Re: OT: Outboard Motor Charging System.

On Thu, 04 Aug 2005 03:16:22 -0700, Derek <joe@xxxxxxxxxxx> wrote:

>On Thu, 04 Aug 2005 02:17:52 GMT, Ken Sterling (Ken Sterling) wrote:
>
>>>
>>>
>>Maybe you could just stick a regulator on it????
>>Ken.
>
>Right now I'm looking at connecting a few LM7812, IC
>Voltage regulators in parallel. Each one is good for 1 Amp, and are
>about $1 each. I'll check the manual and see what the magneto is
>capable of pushing out. The down side is that exess voltage is
>converted to heat. The plus side is that when they overheat, they shut
>down till they cool down.
>

I'd suggest LM317 rather than LM7812 because you'll want about 14.5
volts to the battery. LM317 can be set to any desired output voltage
with a couple of resistors.

A better solution might be a P-channel MOSFET driven by a cheap opamp
(LM324 or so) and a voltage reference as perhaps LM78Lxx. This would
be "better" because it will work with very little overhead -- it could
deliver regulated 14.5 volts output at a couple of amps with as
little as 14.8 volts or so from the alternator The MOSFET could
be heatsunk to the motor housing. The parts are inexpensive.
MOSFET choices might be STP12PF06 for 84 cents or MPT23P06V for
$1.89.

With 17 volts in, 14.5 volts out, at 5 amps your power dissipation
would be about 12.5 watts so you wouldn't need much heatsink. I
doubt if an old outboard motor alternator can deliver much over 5
amps. As your battery gets charged, the load on the alternator (and
hence the motor) would decrease because the voltage
is regulated so current draw would diminish.

Email me if you'd like a schematic.
.

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