You're probably right, so it looks like the AC adapter gives one of its four available DC voltages based on the resistive load at the MacBook input. Hi, thanks again for your answer: You're probably right, but in order to understand my doubt you'd need to read that article 'Teardown and exploration of Apple's Magsafe connector' (it's a well known article, the only out there about Mac magsafe adapters that I know, you'll find it on Google). The controller chip in the adapter reads the value of this resistor by applying a known current through it and measuring the voltage drop across it, then regulates the output voltage based on this. How variable voltages are usually done is you have a resistor inside the device, between two pins (or one pin and ground). The laptop then has a pile of DC-DC convertors (with their own regulation) to give it the voltages needed for its components from either the adapter or the battery, and to charge the battery. 18V is done in the adapter - it's just a simple switch-mode power supply. I don't know about macs specifically, but usually all the regulation to e.g. It's an AC adapter in the sense that it takes in AC power.
The voltage regulation is done by the controller in the laptop.
