Eugene's observations

When I am given a laptop to repair I always ask for the power adapter.  This is not because I don't have a perfectly good universal adapter of my own, but so that I can measure the voltage and check the current.  It's a bit like air dusting and vacuuming the internals and alcohol wiping the screen - little extra services that usually go unnoticed but differentiate between the mass market behemoths and the little guy who cares.  Ok, enough self promotion already, why does the power matter ?

When you need to replace your power supply any one will do.  All you need to check for are the following 

• The acceptible input (alternating current) voltage matches that of your supply.
   
• The physical shape of the plug must be such that it fits in the socket.  It must be a snug fit so even the slightest wiggling around is not good as arcing accelerates decay.  And if you need too much force to get it in you will be damaging either plug or socket.

• The polarity must match.  Typically the outer ring is negative and the centre pin positive.  If you get this the wrong way around you will probably trash the computer and adapter.  If you hear a pop and the computer only works on battery power, you got this wrong. 
  

  
• The voltage must match exactly.  Yes I know a < 5% difference between that stated on the computer's label and that of the adapter will probably work most of the time but you are not leaving any scope for an unclean supply or cheaply assembled components that are intolerant of deviation from the reference value.  Too low a voltage and the machine just won't work.  Too high a voltage and it will at best show a warning in the system tray or event log while the protective circuitry lasts, and at worst go bang and leave you with an expensive repair bill - probably more than the value of the computer.  Both will accelerate the demise of the battery.
  There are some high end (often Dell) laptops that can cope with a small deviation by design, but these are few and far between.
   
• The amperage of the power adapter must meet or exceed that shown on the laptop.  If it does not then depending on the amount of difference you may smell melting rubber as the cable heats to dangerously high temperatures; or you may find there is just enough power for simple tasks on the running PC but it will not charge the battery or may fail abruptly when heavy duty computing is required.

Ignore the wattage (volts multiplied by amps) this is only of import to the electricity bill payer, and only then if they are the king of parsimony.   The moral is therefore if in doubt about the quality of a power adapter measure it before use or find a man who can.