1 USB Port able power 2 desktop fans?

Hi all, I'm planning to self build a USB cooler of sorts for my laptop.

Initially my plan was just to cut a USB cable, use the black/red to join with the black/red of a single fan. Straightfoward. Now I'm just thinking that I might be able to do a similar set up with 2 fans?

I'm not too good with electrics and energy, so this is where my question comes in:

Since a USB port typically is able to output 5V and a max of about 480mA,
if I had say 2 idenfical desktop fans, each requiring 5V and 100mA each,
will I be able to to use 1 USB port to power these 2 fans simultaneously?

If it is possible, what would be the best setup for wiring to have these two powered simultaneously?


Thanks in advance.

 

Normally the USB port outputs only 100mA. Only after the device communicated with the port and said 'hey, I need more' it's allowed to consume up to 500mA.
Still, even without such a communication, most USB ports deliver those 500mA.

If you connect both fans parallel, then each device will run at 5V, consuming 100mA=>both fans will consume 200mA. So this should work without a prolem.

 

Ok, thanks.

Correct me if I'm wrong, but the USB port delivers more than 100mA upon request for devices that sends signals back and forth, right?

So in this case where it's just 1 way power output, then how will this set up draw the 200mA it needs?
Will it simply 'force' more current out of the port?

If it works, it works.. but I'm just curious whether this will have any negative impact on my laptop as a whole, or the usb port, or even the fans?

 

The hub can provide at most 500mah per port. Since both of your fans combined are 200mah, your system will be able to provide 200mah to power your fans. As Frank mentioned, when a load is applied the USB device communicates to the hub, where it can output more current to the device.

Running a continuous 200mah load will not hurt the system in any way

K-TRON

 

Thank you both for your replies.

It is clear now that as long as its under 500mA, it should be fine.
I still got one question/issue, though.

I've been reading alot about this on the net, and the only thing that still disturbs me is that the USB specifications mention something along the lines that a 'compliant' device should not draw more than 100mA upon start-up.

So if my contraption draws 200mA or more upon plugging it into the USB port, will this cause any sort of surge or potential damage to my laptop in the long run?

I couldn't care less if my contraption dies on me after a year or so... but my laptop is a little more precious. ;P

EDIT:
Quoting some text from a website called USB Made Simple, should these be any concern for me?

"Firstly it is important to realise that if you pull a plug out at the far end of the cable from the device while current is being drawn, then the cable will develop a potentially large flyback voltage across your device."

and

"The second thing to consider is that when you plug your device in, any capacitance between Vbus and GND will cause a dip in voltage across the other ports of the hub to which you are connecting."

 

USB is 5v most desktop fans are 12v though some run under lower down to about 7 but 5 is about out of the question.

You will need a special fan to do this, not just your general desktop fan.

My cooler had a dual mode, using a fan that can run at 5v it would run on usb power but it was wired to a dc adapter for 12v from the wall. So if I was near a plug I could plug it in for high speed mode.

I plan to maybe build one again out of a Cryo cooler if i can find one cheap.

 

Most 12V fans can run to about 5V, but the performance is about 1/4 of what it would be at 12V. Some fans can run all of the way down to 3V from 12, and some may only go down to 9V from 12V. Its all in the way the fan was designed.
Say your fan was 12V at 1Amp
When you run it at say 5V, the amperage required to run it would be say 0.4amps
The amperage needed drops as voltage decreases.

K-TRON

 

The other factor is the "startup" voltage some fans may run at 5v but need 9v to start spinning, its usually a separate stat often given on fan specs.

I have not had a desktop fan than runs on 5v, Using a molex in my PC lowest I could go was 7v.

 

Each of your fan consumes maximum of 100mah of power at 5v. The actual energy used is much less than that. You're fine with 100mah @5v.

BTW.. to above poster, as voltage drops, the amperage will increase, just not in a linear function.

With either a drop of amperage or voltage, the fan speed will drop.

95% of 80mm fan will start under 5v whereas only 50% of the 120mm fan start at 5v.