Post by thread:[PICLIST] [OT] [EE] globe failure

20/9/2000 Interesting question posed the other day regarding some faults that have been detected lately. If a globe (Running on 12VDC) fails and the glass goes black, what has occurred? Is it overheat? Is it overcurrent? Is breakdown in the inert gas? Is it a failure at startup high current inrush (Thermal shock)? or Is it a mix of all? Ideas:-? Etc Dennis -- http://www.piclist.com hint: PICList Posts must start with ONE topic: "[PIC]:" PIC only "[EE]:" engineering "[OT]:" off topic "[AD]:" ad's

> Interesting question posed the other day regarding some faults that > have been detected lately. If a globe (Running on 12VDC) fails and the > glass goes black, what has occurred? Is it overheat? Is it > overcurrent? Is breakdown in the inert gas? Is it a failure at startup > high current inrush (Thermal shock)? or Is it a mix of all? > Ideas:-? Etc Yes interesting. Just replaced a 12V 55W halogen lamp in in my car yesterday. The guy at the shop said it looked like overvoltage because of the way the filament had broken in one place, the broken ends had melted into tiny blobs, and the glass was still clear. Sounds like a great opportunity to get a box of lamps, (globes, light bulbs, incandescent illuminatory thingies, whatever) and test them to destruction under a variety of conditions. Brent Brown Electronic Design Solutions 16 English Street Hamilton, New Zealand Ph/fax: +64 7 849 0069 Mobile: 025 334 069 eMail: spam_OUTbrent.brownTakeThisOuTclear.net.nz -- http://www.piclist.com hint: PICList Posts must start with ONE topic: "[PIC]:" PIC only "[EE]:" engineering "[OT]:" off topic "[AD]:" ad's

Plunkett, Dennis wrote: > > 20/9/2000 > > Interesting question posed the other day regarding some faults that have > been detected lately. > If a globe (Running on 12VDC) fails and the glass goes black, what has > occurred? > Is it overheat? > Is it overcurrent? > Is breakdown in the inert gas? > Is it a failure at startup high current inrush (Thermal shock)? > or > Is it a mix of all? > > Ideas:-? > Etc > > Dennis Usually when the glass goes black that shows a high level of vaporisation of the element wire, in a fuse this would indicate very high inrush current but in a globe it almost certainly indicates failure of the atmospheric seal and air was in the globe when it powered up. If you don't believe me drill a little hole through the base of a new globe and power it up and see what happens to the filament. :o) Roman -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

>If you don't believe me drill a little hole >through the base of a new globe and power it >up and see what happens to the filament. :o) One guy I worked with would take an MES lamp and gently squeeze it with a pair of pliers to crack the glass. Then apply a low voltage to it and the filament very slowly vaporizes leaving a white film on the glass. After doing this he would put it up his nose and watch the discusted expressions on peoples faces. I suspect for the glass to turn black the lamp has to be at operating temperature and have a very minute leak. -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

There are two ways the inside of the globe can become blackened. 1) Long-term blackening is gradual and caused by evaporation and condensation of the metalic filament. This is not what is happening in your case. 2) If excessive voltage is applied (which, of course, then causes excessive *current* to flow), then the center of the filament can melt. As it opens up there can be arcing, and it is this arcing that deposits a black coating on the inside of the globe. I believe that THIS is what is happening to your globes. You can reduce the tendency of the lamps to fail by: A) Reducing the voltage. This will also reduce the running intensity, of course. B) Limiting the initial turn-on current. The cold filament draws much more current than a glowing filament. This causes the initial turn-on current to be many times higher than the running current. The center of the filament tends to wear out more rapidly than the ends of the filament. Over time the center of the filament can become so weakened/eroded that when the initial turn on current comes through, it will cause the center of the filament to drop most of the voltage across that inner higher resistance section of the filament. This eventually causes the center of the filament to fail. As it blows open it arcs, and the inside of the globe gets blackened. To limit the initial turn-on current you can use a varistor of the proper value. If using a transformer, you can place a single varistor in series with the primary. This causes the transformer to initially turn on with a higher than normal impedance. As the varistor warms up it lowers its resistance and the lamps will then reach normal brightness. Fr. Tom McGahee {Original Message removed}

>Usually when the glass goes black that shows a high level >of vaporisation of the element wire, in a fuse this would >indicate very high inrush current but in a globe it almost >certainly indicates failure of the atmospheric seal and >air was in the globe when it powered up. If you don't >believe me drill a little hole through the base of a new >globe and power it up and see what happens to the filament. >:o) It's been my general experience that a globe powered up from cold with a sealing leak will produce yellowish white internal smoke (an oxide of Tungsten?) and coat the bulb with a white layer. A friend in the auto servicing trade opined that the inner dark coating sounded like a high temperature fi;ament failure while operating. RM -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> >It's been my general experience that a globe powered up from cold with a >sealing leak will produce yellowish white internal smoke (an oxide of >Tungsten?) and coat the bulb with a white layer. Tungsten oxide probably, >A friend in the auto servicing trade opined that the inner dark coating >sounded like a high temperature fi;ament failure while operating. Tungsten that evaporates from the filament and deposits on the (comparatively) ice cold glass. -- Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9 -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

I'm worked on a project that used an 125W halogen lamp as a heat source to "test rate of rise" heat detectors, and after a lot of research we came to the conclusion that inrush and filament migration was the cause of most lamp failures. Interesting point about the voltage applied to a 12v 20w lamp was lux output over a voltage range 11 to 18 volts was that at about 12v the max lux output was reached an the current about 1.5 amp if the voltage was increased above 12v the current shot up with out any increase in light out put. We controlled the lamps with two FET's to form a Bilateral Switch. I'm still waiting to see other's use this method of control. Regards Art {Original Message removed}

In my experience black glass means oxygen in the bulb (loss of vacuum). Metallic black is caused by massive overvoltage (filament evaporation and arcing). White fogged glass means oxygen in great quantities (basically sea level press. air in the lamp). If you want some real nice fireworks (as required when doing film effects) from a blowing bulb use a fluorescent ballast in series with the bulb and power from a variac. Some experiments are required until it works 'right'. Use the ballast together with the original capacitor from the fluorescent fixture. This is essential. Make sure that there are no ignitable materials around because the bulb will likely explode eventually. Did I mention the fast blow breaker on the variac ? Peter -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.