I recently bought this 12 V universal laptop power adapter. It claims to auto-detect the voltage required by the laptop, and that is also what’s written on it:
INPUT: DC 12V-16V OUTPUT: 18V-5A / 19V-4.74A / 20V-4.5A
The adapter has a plug that seems to be identical to the ones used by Fujitsu Lifebooks, and it comes with different tips that convert this plug into the different kind of plugs that are used by different models of ASUS laptops.
My questions is, how does the automatic detection of output voltage work? In practice, apart from trying to understand what it going on in the electronics of the power adapter, I would like to find out whether I can also use it for a Fujitsu Lifebook (19 V) without using a tip, even though it is meant to be used only with ASUS laptops, using a tip.
Even though adapters like this seem to be common nowadays, I can find almost no technical information about them online. The only things that I could find were some people speculating that some mechanism in the tip tells the adapter which voltage to output. I can barely imagine this, as the tip has only one plus and one minus connector, and I measured the resistance within the tip at both the plus and the minus port as 0.0 Ω.
The output voltage without load both with and without a tip is at about 19.5 V right now (the input voltage is about 12.5 V). I remember measuring it on a different battery before (with a possibly different input voltage), where it was at around 21 or 22 V. Unfortunately I cannot measure the voltage under load, as I would have to cut open the cable.