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Laptop AC Adapter with fluctuating DC output

Discussion in 'Electronic Basics' started by [email protected], Jun 29, 2007.

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  1. Guest

    I have a dell laptop that won't power on or charge the battery but
    ran fine on battery until the battery lost its charge . I checked the
    DC output of the AC adapter with a multimeter and it constantly
    fluctuates from 18.5v - 19.5v.

    I have another laptop a toshiba which happens to have an adapter with
    the same output 130w 19.5v 6.7a and when I measured it the output
    stayed at exactly 19.3v.

    The toshiba adapter has a 2 prong input from the AC plugin and the
    Dell adapter has a 3 pronged plugin on the AC side. Is the fluctuation
    on this type of Dell ac adapter normal? Or is the fluctuating DC
    output the reason that the laptop won't start or charge the battery?
  2. Meat Plow

    Meat Plow Guest

    Fluctuation is not normal backed up by the fact that the adapter won't
    charge the battery or provide running power.
  3. Bob Masta

    Bob Masta Guest

    Did you measure that fluctuating Dell adapter output with
    an appropriate load applied? I have seen this sort of behavior
    from an unloaded adapter, which technicians decided was OK
    since it was the right voltage range. But as soon as a decent
    load was applied, the output dropped to zero.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!
  4. Guest

    I did not test the output under load. I would like to so I can see
    what happens but I am not sure what I should use as a load. Using the
    laptop is very hard since the motherboard has to come out in order to
    have access to the pins for testing. The adapter output is rated for
    19.5 volts 6.7 amps. Do you have any suggestion for what I could use
    as a load so I can test the adapter
  5. geoffturner

    geoffturner Guest

    Use the laptop as a load
  6. Bob Masta

    Bob Masta Guest

    You don't need to draw the full 6.7 amps, just something reasonable.
    Even pulling an amp or less will probably be enough to show if the
    adapter is shot... at least it was in my case.

    The biggest problem is often making connection to the output of the
    power adapter. In my case, I just stuffed the leads of the resistor
    into the holes of the adapter, with the meter clipped onto the leads.

    If you have a power resistor (bigger than 19.5/6.7 = 2.9 ohms but
    less than say 20 ohms) you can try that, or wire a bunch of
    lesser-wattage resistors together. You won't need to test for more
    than a second or so, just enough to get the reading: Low-wattage
    resistors may get hot really fast, but you can get a reading just
    before you burn your fingers. <g> (And getting hot is a Good Thing,
    since it tells you the adapter is at least putting out juice... mine

    Note that when you combining low-wattage resistors to get a
    higher power rating, it dosen't matter (to the effective power
    rating) whether they are in series or parallel. So 10 x 1 ohm
    1/4 watt resistors in series gives 10 ohms at 10*(1/4) = 2.5 watts.
    Or 10 x 100 ohms in parallel gives the same thing.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!
  7. neon


    Oct 21, 2006
    gee this is very hard to answer a flatuiting battery . change the battery battery dn't do that if they are good
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