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Proper voltage setting for laptop car adapter?

Discussion in 'Electronic Basics' started by Captain Infinity, Jan 4, 2005.

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  1. I know nothing of electricity except that if you stick your tongue in a live
    lamp socket you'll get an instant perm. So, I need some help.

    The vice president of the company recently bought a laptop, and with it a
    car adapter that the salesman recommended. I have to get these two devices
    to work together.

    The laptop AC power converter says that its output is 18.5 volts. The car
    adapter will output to a range of voltages: 15, 16, 18, 19, 20, 22, 24.

    Clearly I need to set it to either 18 or 19, but beyond that I'm stumped.
    Will 18 be not enough? Will 19 be too much? Which is the better choice if
    I want to avoid damaging the laptop? Should I just chuck this adapter and
    buy another that has an output of exactly 18.5? Do these exist?

    Any help would be appreciated.

    Captain Infinity
  2. Rich Webb

    Rich Webb Guest

    I'd measure the actual output voltage of the existing laptop adapter,
    under both light and heavy (charging) load conditions and then see which
    setting of the multi-output adapter results in the closest match. The
    checks need to be under load, since the regulation of the power supplies
    won't be perfect.

    A "breakout box" can be made with a plug/receptacle pair. Don't try to
    get live readings by sticking wires into the laptop or cutting the

    You may not hit exactly the same readings but close enough is probably
    OK. There is a DC-DC converter inside the laptop which should have
    enough headroom in either direction to manage within 0.5 V.
  3. Once Upon A Time,
    Thank you but as I said I know nothing of electricity, how to measure it, or
    what a breakout box is.
    OK, that sounds promising. Which would be better, to underrun it by 0.5V or
    overrun it by 0.5V? Or, to put it another way, which way is less likely to
    damage the machine?

    I've been unable to find any car adapters that output 18.5 V.

    Thanks again.

    Captain Infinity
  4. Rich Webb

    Rich Webb Guest


    Well, starting from scratch and given that you're without tools or prior
    knowledge, the best thing that I'd recommend is to carry them both back
    to that store and get the salesman to (a) state which setting is correct
    and (b) state that he warrants the laptop if his recommended setting
    causes any damage.

    For my own in-car use, I've always gone with a DC-to-AC inverter, into
    which the laptop's own, normal power cord can be plugged. There are some
    losses in efficiency by introducing the extra conversion but there are
    gains in flex ability -- other stuff could be used, including a later
    generation laptop someday.
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