ac fans like said run off the wall, which is ALTERNATING CURRENT, which kinda oscillates back and forth...if you hooked a meter up to it...not sure which meter, but i have seen it.
dc, is just straight current...no oscillation.
and since things in a normal computer run on dc, which is what comes out of the powersupply, dc fans are the way to go.
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DETAIL MAN AGAIN!
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-DAvid
-Live, Learn, then build your own computer!-