Smart AC Adapter

I'm sure the designers thought it was a good idea to build smarts into a lowly notebook AC adapter. My HP notebook AC adapter comes with a 3-pin power plug. As an electrical engineer, I had guessed that the center pin was a sensing pin to compensate for the IR drop of the wires. I measured the voltages: the inner ring is 20.0V and the center pin is 15.4V. I did not think too much of it why HP thought was necessary to have a center pin until a few months ago. My computer ran slow and took a long time to boot. Of course, I suspected software problems, maybe virus. I reinstalled the OS and the problem seemed to go away, but only reappeared later. It was very frustrating. One day I noticed that the computer would run fine when I unplugged the power adapter. It was very repeatable; I could see the CPU loading changing as I plugged and unplugged the AC adapter. I could not believe that the running on AC adapter could cause the computer to slow down. A quick search on the web showed other people had the similar problems. HP called the power adapter smart AC adapter. Apparently information is communicated between the AC adapter and the notebook PC and the PC throttles the CPU accordingly. When there is a connection problem on this center pin, the notebook gets stuck in the slow mode. I could juggle the wire to make the problem go away. The technical support at HP seemed unaware of this particular problem. (In general, HP tech support just isn't very good.)

Power supply failures seem very common on the computers. Now the smart AC adapter adds a new way for the power to fail.