# How do power adapters automatically detect the output voltage?

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.

• I would guess it just supplies ~19V regardless. ;-) – Spehro Pefhany Oct 1 '16 at 18:01
• @gbulmer Keep in mind that laptop chargers are 100% integrated inside of the laptop. In all cases, you have the same power, so I too wouldn't be surprised if it was actually just giving 19 V all the time. – AndrejaKo Oct 1 '16 at 18:17
• @AndrejaKo - yes, I understand that. I was just trying to suggest that point for the OP to consider, but clearly it didn't work. I'll delete as it'll likely mislead others. – gbulmer Oct 1 '16 at 18:20
• They do not have only two conductors. Many have three. My dell laptops have three conductors and don't work correctly with all power supplies. I suspect the universal supplies don't detect voltage, but they have special tips that somehow convince the laptop that it is OK to charge. – mkeith Oct 1 '16 at 19:37
• Interesting, actually, the tip that I’m using also has three connectors. Measuring the middle pin with a multimeter, it has about 200 kΩ resistance to the plus pin. So maybe there is at least some kind of logic going on in the tip to tell the laptop that it is okay to charge, but I still doubt that the tip is somehow communicating with the charger. – cdauth Oct 2 '16 at 14:58