A few days ago, I picked up my USB extension cable which was connected to my PC, and the other end was not connected to anything. The cable was really warm. I then plugged it into one of those "USB Doctors", and noticed it was drawing 4A+. I realized there was a short at the other end of the USB cable, as the connector was not the molded kind, and it felt loose. Jiggling it in certain directions removed the short.
My question is, I thought USB ports were designed to protect from over current conditions. It was a USB 2.0 port, so I believe the limit was 500mA. So why did the circuitry not detect the fault and prevent the port from supplying the large amount of current?
PS: The port did not incur any damage, it still functions properly.
USB is supposed to be current limited. Spec says 500mA, with devices able to negotiate with the host to get higher current (up to 2.4A, though this seems to be higher than what the battery charging specs from the USBIF actually permit), and dumb chargers able to tell the devices that they are capable of supplying this current by putting a voltage on the data lines with resistor dividers.
However, frequently current limiting is not implemented rigorously. In the most egregious examples, like you've described, it sounds like the 5v rail on the USB ports is connected directly to the 5v power supply rail, possibly without so much as a PTC fuse to limit current...
I was under the impression that USB allowed max of 100mA without any communication? ie light and mini fan using USB with no additional circuitry to talk over USB. For all USB 1.x and 2.0, 500mA is max for computer and USB hubs not designed for powering smart phones and other high powered devices. Only USB 3.0 has higher max current limit.
I was just looking at the datasheet of an FTDI chip and here is what it says about USB bus powered devices
i) On plug-in to USB, the device should draw no more current than 100mA.
ii) In USB Suspend mode the device should draw no more than 2.5mA.
iii) A bus powered high power USB device (one that draws more than 100mA) should use the
CBUS pin configured as PWREN# and use it to keep the current below 100mA
on plug -in and 2.5mA on USB suspend.
iv) A device that consumes more than 100mA cannot be plugged into a USB bus powered hub.
v) No device can draw more than 500mA from the USB bus.