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Universal laptop power supply?

  1. Aug 4, 2006 #1
    "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?
  2. jcsd
  3. Aug 5, 2006 #2


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    There's normally a switch to select the desired voltage. Perhaps you just happen to have it set to a position that allows everything to work.

    - Warren
  4. Aug 5, 2006 #3
    chroot, could you please elaborate ?
  5. Aug 5, 2006 #4
    I am certain there is no switch. I can move this back and forth between two different laptops, requiring different input voltages, without issue. This is in line with the advertising.

    The mystery remains... :confused:
  6. Aug 5, 2006 #5
    maybe the switch is in the changeble part?
    maybe it recognisies diferent voltages electronicly???
  7. Aug 6, 2006 #6


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    I'd think this would be rather easy since the invention of transistors, but my assumptions come from theory, not practicality.
  8. Aug 6, 2006 #7


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    It's possible that the computer and charger actually communicate with digital signals. When you plug the charger in, the laptop sends digital signals down the wires to the charger, telling it what voltage to generate. The charger then springs to life, selecting the appropriately internal circuitry to produce that voltage.

    Newer power supplies do this kind of thing, but I don't believe older laptops have the appropriate circuitry.

    My bet is that the "universal" supply is just producing a voltage like 20V that is within the tolerances of most laptops. A laptop doesn't need exactly 16V -- the battery charging circuitry has power electronics to regulate and filter the incoming voltage anyway. It can probably tolerate 4-6 volts over its nominal 16V input.

    If you can provide us the make and model of the supply, I can look up how it works.

    - Warren
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