On Friday, May 22, 2015 at 9:27:21 AM UTC-7, wrote:
The discussion ...jogged
my memories of the Polyswitch, a protective device whose resistance
jumps up when sufficiently heated (via current), but goes back down
when it has cooled off.

http://www.te.com/usa-en/products/ci...e-devices.html

We ended up not using them, but now I am curious if they show up in
damaged equipment.

Yes, there were quite a few damaged polyswitches in the 1990's vintage
SCSI (computer bus) systems; these were used in powering the required
resistor arrays (called terminators), and were a big improvement over
the previous generation (which used fuses).

The problem, is that you couldn't power the bus without taking precautions
because other bus-resident boxes could have different +5V levels, or could
be powered off, or have unstable voltages, and were ALSO connected to
that same wires on the bus.

The issue I saw was usually brittle fracture, and mainly on the surface-mount
types. Before polyswitches, there were lots of fuses (one inside each CD or
hard drive mechanism) and when a fuse went out, the whole bus could lose
its logic power (or not, depending on which particular devices were plugged in).
A surge could pop two fuses, and leave the third device (like, a scanner) as the
only bus power source. When you unplugged that scanner, the hard drive
became unusable.