Let's Make Robots!

USB as power source for small robot

I want to use my PC usb port to power a small robot
It contains a buzzer and two solar motors

I have measured that the robobt drain a maximum of 250mA 
Could be there any issue for my motherboard doing this think?

I am very scaryed about to fry the mobo :O

what hapne to usb hub in case of abnormal current drain / short circuit? 

Is there a robust overcurent protection in modern 2.0 usb hub?

placing a fuse like this one could be a good idea?
http://shop.puntoluce.net/open2b/var/catalog/images/1537/0-d0ee4702-300.jpg

thank you all

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I would always prefer using an external USB hub with overcurrent protection. Spend some more money for a good USB hub. I have killed a few of the cheaper USB hubs without any protection like this one:

At the moment I use this model and it works fine. It must have a hardware overcurrent protection inside. If a device draws too much current it will shut down and you will need to repower it.

yep., but my robot is not the only device attached to usb hub of the mobo

so i can not be sure about the total current drain on the hub

what happens if the current limit is exceeded?

 

I can’t see anything bad happening. As far as I can tell the limit of the USB port is 500mA so if you would hook up a motor that draws 1A, it would only run at half speed. (but be advised to use diode for things like motors or solenoids).
So if you have an external disc on the same port (via a hub) and that draws 300mA, that would leave you with 200mA for your device. 
Why don’t you build yourself a current probe like this http://letsmakerobots.com/node/23097 and measure the current?

http://forum.sparkfun.com/viewtopic.php?t=7795

... so it seems that usb is not  electronically short circuit protected, but there is a software monitor that try to shut down the usb ...

 

mh it semms not so safe ..

 

According to Murphy’s law: "A transistor protected by a fast-acting fuse will protect the fuse by blowing first".

That said it can’t hurt to have a fuse in your circuit. But there are a couple of other things I would have done.

1) Add a big electrolytic capacitor to support the motor with some extra current at startup.
2) Remember to add a diode on your motor as the back EMF if anything could kill your USB port.

No, the motherboard should be able to source up to 500mA of current. a 250mA PTC Fuse would be a good idea.