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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.

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    \$\begingroup\$ I would guess it just supplies ~19V regardless. ;-) \$\endgroup\$ – Spehro Pefhany Oct 1 '16 at 18:01
  • \$\begingroup\$ @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. \$\endgroup\$ – AndrejaKo Oct 1 '16 at 18:17
  • \$\begingroup\$ @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. \$\endgroup\$ – gbulmer Oct 1 '16 at 18:20
  • \$\begingroup\$ 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. \$\endgroup\$ – mkeith Oct 1 '16 at 19:37
  • \$\begingroup\$ 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. \$\endgroup\$ – cdauth Oct 2 '16 at 14:58
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Technically speaking, there is a mechanism how laptops identify capabilities of power supplies and act accordingly. For example, DELL laptops will refuse to take power supplies made by HP, and vice versa. This is done through a proprietary serial protocol that goes through the center pin and gets information from specialized secure serial EPROM. So it can be imagined that an intelligent PS can decode the serial requests, determine the kind of laptop, and set proper output voltage.

But I would consider this as a very remote possibility, and the label on your PS simply indicates that at 18V it can supply somewhat more current than at 19.5V, creating a confusion. I will be glad and surprised if it proves otherwise.

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    \$\begingroup\$ My interpretation of the load label: if a laptop takes less than 4.5A, the output will be 20V. If a laptop takes 4.75V, voltage will drop to 19V. If it takes 5A, the voltage will drop to 18V. All these voltages are in normal ballpark for laptop's input. It looks like a pretty ordinary low-quality DC-DC converter. \$\endgroup\$ – Ale..chenski Oct 1 '16 at 18:50
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I don't know how it's done but I do know that it is somehow done since I had a really nice sturdy and slick looking intelligent universal adapter with a blue led display which I would use to test customers laptops and sometimes use as my primary psu.

Firstly the tips were multi pronged and would switch the voltage to the default estimate based on notebook tips, BUT then after that, if it found that the voltage requirement of the laptop was different from what the tip indicated, it automatically stepped the voltage up or down, the display also indicated that it calculated amps too. Hope someone else with more knowledge can add to that.

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