Weird fan

Ok, I made this old 120mm fan run off USB. I plugged it into my Ubuntu desktop, and it worked fine. But as soon as I plugged it into an old Dell, the Dell shut off. And this was a weird kind of shut off. The green power light was still on, but the screen was totally black. I shut it off by holding the power button, and restarted it. It ran chkdsk, and everything was fine. Any ideas on what happened?

 

Sometimes, when devices draw more current than the system can provide, the computer automatically shutoff. In this case, your fan is using too much power that a single usb port can provide. Normally USB port can only give your fan 0.5W of power which is not enough thus causing a restart.

 

See above post. Anyone who says other than this doesn't know what they're talking about.

 

Well, that explains it. But any idea why it worked on another computer? Is a weaker PSU the culprit?

 

You're lucky it didn't damage the other computer, honestly.
The other computer probably has one power rail that gets divided up between multiple ports.
ie, there are four ports and a 2A rail, gives a max of 500mAH per port which is the standard max for a USB device. If you had other devices plugged into nearby ports, I suspect they'd have malfunctioned.

 

OK, I think I get it now. The computer that shut down did have thing plugged into other nearby ports. Well, wont be doing this again!

 

I am always verrrry careful when making anything that runs off of a USB port's power. You can easily damage the motherboard unless you're very precise in your power draws.

 

Also... when you're talking about a device with motor, the instant it starts, it needs a surge of energy to get it going. Sometimes a 120MM fan will will need in excess of 1A at 12V (10-12W) to get it spinning.

Also.. according to the usb 2.0 standard, usb controller are only suppose to give your device 100mA at 5v usb power for unidentified devices. Once the computer "talks" to the device, the usb controller will supply the full 500ma at 5v. There's also new standards called "battery charging" usb specification where a device can draw more than 1.5A at 5v off the usb port.

Anyways.. it's not recommend to attach a fan to a usb port as it's not safe. Long term ussage can cause you to blow your computer capcitor that's reponsible to supply the power. After that, your usb port will stop working unless you do a hardware mod. I learned this the hard way BTW.

Typically, a desktop PC powersupply can only supply around 2-5A @ 5v from the VSB rail. For a normal laptop, that figure will be significantly reduced unless it's using the newest battery charging standard. Usually, the figure is around 1.5A-2A.

Since your 120mm fan requires around 10W of power to spin up, that's probably over the limit of the laptop's powersupply thus causing it to safety protection signal to halt the comptuer's operation. This signal majority of the time causes BSOD to generate a specific error code (forgot which one), but sometimes it just halts the computer without the BSOD.

 

Got it. Fan+USB=Bad

I feel really lucky right now!