What did I burn out by touching VCC to the USB GND?

[nyarlathotep]

I accidentally touched a 4.5V line to the USB housing on my pinned Teensy 3.0, releasing the magic smoke. It runs fine if powered by the VCC and GND pins, but cannot draw power from USB for programming. I've successfully programmed it by placing an alligator clip on the USB housing that runs to GND though.

The Teensy 3.0 schematic one running through a ferrite bead. Is that because the USB cable sheath picks up noise? Or is the ferrite bead attached to some second ground wire inside the cable?

Have I likely burned out the ferrite bead or just some wire? Should I solder another ferrite bead going from the USB housing to the GND pin? Along with maybe an extra Bourns resettable 500mA fuse?

Or should I just solder a wire from the USB cable housing to GND? As I said, this works for programming it and reading the IR code responses, not sure if bypassing the ferrite bead has other consequences.

 

[PaulStoffregen]

Maybe the ground wire from pin 5 of the USB connector burned, and then the ferrite bead burned after that?

Normally the PTC "fuse" (actually it's a resistor that changes value, not really a fuse) should protect against this sort of burning, unless the short occurred to a location before the PTC.

There's little harm just shorting the ferrite bead with a wire. It might radiate a bit more noise since the shield would be conducting the power instead of using the wire inside the shield, but it's probably not a big deal... as long as the USB host has the shield connected to ground (virtually all computers have this).

 

[nyarlathotep]

Right, I'd noticed the wire and ferrite bead going to GND in parallel on the schematic, figured one might be the sheath and one an internal GND wire. I shorted my 4.5 V line to VIN directly to the USB sheath, but probably not to any wire inside the connector. So probably my USB cable ignores the internal wire or maybe my MacBook Air ignores it, or maybe the Teensy's USB connector connects an internal GND to the sheath. So maybe another USB cable that uses the internal connector wire would permit programming the Teensy. Thanks!