AVR-Chat

> >I have put in a "bow thruster", which is a huge 5.0 KW, 12 VDC motor that
> >draws about 400 A (which is another story).   This needs to be run from its
> >own local 12 V battery (an Optima AGM starting type) because to run it from
>
> Maybe not your best choice of battery.  What's your on time and frequency
> look like?

12V?  I^2R losses will eat you alive!  48V would be better, but I'll
assume you're stuck with it.
Turning off this monster will be interesting.


> You've forgotten your local capacitor an/or buffer op-amp.  You could
> easily run a 10M resistor divider and follow it with a voltage follower to
> bring the effective impedance down.

You need to check the opamp's input offset before using such large
input resistors too.
The input offset current into 10M could yield a large error.

> You can also power the circuit only when it needs to be (the whole thing,
> not just the voltage divider).

True.


For a telco application, I needed to see the voltage across the pair.
The part 68 spec didn't allow us enough current to light up an
optoisolator in constant mode, but what we ended up doing was to
sample the line momentarily, which sort of falls through a hole in the
spec. There's many millions of them out there now, and no country's
telco approval has objected..  Instead of trying to draw less than a
microamp, we draw about a milliamp, but only for a few millisec.