A
Andrew Gabriel
Guest
In article <gst1vb$453$1@news.motzarella.org>,
James Sweet <jamessweet1@trashmail.net> writes:
even a coiled coil doesn't enable the filament to run hot enough.
They have to be under-run as compared with gas-filled lamps to get
a reasonable life, although it's still only 75% of the life of gas-
filled lamps. Needless to say, they're horribly inefficient too.
They are standard shape (GLS as we call them) light bulbs of 25W or
less. (This means they can be used in the rain without risk of glass
shattering, because without any gas-fill to convect heat to the glass,
it runs much colder. Commonly used to make up strings of coloured
carnival lamps.)
Also the various formats of filament strip lamps, although these
are only single coil because they need the filament stretched
along the whole length of the tube. (Appalling things which should
have gone from the marketplace years ago.)
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
James Sweet <jamessweet1@trashmail.net> writes:
Yes. We do have some lamps where a vacuum has to be used, becauseAndrew Gabriel wrote:
Designing a full range of filament lamps to operate over a
wide range of voltages is a challenge rather like trying to
solve a set of simultaneous equations when there aren't
enough variables, so you end up with compromises, such as
lower efficiency.
To make a 240V filament, you need a very long thin wire.
It has far too much surface area to get to the 2700K operating
temperature without radiating 100W away at a lower temperature.
The way 240V filament lamps are made to work is to double
coil the filament, so that much of the filament is radiating
heat back onto itself, and the effective surface area for
radiating heat away from the filament is significantly
less than the filament's real surface area. This isn't as
effective as using an optimum thickness filament in the first
place, i.e. you have just the right surface area to radiate
100W when it's reached 2700K, and you still have a larger
surface area for filament evaporation and thinning.
And yes, you're right about heat loss by conduction down the
lead-in wires becoming increasingly significant at low
voltages/high currents. With longer fragile filaments, the loss
from the ends is proportionally less, but you may also require
filament supports, which are additional routes to lose heat,
and you lose more by convection to the gas-fill.
Most 120V tungsten lamps use a coiled coil filament as well, as do some
lower voltage lamps.
A major issue with long thin filaments is convective loss to the fill
gas. If you eliminate the fill gas and use a vacuum instead to solve
this problem, you greatly increase the evaporation rate of the tungsten;
out of the frying pan and into the fire.
even a coiled coil doesn't enable the filament to run hot enough.
They have to be under-run as compared with gas-filled lamps to get
a reasonable life, although it's still only 75% of the life of gas-
filled lamps. Needless to say, they're horribly inefficient too.
They are standard shape (GLS as we call them) light bulbs of 25W or
less. (This means they can be used in the rain without risk of glass
shattering, because without any gas-fill to convect heat to the glass,
it runs much colder. Commonly used to make up strings of coloured
carnival lamps.)
Also the various formats of filament strip lamps, although these
are only single coil because they need the filament stretched
along the whole length of the tube. (Appalling things which should
have gone from the marketplace years ago.)
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]