# What if I try to connect a [email protected] computer case fan to my USB port?

What happens if I try to connect a [email protected] computer case fan to my USB port?

Purchased a USB laptop HDD enclosure lately but it coming with no fans.

So I thinking of putting fans around enclosure opening for keeping cool.

[email protected] = 1.92W


So if I providing fan with 1.92W it not care if source voltage not 12V but maybe less?

Like, if connecting to 5V, it draw 1.92/5 = 0.384A or say 0.4A that most USB hardware port able to source?

Am I taking risk or doing my calculating correctly and so nothing to worrying about?

There is similar question, but I also interest in knowing why a 12V fan might not start up at 5V EVEN if I able to supply it same power?

In addition to the starting problem Roy mentioned:

• The fan wouldn't draw 384mA from a 5V supply by something more like 66.7mA (the impedance of the fan is 12/0.16 = 75 ohms). Given the nature of fan motors this likely isn't precise but point is, you won't get 'full power', you can't 'force' more current into the fan than it draws based on its impedance and the supply voltage, that is not how electricity works.

• The USB 2.0 spec only allows supplying 100mA 'freely' to draw more a negotiation procedure has to occur, that is you'd need an actual USB end point to ask for more current. The current is usually physically limited by the host controller to protect itself. Thus you need a 5V fan that draws < 100mA, a fan with the controller circuitry to ask for more current, or a 'USB wall plug' type device as they often will supply current without any negotiation.

• I read USB papers and they saying you can draw as much current as the port can physically sourcing. Computer port actually never limit current but if you not negotiate higher current, your device not USB compliant and you cannot put USB mark on it. But nothing stopping you from drawing max current without negotiation. Please correcting me if I misreading papers. Commented Sep 20, 2011 at 15:57
• @SGhosh - USB ports do limit the current. 500 mA is the quoted USB spec per port, which is why some external drives powered by USB require 2 plugs. Commented Sep 20, 2011 at 16:03
• @SGhosh depends on the host controller but the most of the implementations i've seen, and virtually all laptop implementations do limit current draw.
– Mark
Commented Sep 20, 2011 at 23:48

The good thing is, trying a 12v fan in there should not harm your computer in any way, it just won't be likely to start.

As it says in the question you linked to, most 12v fans will not start up with 5v - they require a higher voltage to overcome initial friction. This is why they say 12v. If you get yourself a 5v fan, with a current requirement below the limits of your USB port you will be able to run it.

In response to your queries re current draw:

USB devices may draw up to 100 mA before "negotiation" with the host, after which they may draw up to what the host gives them permission to draw, which is limited to 500 mA due to physical interface requirements (USB cables, connectors, etc are designed for no more than 500 mA)

• Not likely to start as impedance of fan not allow it to source same power @5v as it can @12V? ONly this reason or other reasoning also? Commented Sep 20, 2011 at 15:58