Why use DC?

In my limited experience I've found DC fans to be far noisier than AC fans. Why not use ac fans in computer cases? Is it just a matter of energy efficiency?
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  1. can we even get AC fans small enough?

    The only AC fans I've ever seen are 170mm 60Dba monsters that push 240 cfm :)
  2. One of those puppies blowing right on the motherboard would do the trick! Just let the air vent out the front and back.

    -- If you recycled your computer, would you get enough money to buy a pizza? --
  3. Actually the DC fans could be made alot more quiet if they used metal ball bearings - reason they don't - more than double the cost to produce. All the vbration/noise is transmitted to the fan shaft & then to the housing, etc. through the plastic bearings (which aren't highly toleranced & "jump" about) AC fans are usually higher powered and require metal bearings to take the load.
    hence quieter... ah well in a "money no object" world...
    sorry i'm a bit slow keeping up with threads - just not enough days in an hour (:^D)

    Advice - He who giveth, has probably taken his share already.
  4. I for one would be more than happy to pay $20 instead of $10 for a DC fan as quiet as an AC fan.

    -- *Ding Ding* Knowledge for the clueless!!! --
  5. The reason is power, and efficiency just happens to be a by product.

    AC can transfer a heck of a lot more power than dc, that's why your 'friendly' power company transfers electricity to your local sub-station at +200,000 volts (AC).
    If you ever look on the blades of a 120mm fan, the AC ones have a steeper pitch than the DC ones. The AC fan can turn at a lower speed, but pull more torque, and consequently more volume per rotation.
  6. I'll agree with that, but it doesn't answer the question, why use DC fans at all? AC seems a better alternative. An AC connector on a power supply would do the trick as well as save the DC current for the processor, mboard and video card.

    -- *Ding Ding* Knowledge for the clueless!!! --
  7. Because it's easier to hook up a molex connector than to wire a relay or splice into the power cord. I'm assuming that you'd want the fan on only when the computer is on.......
  8. You're right (as usual) - that fan hook up would cost . I mean a fan on all the time blowing hot air around all the time would be alright
    No... Wait! Do I need a fan for that.. :)
    I'm sure a fan manufacturer will respond to market demand for a quieter (more robust) unit when they realise people are more concerned with noise/power than say power/cost/weight.
    That was the original objective yes?

    Advice - He who giveth, has probably taken his share already.
  9. A long time since I studied electrical engineering and my knowledge of IEEE regs is out of date but...

    I'm pretty certain you can't have mains AC in a PC case (outside the PSU), sure wouldn't get CE approval so it probably goes for America too.
  10. I can remember one post several months ago, and someone was talking about a German manufacturer who made rather large DC fans that were extremely quiet. I didn't really bother with checking it out, as the shipping to the US would be the same as the fan.

    It's not tough to wire up a AC fan, especially if you use a surge supressor to turn your computer off/on.
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