering in an erratic pattern. Would you tell me exactly what causes this?
Thank you.
What do you think?
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Fluorescent tubes work by heating cathodes so that they emit electrons and allow an electric current to flow from end to end in the tube. The tube is filled with a gas (usually argon) at low pressure, which becomes ionised and conductive, and mercury vapour resulting from energy imparted to the small amount of mercury in the tube. The electrons and argon ions collide with the mercury atoms which excites them into a higher energy state. The atoms subsequently return to a lower energy state releasing a photon from each atom in the UV region. The photons are absorbed by fluorescent coatings on the inside of the tube and, by a similar method, re-emit light at visible wavelengths.
Problems that have to be overcome is starting and maintaining the process. The cathodes have to be heated sufficiently to get the emission of electrons and then a high voltage applied between the ends of the tube to ensure that the gas ionises to allow a current flow. The voltage that has to be applied to start the process is higher than the 110V or 240V available, so has to be generated though, once the process has started, can be reduced to a lower value. The process is controlled by having a ballast inductor to limit the current - it provides a series impedance to the AC supply.
There are a number of methods to start the tube. A common one employs a small neon lamp which is in series with, and between, the cathode heaters. When voltage is applied the neon lamp strikes and heats its neon gas. The lamp is contructed with bi-metallic strips for its electrodes so that this heat causes the two electrodes in the lamp to eventually touch. When this happens it provides an increased current to the Fluorescent tube cathode heaters and they get extra hot and emit electrons. The neon eventually cools and its electrodes separate; this causes the inductor to generate a high voltage that "strikes" the tube. From then on the tube is conducting and of a low enough impedance that the starter neon (in parallel with the tube) will not light again and the tubes cathodes can maintain sufficient emissions without the extra heat.
Now to answer your question. When a tube gets old a common failure mechanism is that the cathodes become inefficient at emitting electrons. The result is that the tube strikes but then cannot maintain that state. As soon as it goes out, the neon starter tries to re-initiate the start sequence and this can repeat indefinately. The timing of when the neon breaks relative to the voltage across the tube at that time (remembering the voltage is AC) produces inconsistent and seemingly random results and the observed flashing.
I'm sorry this is so complex, but I can't think of an easier way to explain it.
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I didn't think modern fluo tubes and starters had any moving parts. I thought that the small neon, in series with the heaters, would no longer conduct once the main tube had struck and there was sufficient voltage drop across the ballast and this cut the heater current. I don't think that a bimetal strip is needed with this elegant method.
I seem to remember ancient starters having some clanking mechanism inside them but I was too young to make sense of what was inside!
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SC, my knowledge may be out-of-date but as far as I know this is the way the starters work on the typical 5 foot tubes in offices (and in my kitchen). The latest tubes have more sophisticated electronic starters and the problems are different with the small, light-bulb replacement tubes. Many modern offices have more sophisticated systems now, but I was trying to explain the principles in order to answer the question.
The neon does not conduct once the tube has struck, which I thought I had explained... "From then on the tube is conducting and of a low enough impedance that the starter neon (in parallel with the tube) will not light again [...]".
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So what does the bimetal strip achieve? Once there is an established arc, the heat to release electrons at each end is generated by the flowing current.
The heaters are switched off and the tube is operating. I can't remember having seen a bimetal strip inside a broken tube; where would it be (I didn't look in detail)?
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It is not a standard neon in the starter. One of the electrodes of the neon is a bimetal strip. The purpose is to allow time to pre-heat the FT's electrodes until the neon shorts, then on cooling and separating, to break the flow of current and cause an increase voltage from the ballast inductor. Please read the third paragraph as I thought I had explained it. When I refer to the "lamp" I am talking about the neon lamp in the starter which is maybe where it is confusing.
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Oh, I see what you mean.
I still didn't realise what you were getting at about the action of the bimetal switch until I looked it up (always a good idea to do this, I find). The break in the heater current induces an excess voltage pulse from the series inductor and that helps to to strike the main arc in the FT.
This wouldn't have been how the original FTs were started, because many of them just used filament bulbs in series as ballast (no inductor). My original description would apply to that system. They were probably much worse at starting, for that reason.
Interesting!
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The fluorescent lamp was the first major advance to be a commercial success in small scale lighting since the tungsten incandescent bulb. Its greatly increased efficiency resulted in cool (temperature wise) brightly lit workplaces (offices and factories) as well as home kitchens and baths. The development of the mercury vapor high intensity discharge (HID) lamp actually predates the fluorescent (the latter being introduced commercially in 1938, four years after the HID). However, HID type lamps have only relatively recently become popular in small sizes for task lighting in the home and office; yard and security area lighting; and light source applications in overhead, computer, and video projectors.
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