Why is my homemade usb fan not working? I need a rocket scie

bongoyvr · Post by **bongoyvr** » Sat Apr 04, 2009 10:41 am

so i'm trying to make one of those usb powered fans that are all the rage on youtube

i've got the black and red wires connected but when i plug it in the blade moves maybe 1cm and stops. really the movement is hardly nothing at all. whats up? any ideas?

i also tried switching the wires - red to black and black to red and nothing.

i don't need to be a rocket scientist for this, do i?

pic at

Vicotnik · Post by **Vicotnik** » Sat Apr 04, 2009 11:03 am

Are you sure 5v is enough to start the fan? You could try connecting it via a molex first to verify.

bongoyvr · Post by **bongoyvr** » Sat Apr 04, 2009 11:08 am

actually i don;t think 5 volt is enough to make it go 100%.

im a super newb and did not take into account voltage.

i need to run it from 110 5volt - any way to make a 12 volt fan do that? i tried playing with the wires but no joy.

i did red to red and black to black = nothing

i did red to red plus yellow to red and black to black = nothing

i did red to red plus blue to red and black to black = nothing

Mr Evil · Post by **Mr Evil** » Sat Apr 04, 2009 12:32 pm

It's possible to step 5V DC up to 12V, but I don't know of any off-the-shelf products that will do that, so I would suggest buying one of those handy Molex PCI brackets so you can plug the fan straight into 12V.

thepwner · Post by **thepwner** » Sat Apr 04, 2009 2:13 pm

Ya, but look at it like this, the max power from 1 usb port is 500ma if I am correct. So that's 5 * .5 = 2.5W If you step up 5v to 12v, assuming you can get 100% efficiency, you get only 203ma. Now I don't know about the current draw of that Arctic Cooling fan, but I just picked up a spare ADDA 120mm I had laying next to me (used to be in my Corsair 400w) uses .33A or 330ma. So as you can see only a low power fan can run off of 5v stepped up to 12v at 100% efficiency (which I don't think is possible.)

Now if you plugged into 2 USB ports, you could get that up to 406ma. Or just run the USB ports in a series (can you do that without them blowing up? I doubt it) and get 10v at 500ma, that sounds like your best shot.

Mr Evil · Post by **Mr Evil** » Sun Apr 05, 2009 9:08 am

thepwner wrote:Ya, but look at it like this, the max power from 1 usb port is 500ma if I am correct. So that's 5 * .5 = 2.5W If you step up 5v to 12v, assuming you can get 100% efficiency, you get only 203ma. Now I don't know about the current draw of that Arctic Cooling fan, but I just picked up a spare ADDA 120mm I had laying next to me (used to be in my Corsair 400w) uses .33A or 330ma. So as you can see only a low power fan can run off of 5v stepped up to 12v at 100% efficiency (which I don't think is possible.)

Now if you plugged into 2 USB ports, you could get that up to 406ma. Or just run the USB ports in a series (can you do that without them blowing up? I doubt it) and get 10v at 500ma, that sounds like your best shot.

Now you mention it, it is cutting it a bit fine. DC-DC converters are very efficient though, with >95% efficiency being easy to achieve and >98% not being unheard of, and most fans seem to use less than the specified current under steady-state, so it would probably work with most standard fans.

You can't connect two USB ports in series because they are not isolated from each other - you'll just short one of them out.

angelkiller · Post by **angelkiller** » Sun Apr 05, 2009 12:09 pm

This is just a guess...

Is it because it's a PWM fan? IIRC, PWM fans run at full voltage (12v). So they may not be as flexible as most fans are in terms of starting voltage. Maybe you should use a non-PWM fan.

ntavlas · Post by **ntavlas** » Sun Apr 05, 2009 1:58 pm

That`s correct, 4 pin fans are meant to run at 12v only and are much more likely to stall at low voltages. Most 3 pin fans however have little trouble starting at 5 volts.

In spcr`s recommended list you can see how different fans run @ 5volts, pick the one whose speed suits you best.

jhhoffma · Post by **jhhoffma** » Mon Apr 06, 2009 11:08 am

angelkiller wrote:This is just a guess...

Is it because it's a PWM fan? IIRC, PWM fans run at full voltage (12v). So they may not be as flexible as most fans are in terms of starting voltage. Maybe you should use a non-PWM fan.

Ding ding ding!!!

We have a winner! The nonPWM version of this fan has a starting voltage of 5.3V, but also runs at a slower speed (1000rpm).

As angelkiller/ntavlas suggested try a standard fan and see how that works.

Plekto · Post by **Plekto** » Wed Apr 08, 2009 5:14 pm

Also, trying to power devices like this directly from your computer can and will eventually burn out your USB ports. This can work pretty much as-is, though, from most powered USB hubs. They tend to support up to 5-10W on average, which is far more than you'll need for this.(but it's really then, just drawing 5V from a power brick... not quite as cool and you might as well jsut run the fan directly off of the brick...)