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I've got a lighting fixture that says it wants 5v .6w bulbs. literally every one burned out, so he two spares aren't enough. All the other ones I see now are 2.5 V. Can I swap these in? My guess is yes - they'll just draw whatever current makes sense, and since they're lower voltage won't generate more heat. Just want to make sure, for safety reasons.

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    There's a reason they all burn out. Your fixture sounds very old and it may be time to rewire it for modern bulbs, preferably LEDs. – Harper Oct 31 '18 at 16:11
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    How does one arrive at a state where every bulb is burned out? Did you actually leave the string in service as they, one-by-one, went dark? Or did something in the string fail, like a fuse? – isherwood Oct 31 '18 at 16:42
  • you could rewire them, putting two 2.5v in series to make a 5v one. that sounds like a lot of work for cheap lights though... – dandavis Oct 31 '18 at 17:00
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    @isherwood I've had a couple of strings fail in a similar way. As each bulb fails, it's supposed to short out so it doesn't interrupt power to the rest of them. The problem is that this decreases the total resistance of the string, increasing the current through it. This increases the failure rate of the rest of the bulbs. Once you lose enough bulbs, the failure rate goes very high and the remaining bulbs die pretty much all at once. If you buy cheap lights (like I do), one of the bulbs likely won't short properly and a few bulbs survive. – mrog Oct 31 '18 at 18:03
  • @Harper it’s not that old. It’s more cheap than old. Not made for 24/7 use. – Rich Homolka Nov 4 '18 at 15:40
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My guess is yes - they'll just draw whatever current makes sense, and since they're lower voltage won't generate more heat.

No, that's not how it works.

Bulbs that are made for 2.5V can't be supplied with 5V, they'll draw twice as much power and burn very bright, but will probably burn out very quickly.

If you underpowered the bulbs - used a 2.5V power supply on 5V lights - that would likely just make them glow dim - but they'd probably last a long time.

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    The constant (or nearly constant) is the resistance. V = IR so if you double the voltage, you double the current. Power = VI so if you double the current, you quadruple the power. – batsplatsterson Oct 31 '18 at 16:26

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