MadSci Network: Engineering

Re: Why do flourescent lights stop working over time?

Date: Tue May 29 20:00:05 2001
Posted By: Michael L. Roginsky, Staff, Avionics, Honeywell Defense Avionics
Area of science: Engineering
ID: 990669248.Eg

Hello Sterling: The fluorescent lamps you refer to are probably a mix of 
mercury and argon gas. There are many ways that lead to failure because of 
interaction between the ionizing electrodes and types of ballast. Here are 
some pointers for troubleshooting: 
Bad fluorescent tubes: Unlike incandescent lamps where a visual 
examination of the bulb itself will often identify a broken filament, 
there is often no way of just looking at a fluorescent tube to determine 
if it is bad. It may look perfectly ok though burned out fluorescent will 
often have one or both ends blackened. However, a blackened end is not in 
itself always an indication of a bad tube. Blackened ends are a somewhat 
reliable means of identifying bad tubes in 34 or 40-watt rapid start 
fixtures. Blackened ends are not as reliable an indicator in preheat or 
trigger start fixtures, or for tubes of 20 watts or less. Failure of the 
electrodes/filaments at one or both ends of the fluorescent tube will 
usually result in either a low intensity glow or flickering behavior or 
sometimes in no light at all. A broken filament in a fluorescent tube used 
in a preheat type fixture (with a starter) will usually result in a 
totally dead lamp as there will be no power to the starter. Dim glow is 
rare in this case and would probably be confined to the region of the 
broken filament if it occurs. The best approach is to simply try replacing 
any suspect tubes - preferably both in a pair that are driven from a 
single ballast. In fixtures where rapid start ballast runs two tubes, both 
tubes will go out when one fails. Sometimes one or both tubes will glow 
dimly and/or flicker. If one tube glows dimly and the other is completely 
dead, this does not indicate which tube has failed. The brighter tube may 
be the good one or the bad one. The bad tube usually has noticeable 
blackening at one end. It may pay to replace both tubes, especially if 
significant labor costs are involved. In addition, prolonged dim glowing 
may degrade the tube that did not initially fail. In trigger start 
fixtures that uses one ballast to power two 20-watt tubes, sometimes both 
tubes will blink or intermittently dim. Replacing either tube with a known 
good tube may fail to fix this. The tubes may continue blinking or 
intermittently dimming until both are replaced with brand new tubes. This 
sometimes indicates borderline low line voltage ("brownout," etc.), non-
ideal temperatures, or a borderline (probably cheaply designed) ballast. 
Bad starter (preheat fixtures only): Faulty fluorescent tubes continuously 
trying to start unsuccessfully may damage the little starter. It is a good 
idea to replace the starter whenever tubes are replaced in these types of 
fixtures. One way that starters go bad is to "get stuck". Symptoms of this 
are the ends of the affected tube glowing, usually with an orange color of 
some sort or another but sometimes with a color closer to the tube's 
normal color if arcs form across the filaments. Occasionally, only one end 
arcs and glows brightly, and the other end glows dimmer with a more orange 
color. Note that this is hard on both the tube and the ballast, and the 
defective starter should be immediately removed. Should one or both ends 
glow with a bright yellowish orange color with no sign of any arc 
discharge surrounding each filament, then the emissive material on the 
filaments is probably depleted or defective. In such a case, the tube 
should be replaced regardless of what else is wrong. If both ends glow dim 
orange colors, then the filaments' emissive coating may or may not be in 
good shape. It takes approx. 10 volts to form an arc across a healthy 
fluorescent lamp filament. 
Defective iron ballast: The ballast may be obviously burned and smelly, 
overheated, or have a loud hum or buzz. Eventually, a thermal protector 
built into ballasts will open due to the overheating (though this will 
probably reset when it cools down). The fixture may appear to be dead. Bad 
ballast could conceivably damage other parts as well and blow the 
fluorescent tubes. If the high voltage windings of rapid start or trigger 
start ballasts are open or shorted, then the lamp will not start. Ballasts 
for fixtures less than 30 watts usually do not have thermal protection and 
in rare cases catch fire if they overheat. Defective fixtures should not 
be left operating. 
Bad sockets: These can be damaged through forceful installation or removal 
of a fluorescent tube. With some ballasts (instant start, for example), a 
switch contact in the socket prevents generation of the starting voltage 
if there is no tube in place. This minimizes the possibility of shock 
while changing tubes but can also be an additional spot for a faulty 
Lack of ground: For fluorescent fixtures using rapid start or instant 
start ballasts, it is often necessary for the metal reflector to be 
connected to the electrical system's safety ground. If this is not done, 
starting may be erratic or may require you to run your hand over the tube 
to get it to light. In addition, of course, it is an important safety 
Also look-up on the Internet:
Your MAD.SCI Micro.

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