"Universal" laptop power supply? My father had an extra laptop lying around that despite being old and having had it's monitor seperate from it's chassis was in perfect working order. Or such was the case several months ago - I dusted it off today and it refused to start. I got out my multimeter and found the the power brick was only putting out 3V RMS, whereas googling revealed the laptop needed 16 volts. I figured I'd have to buy a replacement power adapter. Then I remember I had a "universal" laptop power adapter with a variety of interchangable tips, one of which fit this laptop. I check it's case, and to my amazement and confusion it claimed to support a variety of voltages at different amperages (ranging from 16VAC at 4.6 amps to 24VAC at 3.1). Furthermore, it can take both 120 and 240VAC input. It works with my laptop. My question is how this is possible? How can the power supply "know" when to produce different potentials?