Mega had been running for many weeks suddenly stopped working. Don't know why. Power supply was ok. Green LED was on. Thought maybe the memory got damaged so I plugged in my Acer laptop to try reloading the program. Laptop suddenly went dead. Noticed the CPU chip on the Mega was very hot.
Is there any way I can protect a laptop from a bad Arduino? Put a fuse in the USB cable?
The USB standard specifies that all USB ports are to be protected against short circuits.
You should be able to short out the USB power on your laptop without causing permanent damage (though a sudden shudown in these cases is not unheardof; a PTC fuse will take some time to reset itself. You may need to pull the battery out and plug it back in to fully reset it). I've shorted USB devices dozens of times, popped diodes off USB, etc... and have yet to cause any damage to the USB port, let alone the computer.
As for what happened to the mega, that's another matter. You'd need to put some time into investigating what happened there... If the chip gets hot, it's been severely abused - too high a voltage getting fed to it, pins shorted to ground or supply while being driven in the opposite direction, etc. Check for things shorting the bottom of the board, or debris getting on it.
The easiest way to protect is to use a USB hub with it's own power supply. They can be had for like $5, so it's no big deal if you do manage to break it
Thanks DrAzzy. I removed the battery and put it back in and the laptop seems ok. I will check the Mega connections, supply, shorts, etc. and get a usb hub.