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Date: Thu, 03 Oct 2002 14:04:55 -0700
From: Charles Edmondson
Organization: Edmondson Engineering
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:0.9.4.1) Gecko/20020508 Netscape6/6.2.3
Subject: Re: Halogen bulb drive
References: <4LPj9.email@example.com> <firstname.lastname@example.org>
X-Original-Trace: 3 Oct 2002 14:04:48 -0700, 10.38.34.223
> "Phil Allison" wrote:
>>>The most significant factor in life (ignoring mechanical vibration and
>>>shock) is applied voltage. Life being inversely proportional to 12th power
>>>of the ratio of specified and applied voltage.
>> ** That is exactly what I have found - silly stories, not facts.
>> The extended life of halogen bulbs mainly derives from the use of
>>thick filaments in the low voltage ones and the same is also true in high
>>powered 120 volt ones like Par 64s et alia.
>> 240 volt low power halogens are both very fragile and short lived
>>unless the colour temp is way down - your 12th power comes into play here.
> Yep there is quite a lot to halogen lamps. The brighter you want the
> shorter they live, some of the small bright ones were rated for only 3-4
> It is also possible to reduce life with under voltage operation, apart from
> discolouring the envelope. There are choices in the mix and concentration
> of Halogens used. One manufacturer told me of problems a customer had
> running projector type lamps significantly undervoltage for extended life
> and the aggressive mix used in them was damaging the filaments running at
> lower than normal temperature.
> What I was actually trying to find out was how bad (or good) an idea it was
> for a brightness regulator to stabilise filament resistance.
Actually, halogens have been in use for decades in theatrical lamps.
The main reason is that failure of a filament is due to evaporation of
the filament from the heat. The hotter the filament, the more light is
produced at the expense of shorter life. The evaporated tungsten then
would condense on the envelope, darkening it and reducing the light output.
So, some genius put some iodine in the bulb, used quartz to handle the
high temperatures, and cranked up the temperature of the filament. The
evaporating tungsten combined with the iodine, so it didn't concense on
the envelope. The high temperature of the filament broke down the
tungsten-iodine molecules, re-depositing the tungsten back on the
filament! So you got long life, more light, better efficiency, and all
sort of other goodies.
The problem is not all the tungsten is re-deposited on the filament.
The support structures are also pretty hot, so some of the tungsten is
deposited there, so eventually you still get filament failures.
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