Function of the Third Pin Inside Laptop Barrel Connector

Question

So I've been doing research on barrel pin connectors, and have been trying to apply this knowledge to understand laptop barrel connectors, specifically. My confusion relates to the function of the third pin that is recessed within the "tip" of the connector.

To give some context, I am using my laptop power connector as an example. It runs 2.1A of current at 19.5V. According to the markings on the adapter, the sleeve is positive, while the tip of the connector is negative. But there is an extra pin that is recessed within the tip. What is the function of this pin?

So far, what I have gathered is that there may be a few possibilities:

First, I have seen some diagrams which seem to suggest that some barrel connectors can use this third pin as a mechanism in a switch. So, would that mean that current only flows to the positive sleeve rather than being rerouted back to the ground when contact is made with that third pin? Could this be a sort of safety feature?

I have also seen some explanation in another post (What to do with third contact in DC barrel plug with only two input contacts) involving a DC barrel plug that mentioned the possibility that this third pin may involve some "swtich" behavior not on the end of the barrel connector itself, but instead on the end of the component which it has been "plugged into". As in, is it possible that contact with the third connector inside of the jack causes a switch behavior that rerouts current within the laptop to indicate that it is "plugged in."

Thank you for your help. Unfortunately, I have not been able to put together a clear understanding from what I have been able to find.

Answer 1

Standard DC barrel female jack 3rd pin is a detect or switched pin. When the male plug is not inserted, the 3rd pin is connected with the center pin. This is used to route a signal around the jack, like a battery pack, or to signal to a controller that nothing is inserted. While I have not seen all laptops ever designed, I have seen many and I have never seen a laptop use the 3rd pin of a standard DC connector. The question you link to is talking about those connectors. Which is not the connector you are asking about.

Newer laptops that use different 3 or 4 pin cylindrical connectors are not using old standard DC connectors. These actually use the extra pin for other signalling like power supply identification (Apple, dell, Lenovo use 1-wire or similar standards for this) or maybe dual voltage inputs. These 3 conductor connectors have a center pin, and the outer barrel is two non-connected sleeves, providing two conductors.