The reduction in life of an incandescent lamp when operated on DC versus AC is
attributable to a phenomenon known as "DC notching." With DC applied, the
tungsten molecules migrate within the filament and the filament takes on a
"notched" appearance. The increased local resistance at the notch points along
with the reduced physical strength result in decreased life. If AC is applied to
the filament, this condition is avoided. A lamp operating on DC may have 20% to
50% of the same lamp operated on AC.
Much research was done on this at NASA-Goddard by their lamp expert Dr. Henning
Leidecker. One of the life-limiting factors on the earlier GOES spacecraft (and
others) was the life of the black bias (incandescent) lamp in the satellite
sensors.
Stu Benner
W3STU
Quoting Martin Ewing <martin@aa6e.net>:
> OK, I've heard this said before. But inquiring minds want to know: why
> should DC be worse than AC for lifetime in lighting applications?
>
> I seem to recall it was an issue for vacuum tube filaments, presumably
> because having a DC gradient on the filament led to unequal
> concentration of emission on the filament. (On the other hand, AC
> supplies could lead to hum.) But in a lighting situation?
>
> I can think of a couple reasons why DC might be better: no AC-induced
> vibration, unity peak to average power, etc.
>
> 73 - Martin, AA6E
>
> >
> > Subject:
> > Re: [RFI] 120v halogen lights
> > From:
> > "Tom Rauch" <w8ji@contesting.com>
> > Date:
> > Sun, 4 Apr 2004 10:23:22 -0400
> > To:
> > <rfi@contesting.com>, "Pete Smith" <n4zr@contesting.com>
> >
> >
> >>I assume that there is no reason why any real transformer that delivers
> >>12.6vac couldn't be used -- perhaps even an AC wall wart.
> >>
> >>
> >
> >That's right, as long as it is AC. If you run DC bulb life is shorter.
> >
> >73 Tom
> >
> >
>
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>
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