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On 31 Dec 2003 17:37:47 -0800,
Dan wrote:
Jim Richardson wrote in message


snip

In order to use a transformer to step the voltage down, you must by
definition be using AC. A transformer doesn't work with a DC level.


Rather than go to the hassle of designing and building your own
powersupplies, you can buy std AT style supplies that take 12VDC as
an input, but usually in limited power forms, don't try powering your
500W game monster machine from one. Expect to pay $200 and up for
one.

For the laptop, most manufacturers make a 12V car powersupply as an
option. If not, then Targus among others, will happily sell you one.

The downside to low voltage DC of course, is the I2R losses in the
cables, 120VAC may lose some in the translation from DC-AC, but it
has the advantage of low loss with relatively small gauge wire.



Jim,
Good catch about the transformers. A simple analog voltage divider
would work.


The down side to simple voltage devider, is that you have to throw away
a lot of power, whether you are doing it with a linear regulator, a
dropping resistor, or a zener diode, any voltage you don't "use" get's
wasted as heat at the rate of "unwanted volts" X amp draw. Nasty if you
are powering a high draw system like a 5VDC logic board, with 12VDC.

In the case of a computer, your best choice really is either an
inverter, or a DC-DC switching powersupply built for the task. Either,
well done, is going to draw less total power, (by wasting less) than a
linear regulator or voltage divider setup. Unfortunately, they are going
to have other problems which have to be dealt with, most importantly,
being switching noise. Which is pretty critical in the case of a system
on a boat with comm gear. You'll need to take the relevent precautions
to prevent noise coming in the power supply of the radios.


On the other hand, if you have the space and inclination, you could
allways build a motor-generator converter although they are typically
used in situations where you are changing a *lot* of power, and number
of phases and frequencies too.. but you could do it I suppose

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--
Jim Richardson http://www.eskimo.com/~warlock
"Even if you can deceive people about a product through misleading statements,
sooner or later the product will speak for itself." - Hajime Karatsu