I assume that there's some UL requirement (or desirement) that says that a wall wart shouldn't burst into flame if you plug it into a dead short. I know that older wall warts were generally fairly robust to that sort of folderol -- they had enough inherent current limiting through CCD (Carefully Crappy Design) that they'd just run a little extra warm.
But what do the newer, slim, sexy, switching wall warts do? Do they gracefully drop voltage when you pull more than the rated current? Blow a fuse? Burst into flame?
I'm kinda curious to know -- it'll save me buying a bunch of 'em and experimenting...
Class 2, IIRC. UL 1310/CSA 22.2 (but check for yourself). AFAIUI, they can be impedance-limited, or burn out through an internal non-replacable fuse, or self-fusing transformer winding, just not burst into flames (or expose voltage through a hole burned in the case or whatever).
The ones I've seen just current-limit nicely, and restart once the short is removed. I'm sure it's a UL requirement that they behave safely, but I can't quote chapter on verse the requirements for AC adapters. Failing instantly is also safe, and I doubt UL/CSA care one way or the other (but your customers might). Maybe you've got a library nearby with the standards?
** AC power adaptors are closely controlled devices in most of the world - see all the badges and logos on them, like a good boy scout. They must pass all the class 2 requirements that apply to low voltage transformers - which includes all degrees of overload up to a dead short WITHOUT failing in same dangerous way.
The most common protection is a thermal fuse in the primary of the transformer that opens if the winding ever becomes too hot. Low powered adaptors may simply rely on leakage reactance to limit current flow to a safe value when the output is shorted.
Some even have a fuse link in the output line.
** Course not.
But most of them scare me and are inherently far less safe than the transformer kind.
There is no requirement to * properly insulate* live parts from the low voltage circuitry - sure there are specified gaps on the PCB of a few mm but this does not preclude events like a leaking or exploding electro from bridging that tiny gap with conductive fluid or foil and so rendering the output live.
No tests are done to establish end of life failure or other failure mechanisms which may result in over-voltage conditions and hence electros exploding.
Also, for EMC reasons, a small ( class Y) cap is often fitted between the live side and the output ground. Experience says that the cap used may or may not be of the required special type and so could fail short.
They are almost all made in China and if that does not worry you, it should.
I tried a duplicate of my current cellphone charger, and it appears to me to roughly regulate some linear combination of voltage and current. And, when hit by a load that is close to a short (around/under half an ohm), it goes into "hiccup mode" - only attempting to supply power at some low duty cycle.
Logos/badges on products are just that, logos and badges. I don't rely on them. Certainly in the 90s products had various safety marks added without the benefit of any testing. If safety is an issue always check the safety certification. UL is easy, just get the UL file number and check online.