BulbCollector Forums
BULB DISCUSSION BOARDS => Modern Electric Lighting => Topic started by: mr_big on February 17, 2005, 03:50:15 pm
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I have recently obtained a used non-working lights of america compact fluorescent flood light with replaceable bulb when I tried to remove the lamp from the ballast it came off but the tube broke I saw that one of the electrode lead wires was melted
I know tungsten has a high melting temprature but it melted and collected on the inside of the glass any ideas what might have caused this?
The lamp is compatible with the ballast
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I'd say that sounds like a normal failure, in a preheat fluorescent lamp the tungsten electrode is heated to lower striking voltage. After a certain number of hours/starts the tungsten becomes thinner to the point where it runs hotter, and allows more current though. So the last time the lamp was started the filament probably broke and arced which melted the tungsten.
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Hello,
The reason why the electrode lead wires (not in tungsten, but in soft iron or nickel-iron) melted is most probably the type of ballast employed to run the fluorescent lamp. In the US, it is know that autoleak transformer 'rapid' or 'instant start' type of ballast can cause complete meltdown of fluorescent lamp electrodes and stems at the end of life.
The underlying reason is the efficiency of electron emission by the cathodes. In new lamps, with undamaged electrodes, the amount of energy required to reach a given lamp current is minimal. This leads to a voltage drop in front of the cathode that is in the range of the ionization potential of mercury (i.e. ~10V), and a dissipated power of Ix10 watt in front of the electrode, where I is the lamp current.
When the lamp is aged and its cathodes worn out, the energy required to extract a given current I is much larger, and the voltage in front of the cathode can reach 50 to 100V. If in Europe the series choke and 230V mains does not allow both a high current and high cathode voltage drop, this situation is however different with autoleak ballasts used in the US. with the latter equipment, the dissipated power in front of the cathode can increase significantly over time. So much in fact that the metal and glass parts of the lamps can melt in the vicinity of the cathodes.
If the ballast in your floodlight is not ferromagnetic, then the cause might be a cheap constant-current electronic ballast. As far as I know, Lights of America is not well known for the oustdanding quality of its products.
Max
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This is a compact fluorescent lamp, which is being driven by an electronic ballast that does not have satsifactory end of life protection. When the electrode runs out of emitter the electronic ballast will not detect this, and continues to supply the same power to the tube which of coruse overheats severely in the region of the expired electorde.
It is a problem which affects European and USA CFLs equally, since most are driven on electronic ballasts today. Most of the reputable manufacturers have since incorporated end of life detection systems in their electronic CFL ballasts, which shuts down the lamp before this problem occurrs. However one popular type of Philips European CFL still remains unprotected, and the glass tubes near the end will often crack at end of life. It's not only the Chinese who make lamps that fail in this way!
James.