Running high RPM fans on 5V instead of 12?

[suszterpatt]

Just wondering, how would a high performance fan on 5V compare to a regular "quiet fan"? Especially in terms of noise levels.

 

[imported_Kensai]

Alot of people do this. Depending on the fan it could be extremely useful or extremely useless.
A fair number of people also get high performance fans and stick them on a fan controller so they can pump more voltage to the fan when they need it more.

 

[Safeway]

But you have to make sure the fan controller you get can do 5V. The Zalman fans tout no motor noise at 5V and still okay stats - 21 cfm, 20dbA.

The Vantec Nexus will only do 7V - 12V. The Sunbeam will do 0V - 12V though. You can maybe buy a modded fan lead with a resisitor to step down voltage too - I know they sell 12V to 7V pass-throughs.

 

[Zepper]

Most PC fans have a working voltage range of 7-13.x Volts so some fans may not start or run on 5V. But some fans will run at 5 if you start them at a higher voltage and then reduce the voltage by one of the usual methods. One of my fans (Sanyo-Denki 120mm) has a very narrow working range of around 10 to 13.x Volts...

.bh.

 

[suszterpatt]

Would simply hooking up a molex connector to the 5V rail of the PSU (as opposed to the 12V one) work? Do fans actually check their voltage and not start if they're under/overvolted?

 

[Fullmetal Chocobo]

Well, it will either work or it won't. I don't know of any fan that has the circuitry for voltage detection, so you don't have to worry about that.
Tas.

 

[suszterpatt]

"Work" as in provide a good performance/noise ratio or "work" as in spin?

 

[Fullmetal Chocobo]

Originally posted by: suszterpatt
"Work" as in provide a good performance/noise ratio or "work" as in spin?

Work as in go roundy-roundy. hehehehehehe. As for a good performance / noise ratio, the best thing to do is either solder in a *rheostat*, or just get a rheobus, and install the fans to that. That way you can change it to how you need it at the time (summer / winter changes, etc).
Tas.

EDIT: Thanks Cerb. Rheo, not potientiometer... Sorry.

 

[suszterpatt]

Well if there's no voltage check then I really don't see a reason why they wouldn't spin, unless I'm missing some technical details (which is very possible btw).


Another possibly dumb question: would a fan that does 2400RPM @ 12V slow down to 1000RPM (2400 * 5/12) @ 5V?

 

[Fullmetal Chocobo]

Think of it like this. If you have a pressure switch that only activates at 23 psi. But you only have the water running at 20 psi, so it doesn't turn on. Same thing. Having a fan at 5v just might not be enough to drive it--so it's operating voltages are 7v - 12v. This is just an example though.

As far as the rpms per voltage, I don't know if it is a linear transition or what. It would be interesting to try it out though...
Tas.

 

[Cerb]

Solder in a rheostat, NOT a potentiometer. Many rheostats can also be potentiometers, but a typical potentiometer isn't going to be made to handle that kind of power. A rheostat would be annoying because you'd have to spec it based on the power the fan uses at full speed.

 

[Zepper]

Fans have to overcome mechanical inertia and friction as well as air resistance to get started - once started, fans don't have to deal with some of those factors any more so they require less power to keep turning - just as a car only needs a relatively small amount of horsepower to keep moving once gotten up to speed.
. Just basic Physics as should be learned in high school. Comprende?

.bh.

 

[Navid]

Originally posted by: Zepper
Fans have to overcome mechanical inertia and friction as well as air resistance to get started - once started, fans don't have to deal with some of those factors any more so they require less power to keep turning - just as a car only needs a relatively small amount of horsepower to keep moving once gotten up to speed.
. Just basic Physics as should be learned in high school. Comprende?

Another example to think about could be a bicycle. You need to exert force to get the bicycle going. But, after you get going it needs a lot less force.

 

[suszterpatt]

Actually, air resistance is one of the things that plays more of a role at stopping the fan from spinning than preventing it from getting started. Also, inertia does nothing but reduces the acceleration (which is true for air resistance too IIRC). Friction and magnetism are the only things that could keep the fan still.

 

[Cerb]

Or how about a motor, since that's what these are, they're just simple DC ones, not ICEs. If they are started without enough oomph (voltage), they just sit there and try to turn. Once started, they can run with much lower oomphiness. However, if it goes too low, they will stall.

So, a 12v fan might need 7v or more to start, but then run as low as 4v without stalling. The stalling is what you have to worry about, especially with faster-spinning fans, which tend to lack a bit in torquiness. Most fans do fine at 7v, though.

 

[rogue1979]

Just hook the fan to a molex in a 5-volt configuration. If it spins, you're good to go, if not simply go the 7-volt variation.