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 -----
From: Plunkett, Dennis <dplunkett@AIRINTER.COM.AU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, September 19, 2000 6:53 PM
Subject: Re: [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:
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>

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