Simple transformers: how do they adjust to different input voltages (100V-240V)

[lemcts]

I use the same transformer for my 20V laptop, in the US and in Europe with just a different adapter.
How does it work? Thanks.

 

[crutschow]

That's not a transformer by itself, it's a switching regulator circuit, which can be designed to work from a large range of input voltages.
It uses a rectifier bridge to convert the AC to DC, and then uses a switching regulator, which operates at a high frequency to allow the use of a small transformer to reduce the voltage to the desired 20Vdc.
The circuit uses feedback to keep the output at this constant voltage, independent of the input voltage.