How can a Lenovo ThinkPad get the power rating from a power supply with only a two-pin connector?

Question

If the question is the same as How smart are laptop power bricks for Lenovo Thinkpads? I apologize, but I'm not familiar with "power bricks".

I have a Lenovo ThinkPad that requires significant power, so it was delivered with a 170W power supply. When I bought the corresponding Thunderbolt docking station that one arrived with a 300W power supply. Both provide power through a rectangular 2-pin (as it seems to me) connector:

Here is a more detailed image of the connector (both plugs are actually the same size, but the wide angle lens distorts the sizes a lot):

Now the point is: One day I was lazy (the 170W power supply was just in reach) and used the 170W power supply to power the docking station (which in turn powers the ThinkPad using the same connector, i.e. not Thunderbold (as that is limited to about 100W for ThinkPads, it seems)). Even thought he laptop was more or less idle (and thus expected to need much less than 170W), Windows complained that the power supply would not be strong enough.

So I wonder: How can the ThinkPad detect the maximum power of a power supply connected via two pins (also through a docking station) while it only uses little power? I mean: I could understand if the ThinkPad would try to draw more power than the supply can deliver (and the voltage goes down), but not in the situation desribed.

Answer 1

Your assumption that the connector has only two pins is wrong.

The square barrel has power contacts on inside and outside, which is very typical for modern connectors for laptops and other devices.

The pin at the center is simply an ID pin which the power supply uses to tell the laptop how powerful it is.

Answer 2

Answer https://electronics.stackexchange.com/a/731677/282833 is correct, but as I wanted to provide some details, I did not want to edit that answer massively, but instead build on that answer, so here it goes:

Even if is seems the two pins are "outside" and "middle pin", there are actually three pins on the "Slim Tip" connector. The second pin is inside the shorter sides of the connector (I had to use a better camera to show):

Looking at the counter-part of the "Thunderbolt 4 Workstation Dock", there are three contacts also:

1 is for the outside contact (ground) pressing inside, 2 is for the inside contact (20V) pressing outside, and 3 is the middle (sense) contact.

As explained in https://superuser.com/a/1246834 a resistor between ground and the sense pin indicates the power rating of the AC adapter.

I measured these values for the two AC adapters mentioned in the question:

• The 170W AC adapter shows 1.9kΩ resistance
• The 300W/20V AC adapter shows 8.7kΩ resistance

Unfortunately things get more complicated when using the "Thunderbolt 4 Workstation Dock", because the AC adapter is connected to the dock, while a "Thunderbolt 4 Split Cable" serves as a power and dock connection for the laptop:

The "output" to the laptop:

The "input" to the cable:

The "Round Tip" connector has three pins also (the middle pin cannot be seen in the photo):

Again both ends use three pins, but the cable itself is "straight through", i.e.: there is no resistor between pins 1 and 3, and on the "Thunderbolt 4 Workstation Dock" output there is no resistor between pins 1 and 3 also.

So how does the laptop know the power rating of the AC adapter then? I can only guess that the "Thunderbolt 4 Workstation Dock" makes the value from the AC adapter available through the Thunderbolt 4 cable.