At 11:37 AM 2/10/2004 -0500, you wrote:
>I have a question about the 6 amp diode actually.  I put in the diode in
>case the power and ground were connected backwards.  Is this diode
>really needed?  The only reason I ask is I need to connect more lights
>(about 10 amps worth) and this is the limiting factor.  It's mounted on
>the PCB and anything bigger will be a TO-220 or stud mounted device --
>ugly and cumbersome but doable.

If it's just lamps, you don't need the diode across the load. If you
do put it there, it will take out the MOSFET and maybe the diode if the
polarity is reversed. The fuse might go first, but I wouldn't count on it.
The IRF5305 is avalanche rated so it can deal with overvoltage from a bit
of stray
wiring inductance all on its own.

Yes, you can leave the 6A diode out too, BUT you MUST put a diode from the
base of
the BJT to ground for reverse polarity protection as thus:



>       (view only with fixed-pitch font)
>
>                           +12
>                             |
>                             |
>                             |
>                             x---x--------------x
>                             |   |  /           |
>                           [1K] /--             |
>                             |   /\ 15V       S |
>                             |   |           |--x P-channel MOSFET
>                             x---x---------| |      IRF5305
>                             |               |--x
>                           [1K]               D |
>                             |                  |
>                             |                  |
>         PIC               |/                   |
>          o-----[Rg/Rb]-x--|                    |
>                        |  |\E          (incandescent lamps - 5 Amps)
>                       ---   |                  |
>              1N4148   / \   |                  |
                        ---   |                  |
                         |    |                  |
                         |    |                  |
>          0V------------x----x------------------x

Of course you'll need reverse polarity protection for the
PIC power supply as well (which could be as simple as using a regulator
that provides it as a feature).

Note that the load will be energized through the MOSFET body diode
if reverse connected, so make sure that that is okay and that it
doesn't overheat under that condition- the dissipation might be
in the 3W-4W range typically.

>P.S. -- Thanks to Sphero for most of this diagram from an earlier mail!

You're welcome. ASCII art has the advantage (or possibly disadvantage)
that it will be archived.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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