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Why use DC?

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April 22, 2001 6:49:48 PM

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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April 23, 2001 11:07:57 PM

can we even get AC fans small enough?

The only AC fans I've ever seen are 170mm 60Dba monsters that push 240 cfm :) 
April 24, 2001 4:48:29 AM

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? --
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May 1, 2001 2:06:34 PM

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.
May 1, 2001 4:38:12 PM

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!!! --
May 1, 2001 6:03:32 PM

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.
May 1, 2001 8:44:34 PM

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!!! --
May 2, 2001 11:53:06 AM

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.......
May 2, 2001 5:05:02 PM

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.
May 2, 2001 6:25:51 PM

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.
May 2, 2001 9:13:21 PM

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.
!