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AC/DC adaptor outputs questions

Discussion in 'Electronic Basics' started by Dan Beck, May 7, 2005.

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  1. Dan Beck

    Dan Beck Guest

    Hello all,

    I am mostly a lurker here, but here are some simple questions--please allow
    me to give some background...

    The AC/DC adaptor for my portable MP3 player died; I measured outputs with
    my DMM to confirm. The outputs on the device are supposed to be 5.0 VDC and
    1.5 amps. I went to Radio Shack to see if I could find an exact
    replacement; no luck, but the guy there set me up with a device that outputs
    4.5 VDC and 700 milliamps. This Radio Shack device simply is one of those
    wall devices you would use to power your desktop speakers. I jury-rigged
    the connector, and this new AC/DC adaptor seems to work fine with my MP3

    The new wall device is much larger in physical size, than the now dead
    device that came with my MP3 player, but yet this new device only outputs
    less than half the stated current, at a LOWER voltage. Can this be correct,
    or are the ratings on the dead device too high? If all the stated ratings
    are accurate, is it correct to assume with the new, replacement device
    recharge times will be longer? Will I wear out the new device prematurely,
    because of excessive load?

    I recognize these questions are probably merely academic, because not a lot
    of money and time has been invested in this little project; I am just

    Thank you in advance for reading.

  2. Lord Garth

    Lord Garth Guest

    The most important factor is the output voltage. Whether it is AC or DC and
    if DC,
    the plug polarity. Add to this that if DC, whether your power supply is
    The output current should be equal or greater than the expected demand.

    Your output voltage is lower because you don't have sufficient current.
    MP3 player seems to be a power hungry beast!
  3. Bob Monsen

    Bob Monsen Guest

    Dan Beck wrote:
    One possible problem would be that you aren't recharging your batteries
    all the way. This will lead to additional charge cycles, which may lead
    to premature battery death.

    As far as the new device is concerned, you should definitely get one
    that has the same rating as the old one. When they say 700mA, they are
    really saying that if you draw much more than that, the voltage will
    droop, and it may overheat. There are usually fuses or breakers buried
    in them that will prevent fires, but the droop in voltage may affect
    your charging even more, causing even more cycles on the batteries.

    I'd get another one. You should have looked harder. You can get them
    here, online (For the USA):
  4. Dan Beck

    Dan Beck Guest

    Hi Bob,

    thank you for the sage advice and URL. A new device will be purchased;
    probably a new battery for the MP3 player as well. It sounds like I may
    have prematurely used up charge cycles :-(

  5. Traditional wall-warts are usually just line frequency (50/60 Hz)
    transformers. Modern wall-warts are often tiny switchmode power supplies.
    This allows them to use lighter magnetics (because they operate at higher
    frequencies), and it also improves the output regulation. The Radio Shack
    unit is unlikely to be of the latter type.

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