On Oct 4, 4:28*pm, Phil Hobbs
wrote:
On 10/04/2012 02:57 PM, George Herold wrote:





On Oct 4, 12:43 pm, Jeff *wrote:
On Thu, 04 Oct 2012 10:03:21 -0400, Phil Hobbs

*wrote:
On 10/03/2012 09:41 PM, Jeff Liebermann wrote:
On Wed, 03 Oct 2012 10:32:57 -0400, Phil Hobbs
* *wrote:

I don't know of any data that supports this common idea, but I'd be
interested in reading about it if anybody's actually done the experiment
carefully.

It's an accelerated life test. *The deration curve of the incandescent
light bulb is well known and assumed to be
* * (Vapplied/Vdesign)^-12 to ^-16 * Life at design voltage
http://www.welchallyn.com/documents/Lighting/OEM_Halogen_Lighting/MC3...
See Fig 5 on Pg 5 for the graph. *Nobody wants to wait 1000 hours for
a bulb to blow. *So, they increase the applied voltage, which
dramatically decreases the lifetime down to reasonable test times.
Using a rack of bulbs, they obtain an average (or median) lifetime at
the higher voltage. *Then, they work backwards on the curve to
estimate what it would be at the design voltage.

You can't run an accelerated life test when the exponent isn't known
more accurately than 12 to 16.

True, but I believe that's the range expected from different types of
light bulbs (nitrogen filled, halogen, vaccuum), and not the range
expected for a given device. *I suspect that more accurate exponent
value could be empirically determined for a given device, and later
used only for that device.

Yep. *As I understand it (possible wrong), AC filaments break in the
middle, mostly from vibration flexing.

I don't think so, because there's no mechanism for that, as I said. *The
wire is fully annealed at all times, so there's no possibility of
progressive fatigue failure.

http://en.wikipedia.org/wiki/Incandescent_light_bulb#Reducing_filamen....
* * One of the problems of the standard electric light bulb is
* * evaporation of the filament. Small variations in resistivity
* * along the filament cause "hot spots" to form at points of
* * higher resistivity; a variation of diameter of only 1% will
* * cause a 25% reduction in service life. The hot spots evaporate
* * faster than the rest of the filament, increasing resistance
* * at that point a positive feedback that ends in the familiar
* * tiny gap in an otherwise healthy-looking filament.

Note the photo of the filament with a break in the middle. *When I was
quite young, I would break burnt out AC light bulbs to see what was
inside. *If the filament was intact, the break was always somewhere
near the middle. *If a piece broke off, one end of the broken piece
was usually near the middle. *In later years, I would look at the
remains of DC panel lights (usually type 47 for old Motorola radios)
and noted that the breaks were always near the supporting terminals,
probably due to metal migration.

I suspect that the notion that cycling is hard on bulbs comes from the
way that the bulb often fails at turn-on, when the thinnest hot spot
vapourizes before the rest of the filament has a chance to come up to
temperature and reduce the inrush current.

Yep. *See my comments on the relatively high failure rate on the
40watt theater marquee lamps due to cycling. *The same lamps in the
lobby and foyer were not cycled and seemed to last forever.

I was actually disagreeing with you. *There are lots of possible reasons
for the marquee lights failing prematurely. *I'm not a tungsten expert
myself, so I'd be very interested in seeing actual data that shows a
dramatic shortening of life due to cycling. *I'm not saying it's
impossible, just that I haven't seen any such data.

So much for my anecdotal data. *My theater marquee experience was in
about 1966. *The theater actually did keep records so that they could
stock enough replacement bulbs, but I don't have copies of any of
that.

I tried Googling for similar repetative on-off tests and didn't find
anything. *If I have time, I'll try again. *I must admit that the lack
of test data does look suspicious. *Perhaps sending the idea to
Mythbusters and have them runs a test?

The filament isn't tungsten-plated, it's pure tungsten or a low alloy..
The brightness drop comes from tungsten condensing on the envelope.

Oops. *I thought it was plated.

And the connecting wire isn't plain steel, it's generally Dumet,
http://www.jlcelectromet.com/dumetwire.htm

which is a 42% Ni steel with OFHC copper or nickel plating.

You're making a lot of that up. *I'd still like to see
carefully-collected data.

No, not fabricated. *It's my reliance on my memory in an area that I'm
not familiar with. *I tried Googling for the wire used, couldn't find
much, and made a bad guess. *The plating came from somehow getting
thorium coated tungsten wire used in vacuum tubes mixed up with light
bulbs. *Sorry for the errors and muddle.

Cheers
Phil Hobbs

--
Jeff Liebermann * *
150 Felker St #D * *http://www.LearnByDestroying.com
Santa Cruz CA 95060http://802.11junk.com
Skype: JeffLiebermann * * AE6KS * *831-336-2558- Hide quoted text -

- Show quoted text -

Hi Jeff, Phil. * First I know nothing about incandescent bulbs.
But how about this as a model of why turning bulbs on and off might
cause them to fail sooner.

1.) I think we all observe that bulbs tend to blow when you turn them
on.
(unless you knock the lamp over or something.)

2.) I assume that the failure is mostly due to the thinner �hot spots�
on the filament. * Thinner regions heat up faster (higher resistance
with equal current).

3.) Now even if the thinner region doesn�t blow, it still gets hotter
and loses a bit more tungsten than the rest of the filament. * (For
that small amount of time that it�s turning on.) *But still this means
that turning on the bulb causes the thin region to become a bit
thinner.

And that�s it. *Repeated on and off means that the thin region has a
higher average temperature than the thick part of the filament. *It
evaporates faster and fails sooner.

George H.

If the effect is real, that sounds like a good candidate for a
mechanism. * Certainly you'd expect that to be important right near the
end of the bulb's life, so maybe it's important throughout.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text -

- Show quoted text -

Yeah, I was thinking about this while splitting/stacking wood
tonight.
If the time to fail goes as some big power of the voltage
(temperature),
then during turn on, small diameter variations (or defects)
get amplfied.

A 'long life' 40 Watt bulb would fail almost as fast as a 100 W'er.

(Of course I've got my 'lifetime supply' of 100W bulbs, and didn't
budget any for research.)

George H.