[aprssig] Shunt for battery current / APRS Telemetry

Steve Noskowicz noskosteve at yahoo.com
Mon Feb 27 14:49:51 EST 2006

  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.
  Message: 7
Date: Thu, 23 Feb 2006 15:13:57 -0800
From: "Phillip B. Pacier" 
Subject: [aprssig] Shunt for battery current / APRS Telemetry
Message-ID: 43FE41B5.8090107 at arrl.net

  I am in need of assistance with building a shunt circuit to measure the 
charging amperage of my solar-powered digi. I have found some vague 
references on the web, but none that tell me how to figure out what 
values of what components I need in order to achieve my desired goals. 
For reference, if it makes things easier, I believe the maximum current 
will never get above 12.7 amps. Therefore, dividing the 255 max 
telemetry range in half gives 127.5, or 12.75 amps. That makes the 
calculations easier - now, how do I build this circuit? I have already 
done the voltages with the divider circuits - that was easy. Thanks in 

Phil Pacier - AD6NH


Message: 8
Date: Thu, 23 Feb 2006 17:19:38 -0600
From: James Jefferson Jarvis 
Subject: Re: [aprssig] Shunt for battery current / APRS Telemetry
To: TAPR APRS Mailing List 
Message-ID: <200602231719.38691.jj at aprsworld.net>
Content-Type: text/plain; charset="iso-8859-1"

Measuring the power source that is powering you can be tricky business without 
an isolated power supply. I suggest you look at the Allegro hall effect 
current sensors:
They are a single part that will take measure your current and give you a good 
output in the 0-5 volt range. I've used them before and for DC applications 
they are pretty darn easy to work with.

Best of luck,

-James Jarvis KB0THN


Message: 9
Date: Thu, 23 Feb 2006 19:17:58 -0500
From: "Robert Bruninga" 

Subject: Re: [aprssig] Shunt for battery current / APRS Telemetry
To: , 
Content-Type: text/plain; charset=US-ASCII

Easiest way is to put a 0.02 ohm resistor in the negative
lead of the battery. Then connect the battery terminal
through a 10K resistor to an analog input and protective
5v zener. When 12 amps is flowing through that
resistor, the voltage will be 0.24 volts and the (0-5v) telemetry
will be a count of 12 (out of 255). This is of course only
1 amp resolution. 0-12 and is not taking full advantage of
the 0-255 range of the A/D, but it is simple. de Wb4APR

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