h a l f b a k e r yQuis custodiet the custard?
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Do you have linky for light bulb shock? |
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For domestic filament bulbs, the problem
is that they run on AC. A capacitor will
just charge and discharge as the polarity
alternates - it won't work. You'd need
some additional circuitry in the bulb (or
switch). |
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isn't this the complaint people have with low energy lightbulbs? (that they take time to fully light) |
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Google for "bulb saver" and you'll find a little thingy that fits under the bulb. It may not do any good, as the start-up isn't really the problem. |
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250ms isn't a noticeable-to-the-eye delay, is it? |
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A quarter of a second is noticeable, but seldom troubling in this application. |
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//It should work. (+)// No, it shouldn't.
See above. You can't just stick a capacitor
across an AC-driven lightbulb, whether in
the switch or elsewhere. It won't work. |
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Most devices billed as "bulb savers" are just rectifiers - that is, they pass only one half-cycle of ac current, so the bulb is dimmer, and uses less power, and theoretically lasts longer. |
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There is a device called an "inrush current limiter," which is a specialized NTC (negative thermal coefficient) thermistor. This operates on ac or dc, but must be sized appropriately for the application. Essentially, this device has high (a few hundred ohms)resistance at low current, and negligable resistance (0.01 ohms) at operating current. Time constant is determined by the thermal mass. |
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If you like the romatic aspect of dim lighting, you can buy dimmers that automatically dim lights over a specific period (5-10 minutes.) |
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If turning an incandescent bulb on and off doesn't decrease its life significantly*, is this idea bad science? |
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//* I remain unconvinced.// Bulbs have
several failure modes. The commonest
are: |
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a) Imperfect sealing, leading to gradual
influx of air sufficient to oxidize the hot
filament |
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b) Slow evaporation of metal from the
hot filament in a vacuum |
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c) Mechanical failure of the filament;
every on/off cycle causes expansion/
contraction of the filament (and, to a
lesser extent, its supports), which will
eventually lead to mechanical failure. |
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Manufacturers adjust the construction
of the bulbs to balance these failure
modes, and to give an expected
lifespan that's best for them (not too
long, not too short). In general, this
means that mechanical failure (c) is the
commonest mode of failure, though it's
exacerbated by (a) and (b). |
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Mode (c) is made worse because the
resistance of the filament is lower when
it is cold - so there is an initial surge of
current, which can lead to a brief
overheating before everything settles
down. This effect would indeed be
eliminated by "soft" switches. |
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I can't honestly remember when I've
seen a light bulb die while it was
on in my own house - they
always seem to fail when first switched
on. |
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On the other hand, my mother's house
had a bad mains supply (I guess it was
at the end of a long line), and bulbs
there would fail frequently, often while
they were on - I think that's because
the power fluctuated, effectively turning
them off briefly now and again. |
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So aside from the dodgy circuitry, it's good science? |
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Yes to all three of those. |
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[admin: I'm ignoring the widely known to exist, since this is different from a normal dimmer switch - it's an automatic process happening at turn-on/turn-off, not a brightness control. I believe this exists - csea's thermistor is pretty close - but isn't widely known or used in home applications.] |
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