Author Topic: Very Hot EOLs  (Read 193 times)
Emersyn
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Very Hot EOLs « on: March 10, 2026, 09:55:10 AM » Author: Emersyn
In the hallways of the older buildings at my school (that aren't there anymore), the 2x F32T8 (retrofitted) wraparounds were plagued with horrible lamp life and often really hot but dim EOLs. The tubes would just go dim and red but then below the ends the cover would start to deform and it would actually like form a drip of plastic.

Is this normal EOL behavior?
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Medved
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Re: Very Hot EOLs « Reply #1 on: March 10, 2026, 11:32:40 AM » Author: Medved
Yes, when the ballast is able to support the excessive voltage drop and does not shut down.
Failing cathode leads to high cathode drop so high power dissipation, so general overheating in that area. This then leads to slow lamp[ leak and oxygen poisoning, leading to the purple color.
It depends on the ballast, how far is it able to maintain the arc current and whether it has any EOL detection and shut down.
Good quality modern ballasts do not allow this condition for more than few seconds, but many early electronic ballasts were lacking any form of EOL protection so may drive the lamps in this dangerous manner.
Because of the long lamp life, it took a few years of these ballast being sold till these problems were recognized as a real safety issue, so there is huge amount of ballasts in the field that are lacking any form of protection, the early designs were just made so robust this condition did not destroyed the ballasts alone, but the lamp socket area overheating and consequent fire risk was neglected.
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Emersyn
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Re: Very Hot EOLs « Reply #2 on: March 12, 2026, 07:07:38 AM » Author: Emersyn
Ah that makes sense, are these the same ballasts that sometimes actually break the ends on lamps at EOL?
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Medved
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Re: Very Hot EOLs « Reply #3 on: Today at 01:02:45 AM » Author: Medved
If you mean break the glass, then I would guess so...

But breaking the filaments and letting that to shut the ballast down is very common method on many cheap ballasts and CFLs.
The higher voltage for maintaining arc at EOL tube means the ballast has to be boosting the voltage by the output curcuit resonance. And when the resonant capacitor is connected via the lamp filaments, the resonance is feeding them by large current, so melting them off. And once broken, the resonant capacitor is not connected to the circuit, so the thing can not boost the voltage anymore, so the circuit shuts down.
The main problem with this method is, some filaments are way tougher than anticipated, so actually refuse to break even at the overcurrent (lamp maker uses thicker filament "to make it stronger", but also a leak into the tube causes excessive filament cooling, preventing them from melting), with the consequence of the ballast not shutting down on EOL and continuing to overhat the tube end assemblies.
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