Re: Why use an FET on battery charger output?



Lee wrote:
> 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?
>
>
> Rheilly Phoull wrote:
>
> > One day nospam@xxxxxxxxx got dressed and committed to text
> >
> >
> >
> >>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.
> > >I'll take a shot.
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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