Do switching AC adaptors limit the current they provide to the rate described?
I have thought that they do not, and that it simply gets dangerous by overheat, but now I am thinking that they might.
I had a power unit with pretty much small capacity (60w) on my mini-ITX computer, and it worked for a while, but after I started to attach usb units and gained current, the computer frequently shut down, so I suspected the power unit's capacity was not enough, and replaced it with a larger one (120w) but kept using the AC adaptor (12V 5A) that was designed for the smaller power unit, and it still shuts down frequently. After calculating the power that I am using, the power unit should be enough, but the adaptor may not. Now I am suspecting that the AC adaptor does not have enough capacity.
A different reason I came to think that AC adaptors limit the current is when you connect a AC-to-usb adaptor to ipad that is not designed for ipad, it does not charge at full speed because ipad requires 2A, which is more than what ordinary usb power supplies.
Best Answer
Many switching power supply controller ICs have a built in current sense input to avoid damage of the switching element when a over-current situation occurs.
Regarding the iPad charging, I read somewhere that there must be a specific resistor combination on D+ and D- lines in order to charge it at full current. This is to avoid over-current situations when connecting it to computer USB ports.