HP 97 VERY hot charger

[brouhaha]

(10-16-2014 02:10 AM)Katie Wasserman Wrote:  the charger is not current limited so it's getting really hot becasue it's looking at almost a dead short

Actually the charger IS current limited. UL will not list it otherwise. However, it is current limited by the design of the transformer, not by additional components.

For wall warts, UL examines the design documentation, and tests with a dead short for some number of hours, and they must not exceed a temperature limit. (I no longer recall the specific hours and temperature.) This is achieved either by current limiting inherent in the transformer design, or additional current-limiting components, or an electrical or thermal fuse.

For UL listing, the wall-wart doesn't actually have to be usable for its intended purpose after the testing, so if a fuse opens, that satisfies the test.

Some higher-quality wall warts are designed so that the transformer is adequately current-limited, AND there is a thermal fuse, as a belt-and-suspenders sort of thing, but cheap ones don't do both since that isn't required. I don't know whether any of the HP calculator wall-warts have a thermal fuse, because they don't normally get hot enough with a shorted output to open such a fuse. I think the only way to determine whether a thermal fuse is present would be to disassemble or X-ray the transformer.