AVR-Chat

Steven Hodge Wrote
>Yes, my application is (sort of) automotive.  Specifically it is a boat
>environment, which is 12 VDC.  My guess is that in some respects it's not
>quite as harsh as automotive, but in others, eg, high power RF
transmissions
>and need to shield magnetic compasses from EM fields, it is far worse.    
I
>have many uses for uC's but the specific one that the post was about is
that
>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?

>the main boat's bank in the aft part of the boat, the voltage drops even in
>large 4/0 cable would kill a lot of the motor's thrust.   

That would be why you run the local battery in parallel with the remote. 

How far are you from your main battery bank?  Inductance may be as big a
problem as resistance.  Actually I rather expect your battery resistance is
comparable to your cable resistance.

>Regarding voltage divider current drains, my assumption is that from the
>data sheets the analog ADC input likes to see 10 Kohms maximum impedance.
>For a maximum input to the ADC of Vcc = 3V this implies a current drain
>through the divider, regardless of amount of dividing, of 0.3 mA.   

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

Robert


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