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Hi,
just tried to power up a little 12V cpu fan using a USB cable. I've soldered the right wires (black and red) to the fan and turned on the PC. The fan worked 50 seconds, and the PC began to make a high-pitched short squeals from the PC beeper that looped infinitely. I've saw all keyboard LEDs blink (like when bios initializes the keyboard). Keyboard with mouse weren't responding, the only thing I could do was reset the PC. When the PC rebooted, BIOS couldn't find the keyboard so I switched the PC off for a while and then everything was fine. I was using the old testbed to test my fan gizmo thing.

Where could be the error? I have read that USB can deliver max. 5 volts and the current drained should be no more than 500 mA. As long I don't have the ampermeter handy, where could be the possible error? Short circuit? Overload?

Thanks for your response.

USB decorations like that are completely against the standard... such USB lights... mug warmers, etc.

I wouldn't mess with that... but perhaps it wasn't properly grounded, perhaps you accidentally put current on one of the data pins.

I've tried to follow Wikipedia's article on the USB wiring - but maybe you're right. Well, I'll try to use a 12V switchable mains adaptor instead of a USB cable.

Hi,

IIRC, USB devices prior to enumeration can only draw 100mA. After enumeration, a high powered device can draw 500mA, as you say (I think some laptops do not provide this and you may also be in trouble if you have multiple devices with an unpowered hub). I think that for a project like yours, an ammeter is absolutely essential (and they are pretty cheap too).

A 120mm fan may draw around 150-250mA, but I assume a cpu fan (40mm?) would draw a lot less, but I still think you need to get siome proper measurements.

Cheers,
Adam