[aprssig] Shunt for battery current / APRS Telemetry

Phillip B. Pacier ad6nh at arrl.net
Mon Feb 27 14:56:12 EST 2006

Thanks for the note.  I guess the calculation really is of no 
consequence whatsoever.  As you say, some simple webpage coding can take 
care of the math.  I'm trying to maximize resolution while minimizing 
the impact of the voltage drop.  I don't need a charge/discharge swing 
meter, as I am only interested in the current coming down from the 
panels to the charge controller.  There will be no current flowing in 
the other direction (unless of course the charge controller fails.. 
oops).  Thanks for all of your help.

Phil - AD6NH

Steve Noskowicz wrote:

>     Phil,
>     A- Your calculation is very "simple" indeed ( x/10) , but if you
>     do one calculation, why is any other (say x/20) more difficult? 
>     Looks like  it'll just a little code somewhere.
>     B- Those current sensors James mentions look neat, but as Bob
>     suggests, a current sense resistor is also common.  The standard
>     (ok its old) way it to use a 100 mv (0.1v) current shunt.  (12A
>     and 0.1 V gives .008333 ohms)  Bob suggests 0.02 ohm, but that
>     gives 240 mv drop @ 12    And  don't know how much voltage drop
>     you can stand.  If you use an OP AMP which has an input common
>     voltage range which includes the negative rail, you can amplify
>     the negative lead current shunt voltage up to a more comfortble
>     level.  An op amp and two or three resistors before your A/D.  If
>     you want to see charge AND discharge, you may want an offset so
>     zero current is half way up yout 0-255 A/D range.
>     You can even just use the ground lead of the battery as your
>     current shunt.  Just use smaller and smaller wire - or longer and
>     longer, until you get your desired 0.1 volt.   (I don't know how
>     accurate you want it, and wire does change resistance with
>     temperature).  With this random or unknown resistance, you can
>     make up for whatever it is by adjusting the gain of the op-amp. 
>     You just calibrate it once with an ammeter.  You can try this by
>     putting a DVM across the ground jumper on your car batters and
>     even "measure" cranking current.
>     73, Steve, K9DCI
>     P.S. the 100mv was common because a common meter movement was such
>     that it was also 100mv full scale.  (50 uA, 2,000 ohms & 0.1 V) 
>     The movement current was pretty much irrelevant since it required
>     micro or mili amps and you're measuring amps.  I think I have  100
>     and  20 amp,  50 mV shunts around here.

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