Why use an FET on battery charger output?

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?
 
One day nospam@vegan.net 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.

Am I missing something here?
Click to expand...
Seems feasible to me, dont forget the diode will drop more volts than the
FET.

--
Regards ..... Rheilly Phoull
 
You could do a LM431 and a few PNP's to pull down the
12vdc to 4.1 . But the LM431 needs to draw thru a 300 ohm R
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 a
"switcher"
or "buck" regulator ....
 
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@vegan.net 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.

Am I missing something here?



Seems feasible to me, dont forget the diode will drop more volts than the
FET.

--
Regards ..... Rheilly Phoull
Click to expand...
 
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@vegan.net 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
Click to expand...
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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