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I have a small desk fan that is currently powered by 4 AA bateries, I have measured the voltage output and it is 5.5VDC which got me thinking. Is it safe to modify the fan to run off a USB port on my PC?

I'm not sure if the fan draws more than 100mA, but it is my understanding that USB will only allow me to draw 100mA by default anyway. Is this correct, or could my fan draw too much power?

I also read this

A dedicated charger can supply maximum 1.8 A of current. A portable device can draw up to 1.8 A from a dedicated charger. The dedicated charger shorts the D+ and D- pins together disabling data transfer and will not send or receive any information, allowing very simple, high current chargers to be manufactured. The increased current (faster charging) will occur once the host/hub and devices both support the new charging specification.

Is this supported by USB 2.0, or will this not be supported until 3.0?

should I short the data wires in my case or just leave them disconnected?

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Why don't you measure the current usage of the fan? Be sure to measure the entire life-cycle including the spin up as this may draw slightly more to start. –  basszero Sep 1 '09 at 19:55
    
Measuring current is definitely worth doing. There also might be a Wattage written on the fan. Anything over 2.5W is suspect. –  hanleyp Sep 1 '09 at 20:24

2 Answers 2

up vote 2 down vote accepted

The USB 2.0 specification allows up to 0.5A of current draw from the 5V pin from each port. You can leave the D+ and D- wires unconnected, just connect power and ground.

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I thought the 0.5A was only in high power mode, which could only be turned on by request from the device? the fan cannot request high power (it has no software), so won't it be limited to low power (0.1A)? –  Tester101 Sep 1 '09 at 18:14
    
That is for an actual USB device. If you are just tapping the power off of the USB port, then the max is 0.5A per port. Functionality may vary depending upon if you are using an unpowered hub or have a bunch of high power USB devices plugged in. All USB power connections have resettable fuses, so, unless you are drawing very high current upon power-up the worst that is likely to happen is the port will be shut off. –  hanleyp Sep 1 '09 at 19:56

You can use a USB Y cable to draw more power. They're commonly used for portable hard drives and DVD burners, requiring more power than can be provided by a single USB port.

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I don't really need more power, I just want to make sure the fan will not try and draw too much. As for the wire. I have already clipped the end off an old USB cable and rewired the fan. This question is about power consumption, not what cable to use. –  Tester101 Sep 1 '09 at 18:40
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+1 I finally understood what the purpose behind these weird cables! –  harrymc Sep 1 '09 at 19:53

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