Scott Dattalo wrote:

>In another message, you express some concern about turning the MOSFET off
>hard.  In your current schematic, the turn off is controlled by a 10k
>resistor. I'm not sure how big Cgs  or Vgsth of the 530 are, but I think
>I'd be concerned too. (I mean this is supposed to be a perfect charger,
>right?) One circuit I have found that works well for capacitive drive is
>a simple push-pull:
>
>
>          ^
>          |
>        |/
>     +--|  NPN
>     |  |\
>     |    v
>     |    |
>in --+    +---- out
>     |    |
>     |    v
>     |  |/
>     +--|  PNP
>        |\
>          |
>          |
>         ---
>        ///

Almost exactly right!  I would connect the collector of the PNP transistor
to the FET's Source terminal - there is no need to swing the gate below S
and doing so only wastes some of that precious boost supply.  Tjaart - you
should be able to use this in place of your PNP transistor and 10K pulldown
resistor.  The 4K7 resistor in the collector of the NPN transistor sets how
much current is drawn from the battery thru the forward biased B-C junction
while the NPN is conducting - it might be prudent to increase that 4K7
resistor somewhat.  Also note that the PWM output from the PIC now becomes
inverted (exactly like mine is!) and the 100K pulldown resistor you have on
the base of the NPN transistor to GND should now go to VDD.

It looks you now have several different pieces of code to try.  I'm looking
forward to hearing how it all owrks out.

dwayne


Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(403) 489-3199 voice     (403) 487-6397 fax