dual-filament light bulbs

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dual-filament light bulbs

Postby spacefem » Mon Feb 22, 2016 1:28 am UTC

I don't know if this is a math, science, or hardware question... figured I'd post it in the highest forum so it gets the most attention.

bit of a debate between a colleague and me.

It all started when I went to change some lightbulbs that had gone out in my cheap little car - one side had a brake light out, the other a taillight. Oddly enough on each side, this involved changing out the exact same bulb. Honda chose to use a dual-filament bulb for the bottom red light, turn on your taillights and one filament lights up, tap the brakes and the other, brighter one does (it's got three wires to it - 1, 2, ground).

My opinion is that this is cheap crappy design. I have to replace a $7 bulb every time either filament fails, so in my opinion I've got a light bulb that's twice as likely to fail, resulting in more money out of my pocket and more light bulbs in the landfills. Sure, out of the factory designers probably loved this idea because they were buying fewer bulbs, wires, and bulb sockets. But all the other light bulbs are $4... I'm buying this expensive one that won't last any longer, and I'm mad about it.

But as life goes, somebody disagreed with me. This person says the dual filament bulb is a great idea, saving me money and reducing light bulbs in landfills because it's only one bulb. You have to replace it after a given lifespan anyway, now you're only replacing one bulb instead of two. I guess he feels like the lifespan of the non-failing filament would always be really well defined, it'd fail shortly after? That's not how lightbulbs are in my experience, I feel like they're really random, some last 2-3 times longer than others. With a two filament bulb you'll never know, you can't take advantage of a filament with an unusually good lifespan because you've got to throw the bulb out!

So okay nerds, what would you rather have? A car that requires a $7, dual filament tail/brake bulb? Or a car that requires a $4 bulb for the tail light and a $4 bulb for the brake light? What does the eco-friendly side of you say? Do you think light bulb lifespans are easy to predict? Would you be frustrated throwing away a bulb that half works?

Is this cheap crappy design or a resourceful idea?

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Re: dual-filament light bulbs

Postby Xanthir » Mon Feb 22, 2016 2:29 am UTC

I'd prefer an LED that can vary its intensity and will last the entire lifetime of the car.

If we're stuck with filament bulbs, I'd have to see the actual lifetime stats to see which one was a better deal. My intuition is that you're right - it's not like you're stressing the bulbs any less, so you should expect the normal lifetime out of each, and that means you lose any "excess" lifetime in the filament that survives whenever one of them dies. The stats'll do it, tho - if the bulbs typically last approximately the same time, then the single $7 is cheaper than two $4 bulbs.
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Re: dual-filament light bulbs

Postby quantropy » Tue Mar 01, 2016 12:37 pm UTC

From http://bits.blogs.nytimes.com/2009/02/11/how-long-did-you-say-that-bulb-will-last/?_r=0
When it’s said that a standard light bulb will last 1,000 hours, that is the mean time to failure: half the bulbs will fail by that point. And because lamp manufacturing has become so routine, most of the rest will fail within 100 hours or so of that point.

This would support you're friend's point of view (but I don't think the author of that article is an expert)

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Re: dual-filament light bulbs

Postby Spike » Tue Mar 01, 2016 12:48 pm UTC

Clearly not. He's confused median and mean, for a start.

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