13

I have a ~60 cm fluorescent tube light that is failing. ~10 cm on one end glows orange, ~10 cm on the other end glows blue-white (normal fluorescent light color) and the middle 40 cm is dark.

Question: From an electrical point of view what exactly is going on such that this particular graded pattern of three colors is produced?

my fluorescent light

There seems to be a "starter" just behind the tube, near the "orange" end.

uhoh
  • 3,399
  • 2
  • 23
  • 63

1 Answers1

13

Your fluorescent tube has filaments on both ends. Those filaments are heated by the starter current and then emit electrons into the gas filling of the tube. To assist that, they have a coating of lanthanides.

Over time, those lanthanides amalgamate with the mercury gas filling and cannot serve their function any more. The tube cannot be started any more then. It will be heated up through the filaments until the starter opens, but as nothing happens then, the starting process is repeated.

What's still there is the filaments. Depending on the amount of lanthanides still in function, electrons are still emitted and excite the mercury gas filling in the surroundings of the electrode. That's the green or white end. At the orange end, what you see is the glowing of the filament.

Janka
  • 13,636
  • 1
  • 19
  • 33
  • I have used thoriated tungsten wire in the past. While thorium is not a lanthanide, is that the same basic idea (work function, electron emission...) – uhoh Jul 30 '19 at 14:25
  • I'm not sure what "until the starter opens" means. The tube always looks like this. By accident I left it on for hours once and there was no change. Perhaps it closes again quickly and I miss it? – uhoh Jul 30 '19 at 14:26
  • 2
    If the tube does not flicker, the starter is defective, too. Often, that's the reason why a tube dies prematurely. Always replace the starter, too. – Janka Jul 30 '19 at 20:08
  • 2
    I think thorium-coated wire got out of fashion because the inadvertible radioactive hazard. – Janka Jul 30 '19 at 20:13
  • Thanks! I'll let you know what I find. And oh that's right, it's the same thorium that was in the (slightly) radioactive lantern filaments. – uhoh Jul 30 '19 at 21:09
  • I finally got around to doing the experiment, and indeed you are correct! Both the tube and the starter were defective. Great call, thanks! – uhoh Oct 12 '19 at 03:19
  • 1
    @uhoh Either the gas ions in the glow switch tube in the starter have plated so much metal from the electrodes onto the glass (a process known as sputtering) that it has shorted itself out, or the capacitor across it is shorted. – Peter R. McMahon Aug 05 '21 at 03:00
  • 1
    @Janka What blackens the ends of tubes and shortens their lives is the bad design of starters allowing the tube to blink on before the cathodes are fully heated. The high voltage between the gas & the cathode causes ions to blast it, dislodging atoms (a process known as sputtering). There is an English made electronic starter in a transparent green case, that fully preheats the cathodes. The 1st tube in my 1984 range hood lasted about 10 years. The 2nd one with one of these starters is still going. I don't remember the brand, but they are available from electrical wholesalers. – Peter R. McMahon Aug 05 '21 at 03:17
  • @PeterR.McMahon Thanks! Now I wish I still had the old starter. – uhoh Aug 05 '21 at 04:12
  • @Janka The glass will be black and silver from all the metal plated on it. Those glow switch starters use cheep argon gas at low pressure, and the current is high to heat the bi metal strip inside. The atoms of heavy gasses like xenon help to block the ions from hitting the cathode in flash tubes. Not only are argon atoms much lighter, but the low pressure to make it high impedance means there is effectively an even higher current density per atom and, sputtering is more than proportional to the current. Proportional to I^3.3 according to a neon lamp datasheet by CML. – Peter R. McMahon Aug 08 '21 at 04:53