Are these two Li ion chargers equivalent, despite their stated spec?

Question

Not being game enough to DIY it (as suggested in some answers to this question) I've bought a 240V charger for my beard trimmer. Despite the singular "240V" listed on the label, the person at the other end of the Wahl support line insisted that it will work on 110V in the US with only a pin adapter, but the original charger (rated for 120V, left) would not work here in Australia on 240V.

I find it quite surprising that the 240V charger would in fact be "world compatible" and not labelled as such. Is it inherent in the nature of switching power supplies that one rated for 240V will work when presented with 120V but the converse would not be true? What are the odds of the magic smoke leaking out if I plugged the one rated for 120V into a 240V socket via a pin adapter?

Answer 1

A small AC-DC converter like this is almost likely a flyback topology.

AC-DC converter rated for 120VAC and plugged into 240VAC will not work almost certainly. There should be an overvoltage protection circuit inside. The purpose of this circuit is not to protect the wall wart, but to prevent a fire. It's designed to trip somewhere above 120VAC, but most likely below 240VAC. The overvoltage protection circuit usually consists of a MOV and a fuse.

AC-DC converter rated for 240V and plugged into 120V might work. But there are failure mechanisms in this case too. To deliver 2.4W, the converter would pull 0.01A off 240VAC supply, or 0.02A from a 120VAC supply. Factor of 2. Granted, additional 0.01A is a very small (probably negligible) increment for a power supply. But in a larger power supply a 2x current can lead to a blown fuse, or higher copper loss in the transformer (and burn it), or higher loss in the switching transistor (and burn it).