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Why use an FET on battery charger output?

Discussion in 'Electronic Basics' started by [email protected], May 9, 2005.

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  1. Guest

    Hi,

    I'm working on a battery charger for a Palm Pilot. This fits inside the
    modem docking case, where space is limited. It basically takes 12v,
    regulates it down to 4.1v with a LM317L, and feeds it to the docked palm
    pda.

    There's a circuit on the web that does this:
    http://www.xs4all.nl/~lous/palmpower/palmvpower.htm

    The circuit has an FET as a switch on the output, to prevent the palm
    from discharging into the LM317 when 12v is not applied. Can this FET
    simply be replaced by a diode? Since the circuit has a variable
    regulator, it can be adjusted up to compensate for the voltage drop
    across the diode.

    If this is right, then a 3 lead part gets replaced with a smaller 2 lead
    part. And the diode actually takes up zero space, as it could be used
    as the output wire with some spaghetti tubing or heat shrink.

    Am I missing something here?
     
  2. One day got dressed and committed to text
    Seems feasible to me, dont forget the diode will drop more volts than the
    FET.
     
  3. Paul Jones

    Paul Jones Guest

    You could do a LM431 and a few PNP's to pull down th
    12vdc to 4.1 . But the LM431 needs to draw thru a 300 ohm
    to start a PNP or the 431 will turn on prematurely at 2.0 vdc
    and there are hundreds of circuits ...
    I don't like the 317 .

    It's easy to add a coil and a diode to make the LM431 circuit
    "switcher
    or "buck" regulator ...
     
  4. Lee

    Lee Guest

    A larger voltage drop is the only difference, right? The drop can be
    adjusted out on the LM317.

    Is there any other benefit in using an FET?
     
  5. kell

    kell Guest

    The mosfet works as a kind of variable resistor. At higher output
    currents the gate voltage drops and its resistance increases, limiting
    current. A series diode won't do that. As a current-limiting scheme,
    the mosfet is about as simple as you can hope for. Also, he says exact
    voltage is critical with lithium batteries; you can get the 317 output
    dialed in very precisely. With a diode on the output, you will have a
    varying voltage drop across the diode according to the current.
     
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