mrgizmo wrote:
> How would I get 7.2V from a 7812 or a lm317 regulator?

That's the wrong question.

> The source voltage is approx 12 V, but will vary as different
> thrusters are used. 5.5W is the cameras power consumtion.

So the real question is how to get 7.2V at 5.5W given that there is at least
a 12V linear regulator in the system.  This of course also implies there is
a higher regulated voltage available.

First do the math.  5.5W / 7.2V = 764mA current the 7.2V source must be able
to supply.  Dropping that linearly from 12V is a 4.8V drop.  4.8V * 764mA =
3.7W.  That's a lot of heat, and will definitely require some thought and
heatsinking to get rid of.  And that doesn't even take into account the
extra 3/4 A load on the 12V regulated supply.  If that's really a 7812, then
it only leaves 1/4 A for everything else.  Even if that's OK, what voltage
is going into the 7812, and can it handle the extra dissipation.  One way or
another by using a linear regulators, you're going to burn up the difference
from the unregulated supply to 7.2V times 3/4 A in heat.  Ouch.

This looks like the poster child for a buck regulator.  My first knee jerk
approach would be to use a 10F204 controlling a high side switch.  In fact
I'll be teaching a course on using 10Fs as switching power supply
controllers next week at Masters.  For a complete example, take a look at
page 2 of the schematic for my QuickProto-01,
http://www.embedinc.com/products/qprot01.  You need a bit higher current
than my design is intended for, so you probably want to use a higher current
inductor for L1 and a 2A diode for D3.  The R7/R8 voltage divider needs to
be adjusted to produce 600mV when the output voltage is 7.2V, not the
roughly 5.5V of my design.


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