RichD
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| Hi. During my college years (about 18 years ago!!) I worked for a small bowling center illuminated with numerous 2x F96T12 IS fixtures (magnetic ballasts - this was the 80's after all!). One of my responsibilities was lighting maintenance. I recall many instances in which one of the slimlines would still light at about 90%, but would blacken heavily at one end and appear to "shimmer". Every time I spotted one of these I would replace it, but my boss (the owner of the center) often questioned my decision.
Anyway, I was wondering if any of you: 1) Know the failure mode I'm remembering. 2)Can explain in technical terms exactly what was happening. I'm guessing electrode failure, but was this caused by the bulb or the ballast? I don't see this type of failure any more in the electronic ballast era.
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Medved
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| I think yes, it was electrode loosing emitter.
The asymmetric behavior might would be linked to the lamp rectification: The total asc voltage in the direction, where the bad electrode was a cathode was bigger, causing the DC current flowing in the opposite direction and migrate the mercury to one end. Furthermore it cause the ballast saturation, so increase of current crest factor, what speed up the blackening.
Electronic ballasts are mostly DC decoupled, so such DC voltage component is across some capacitor and does not cause the DC current component migrating the mercury. And furthermore newer, good quality ballasts sense this rectification and shut down, as in any case such condition overheat the failing tube end and might damage the ballast.
In any way by relamping just when such signature occur you mist likely save some ballasts from overheat...
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buddyboi1979
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| If the bulb on the leg side? burns out it will still light about 90% but flickery, and the other will be at full brightness. Until vacume is lost on the bad tube.
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DimBulb
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| I remember this failure mode on F96 lamps quite well back in the late 70s and early 80s at school. I don't really remember the lamps losing vacuum often back then either, that flickering seemed to last days or even weeks.
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My very first word was LIGHT!
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arcblue
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| I've seen some interesting failure modes in the typical 2-lamp F96 slimline fixtures. I've always wondered why on a series ballast one lamp could be lit at full or close to full brightness but the other one will be out. I believe the lead-lag fixtures are series-sequence start, so if the lag lamp fails, the lead lamp actually starts up and burns at full brightness because it gets full OCV at startup. But I'm not sure if I'm correct on this. I can't imagine at any rate this would be good for the ballast.
The other interesting failure is when lamps will snake & flash wildly, or maybe go out for a while and then suddenly flash for a bit. If left like this until the lamps go out completely, usually when the lamps were changed, the ballast would be dead too.
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I'm lampin...
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Medved
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I've seen some interesting failure modes in the typical 2-lamp F96 slimline fixtures. I've always wondered why on a series ballast one lamp could be lit at full or close to full brightness but the other one will be out.
On series it can not I believe the lead-lag fixtures are series-sequence start, so if the lag lamp fails, the lead lamp actually starts up and burns at full brightness because it gets full OCV at startup. But I'm not sure if I'm correct on this. I can't imagine at any rate this would be good for the ballast.
Lead-Lag contain (by functional mean) two independent circuits, so when one lamp fai, the other stay lit. Of course, the primary of the ballast is overloaded, as it has to handle the reactive part of the current as well (normally compensated out by the reactive current component of the other lamp) The other interesting failure is when lamps will snake & flash wildly, or maybe go out for a while and then suddenly flash for a bit. If left like this until the lamps go out completely, usually when the lamps were changed, the ballast would be dead too.
In magnetic ballast circuit i think this is the ballast, that failed at first (shorted capacitor?), then taking away the lamp. On cheap electronic the lamp was depleted of emissive material on electrode(s), so has very high arc voltage, what lead to large currents in the ballast resonator circuit and transistor hard switching, overheating them as a result. |
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RichD
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If the bulb on the leg side? burns out it will still light about 90% but flickery, and the other will be at full brightness. Until vacume is lost on the bad tube.
Hi Buddyboi.Until I found this forum I knew nothing about the lead/lag sides of a tulamp series ballast  . In any case, your description of "about 90% but flickery" sound right-on. As stated, I always replaced such lamps as soon as I spotted them, so I never had one lose vacuum. Thanks for the info. |
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RichD
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I think yes, it was electrode loosing emitter.
The asymmetric behavior might would be linked to the lamp rectification: The total asc voltage in the direction, where the bad electrode was a cathode was bigger, causing the DC current flowing in the opposite direction and migrate the mercury to one end. Furthermore it cause the ballast saturation, so increase of current crest factor, what speed up the blackening.
Electronic ballasts are mostly DC decoupled, so such DC voltage component is across some capacitor and does not cause the DC current component migrating the mercury. And furthermore newer, good quality ballasts sense this rectification and shut down, as in any case such condition overheat the failing tube end and might damage the ballast.
In any way by relamping just when such signature occur you mist likely save some ballasts from overheat...
Wow...I'm not sure I understand all of that, Medved, but am I correct in understanding that your're saying the failure was due to a bad lamp electrode?
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Medved
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Wow...I'm not sure I understand all of that, Medved, but am I correct in understanding that your're saying the failure was due to a bad lamp electrode?
This is what it look to me... |
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