Overclock.net banner

Volt vs Amps - Custom USB fan question

5K views 10 replies 4 participants last post by  ryan55000 
#1 ·
Hello all,

I've had a recent surge in needs for usb based cooling such as modems, routers, etc. I had bought a 120mm usb fan a while ago and it was simply a case fan converted from what I could see. So instead of buying more I've decided to make my own as I had some usb cables and tons of 120mm fans lying around.

That said, I've done a few test runs and it's obviously pretty straightforward. My concern comes into play with the amperage of the fans I use. I will say this now, I am definitely not an electrical engineer and will probably be asking a simple question I slept through in school here...

I understand 99% of case fans are 12v. I also understand that USB is 5v. I also understand (please correct me if wrong) that the max amps supplied by usb 2.0 (what I'll be using) is 0.5A / 500mA as often listed in my research.

Now one 120mm fan I tested was 12v .22A. On 5v it started up fine, but barely moved any air whatsoever. I had another fan that was 12v .40A, which is moving around the amount of air I'm looking for. On regular 12v power this was actually quite a loud and powerful fan in general It is also LED if that matters (not a requirement for my needs just happened to have it).

That said, where I am confused is on amperage draw. Now .4A is obviously less than the max output of usb's .5A however from my reading as the voltage supplied goes down doesn't the current (amps if I'm not mistaken in words here) required go up? Would this effectively mean the fan that normally requires .4A on 12v require more to run on 5v? The fan is certainly not spinning at near the speed or strength of when it was on 12v and the wire is certainly not hot nor is the port or any sign of issue.

On the flip side of that, if it does NOT cause the current to go up to compensate, does the current drop or does it remain constant? or go lower For example a 60mm fan I'd like to use with usb is actually showing .12v .70A. Which first off surprises me as the .40a 120mm fan is very powerful as is so I'm surprised this tiny 60mm needs .70A. So like I said, would this, if it dropped current on 5v, mean the current requirements would drop under the .50A limit of USB?

Thanks for all the help as always. My main concern here is safety as these fans will be on 24/7 in some cases and a fire or burnt out usb or even fan is the last thing I'd like to deal with.
 
See less See more
#2 ·
Your best bet would be to look for 5 volt fans.

From what I have seen all of these fans take LESS current as the voltage drops(as would a lightbulb and many other things.).

Computer fans generally tend to take less power than the sticker lists as well, but the startup should be a a heavier load.

To get upto 1.57 amps I needed all these fans(I wanted something to load a cheap switching regulator from amazon).



I have an image with them at a lower voltage I will try to dig it out.
 
#3 ·
If it was working as hard at 5V as it was at 12V then amps would go up.
But that on 5V its only working half as hard (well not even that) amps will go down.

If you wanted to increase the fans speed you could get a buck/booster.
If I was you id just get the mult-meter out and measure how many amps its really using since the stickers can be very miss leading.
 
  • Rep+
Reactions: SoloCamo
#4 ·
Quote:
Originally Posted by Nukemaster View Post

Your best bet would be to look for 5 volt fans.

From what I have seen all of these fans take LESS current as the voltage drops(as would a lightbulb and many other things.).

Computer fans generally tend to take less power than the sticker lists as well, but the startup should be a a heavier load.

To get upto 1.57 amps I needed all these fans(I wanted something to load a cheap switching regulator from amazon).

I have an image with them at a lower voltage I will try to dig it out.
Wow, assuming those were all connected that was 10 fans running off 12v and only pulling roughly 1.6A? Were they even moving or was it too little to start them (camera's can be a bit tricky with fan movement)?

Being that I already have tons of 12v fans and spare usb cable I can dedicate to this I'd definitely like to stay this route before buying anything as it's not a big priority either way currently. Either way then, with the fans I listed above on 5v (the .7A listed 60mm and the .4A listed 120mm) they should be perfectly safe? The .40A one is pretty much perfect for my needs as far as cfm to noise goes.
 
#5 ·
Here was the same fans @ 7 volts(different power source, but I was looking at current off the cheap dc-dc[buck only so lower voltage can only go down] converter.).

As you can see the current requirement dropped(remember this is 10 fans).


The idea of running a boost converter works, but WILL take more power(but get you full speed) since you will need more current to get upto 12 volts(and a bit more for converter efficiency). Not that I think any average fan will pull enough to damage a USB port(I have an old dell rated at something like 1.5 or 2 amps.).

It may help to think it this in watts.

A usb port is rated for 2.5 watts(2.5 / 5 = 0.5) while the same 2.5 watts @ 12 volts is only 0.21amps(2.5 / 12 = 0.2083333333333333). Most computer fans are very power friendly(unless you get very powerful units).

EDIT
All fans ran fine once I replaced the transformer that could note handle the load. In the image they are all running(camera just stops them.).
 
  • Rep+
Reactions: SoloCamo
#6 ·
Thanks for the fast replies guys, it makes me rest easy knowing nothing should randomly burst into flames on me. Now off to make a crap ton of these things.
thumb.gif


+rep to the both of you
 
#7 ·
As Nukemaster said you will have an initial current spike when you turn on the fans. This is a characteristic of any inductive (fan, motor, etc) device. The current it draws when running should be less than what it would run at under 12V. However, be aware this initial surge may cause damage to a USB port. I'm not terribly familiar with the internal layout of USBs but I'm going to assume that each has an in line fuse for current protection, this surge could be enough to blow this fuse.
 
#8 ·
Quote:
Originally Posted by ryan55000 View Post

As Nukemaster said you will have an initial current spike when you turn on the fans. This is a characteristic of any inductive (fan, motor, etc) device. The current it draws when running should be less than what it would run at under 12V. However, be aware this initial surge may cause damage to a USB port. I'm not terribly familiar with the internal layout of USBs but I'm going to assume that each has an in line fuse for current protection, this surge could be enough to blow this fuse.
Most usb ports have I seen use a polyfuse type of protection. They just cut out when overloaded, but on the plus side they are somewhat slow to react(like a slow blow).

If you have a 2.5 inch drive it will not start on some boards because of over current on startup.

I would take some fan measurements if you want(test with a board that I KNOW has working current limiting), but my meter would never catch the startup because it is just not that fast.
 
#9 ·
Quote:
Originally Posted by Nukemaster View Post

Most usb ports have I seen use a polyfuse type of protection. They just cut out when overloaded, but on the plus side they are somewhat slow to react(like a slow blow).

If you have a 2.5 inch drive it will not start on some boards because of over current on startup.

I would take some fan measurements if you want(test with a board that I KNOW has working current limiting), but my meter would never catch the startup because it is just not that fast.
Yea you would have to break out the oscilloscope and get a capture for that. Best of luck.
 
#10 ·
Quote:
Originally Posted by ryan55000 View Post

Yea you would have to break out the oscilloscope and get a capture for that. Best of luck.
If only I had one
frown.gif
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top