Why do fluorescent lights burn out?

January 6, 2015 4:22 AM

When fluorescent lights burn out, what is happening? Also, why do they flicker when they're about to go out?

In an incandescent light, the tungsten filament breaks and the current can't flow and heat up the filament to glowing temperatures. This could happen because air gets in.

When fluorescent lights burn out, what part broke and why?

posted by Galaxor Nebulon to Science & Nature (6 answers total)

Wikipedia's article on the Fluorescent lamp gives several end-of-life scenarios for fluorescent bulbs. The first category, using up the emission mix, sounds like what happens to the CF tubes in our laundry room, which are usually on for just 10-20 minutes at a time. I also found Mechanisms Of Fluorescent Lamp Failure At End Of Life and Troubleshooting of Fluorescent Lamps and Fixtures which didn't at first glance name any other interesting failure modes. (incidentally, my search term for all of this was: fluorescent light failure modes)
posted by jepler at 5:32 AM on January 6, 2015

This Instructable goes into what's inside a CFL, and covers some of the reasons they fail. TL;DR: Typically, the ballast, - electronics that convert the wall current into power the tube can use - has components that have a duty cycle (the number of times it can do what it does before they fail) - when that duty cycle is up, the component, usually a capacitor or diode, can literally burn out.
posted by Slap*Happy at 5:32 AM on January 6, 2015

As a corollary to Slap*Happy's answer, it's worth noting that in tube fluorescents (as opposed to CFLs) the ballast is in the housing, not the tube itself, and is typically hardier (a CFL's ballast is unusable after the tube burns out, so there's no reason to make them long-lived; independent housing ballasts are supposed to be used over and over again with new tubes, so making them workhorses makes sense). For a CFL, a ballast EOL scenario is identical in effect to a gas/phosphor/etc. EOL: the bulb stops working. But for a tube fluorescent, the difference is quite conspicuous: no new tube will work until the light housing is repaired or replaced.
posted by jackbishop at 5:41 AM on January 6, 2015 [1 favorite]

Slap*Happy: a duty cycle (the number of times it can do what it does before they fail)

Trivial correction: What you are describing is an "expected lifetime".

The "duty cycle" is what percentage of the current cycle that the device is on. For instance, a perfectly symmetrical square wave has a duty cycle of 50%. It's only related if the switching causes deterioration - which in the case of high-voltage components that exist inside fluorescents, it does.
posted by IAmBroom at 7:22 AM on January 6, 2015 [2 favorites]

A burn out tends to be of the electrical components (ballast), or the sealed glass tube leaks and air gets inside, causing flickering as the ignition faces higher resistance. On top of this more / harder ignitions erode the cathodes (the metal ions jump right off!) at either end of the tube, and this stops ignition from ever working.
posted by nickggully at 8:39 AM on January 6, 2015

In an incandescent light, the tungsten filament breaks and the current can't flow and heat up the filament to glowing temperatures.

Fluorescent lamps have tungsten filaments as well. You can see a closeup of one here. There is one filament in each end of the fluorescent tube. In the picture you will see a white coating on the tungsten filament. This is a mixture of barium and strontium oxides. The mixture assists in allowing the filament to emit electrons which flow through the gas in the tube to the filament on the other end of the tube. The filament's purpose is not to get white hot like an incandescent bulb, but instead, it gets orange hot so that it emits electrons from the emission mix on the filament surface.

Every time you turn on the lamp, some of the emission mixture sputters off the filament. You can see this as darkening on the ends of the tube. Eventually all of the emission mix is gone and you no longer emit enough electrons from the filament to operate the tube steadily. It will flicker as it tries and fails to ionize the gas in the tube to maintain an arc.

So in an incandescent lamp, the tungsten becomes hot enough that the tungsten itself gradually boils off the filament, thinning it until it breaks. In the fluorescent lamp, the filament doesn't get hot enough to burn out, but hot enough to gradually boil off the emissive coating on the filament so it no longer works.
posted by JackFlash at 1:06 AM on January 7, 2015 [4 favorites]