How do Power Tips work?
 

Like those for the Kensington travel supplies?

The devices that travelers carry obviously take a variety of DC voltages.

How does a travel power supply distinguish between the requirements of a 15v DC Toshiba laptop and, say, a 19.1V Sony Vaio?

IIRC most of these power tips devices are designed for small electronics, not notebooks.

I have a universal notebook power supply that use tips.

I also wondered what the implementation was, so I cracked open one of the spare tips. They did what I would have done - just used a couple of resistors. There are 4 contacts on the connection to the "brick", but of course only 2 to the laptop.

There are 2 ways to do it - analog and digital (didn't put a 'scope on it to see which they used) Digital - use resistors to digitally encode at least several hundred tip ids using resistor combinations - there would be a small CPU (something like a Cypress PSoC device) in the brick that measures the resistors, and figures out which tip has been connected, and sets the polarity and voltage.

The Analog method is dumber - use the resistors as a simple feedback divider.

The Kensington device I have uses different color plug-ins (probably resistors) for different voltages. You have to insert the proper plug-in for your laptop. The power tips are totally passive.