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Switching power source on plugin for battery charger

Discussion in 'Electronic Basics' started by Zebov, Sep 26, 2007.

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  1. Zebov

    Zebov Guest

    I'm designing my first battery charging circuit for a device and need
    some assistance. As a preface, I am charging a 3.7V 850mAh Polymer
    LiIon battery from a 5V wall-wart through a MAX1555.

    I have a battery charge circuit and I have the rest of my circuit
    which runs off of 3.3V (3.7V down-converted to 3.3V via a MAX710 step-
    down) pulling about 100mA. My problem comes when I merge the two

    I don't want to overcharge my battery and would like the circuit to be
    running while the battery is charging. The problem is, if I just
    connect the battery + to the +rail of the circuit, it will draw 100mA
    +battery charging draw. Will this confuse the MAX1555 and make it not
    turn off when it's supposed to?

    My first thought is to somehow switch power sources from the battery
    to the 5v wall (through a step-down to get 3.3V) whenever the 5V
    supply is plugged in. I'm trying to think of the best way to do
    this. I was thinking of just putting steering diodes between the
    battery and wall rail but don't think this will work well because the
    battery will be switches on once it is charged and I'll run into the
    same problem as if I was just running off the battery charging rail by
    itself (as mentioned previously). Any suggestions? Thanks ahead of
    time for the help.

    - Wes
  2. Martin

    Martin Guest

    If the power supply is plugged in via a plug and jack, consider using
    headphone type plugs and jacks. Some (not all, not even most) of them
    contain a terminal that is disconnected when the plug is inserted.
    They are meant to be used with devices with speakers, where the
    speaker is disconnected when the headphone plug is inserted. I don't
    offhand remember what this feature is called.

    Please note that if you do this your battery will be totally
    disconnected (i.e. neither charging nor discharging) while the device
    is on the suppply

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