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The rating of the wall wart is probably nominal; there's little to say it can't give you more current if you didn't ask for it. The smoke means you were turning the potentiometer into a small heating element, which would make sense if say suddenly you were running several amps through it. A quarter watt is not much at all (12V at .3A is 3.6 watts.)
I always find DC sort of boggles the mind.
But it seems like for your purposes you could set aside the 12V wall wart and the (now-smoke-faerie-free) potentiometer and just cut the plug off the charger for a cell phone that you don't have anymore. Probably 5V. Probably spins the fan substantially slower, maybe slow enough (might overheat? who knows). Or run the two fans in series. As far as I can see the premise of a DIY yeast-stirring contraption is to just make whatever and call it good enough.
ETA: A really handy source for low-RPM cheapo motors is thrift-store citrus juicers. They come with gearing to get it down in the 10RPM range (grandma doesn't want her OJ on the wall, after all.)
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03-28-2013, 06:53 PM
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