<BLOCKQUOTE class=replbq style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #1010ff 2px solid"> <div>Phil,</div> <div>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.</div> <div> </div> <div>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.</div> <div>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.</div> <div> </div> <div>73, Steve, K9DCI</div> <div> </div> <div>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.</div> <div> </div> <div>Message: 7<BR>Date: Thu, 23 Feb 2006 15:13:57 -0800<BR>From: "Phillip B. Pacier" <AD6NH@ARRL.NET><BR>Subject: [aprssig] Shunt for battery current / APRS Telemetry<BR>To: APRS SIG <APRSSIG@LISTS.TAPR.ORG><BR>Message-ID: <A href="mailto:43FE41B5.8090107@arrl.net">43FE41B5.8090107@arrl.net</A><BR></div> <div>I am in need of assistance with building a shunt circuit to measure the <BR>charging amperage of my solar-powered digi. I have found some vague <BR>references on the web, but none that tell me how to figure out what <BR>values of what components I need in order to achieve my desired goals. <BR>For reference, if it makes things easier, I believe the maximum current <BR>will never get above 12.7 amps. Therefore, dividing the 255 max <BR>telemetry range in half gives 127.5, or 12.75 amps. That makes the <BR>calculations easier - now, how do I build this
circuit? I have already <BR>done the voltages with the divider circuits - that was easy. Thanks in <BR>advance!<BR><BR>73<BR>Phil Pacier - AD6NH<BR><BR><BR><BR>------------------------------<BR><BR>Message: 8<BR>Date: Thu, 23 Feb 2006 17:19:38 -0600<BR>From: James Jefferson Jarvis <JJ@APRSWORLD.NET><BR>Subject: Re: [aprssig] Shunt for battery current / APRS Telemetry<BR>To: TAPR APRS Mailing List <APRSSIG@LISTS.TAPR.ORG><BR>Message-ID: <200602231719.38691.jj@aprsworld.net><BR>Content-Type: text/plain; charset="iso-8859-1"<BR><BR>Measuring the power source that is powering you can be tricky business without <BR>an isolated power supply. I suggest you look at the Allegro hall effect <BR>current sensors:<BR>http://www.allegromicro.com/hall/currentsensor.asp<BR>They are a single part that will take measure your current and give you a good <BR>output in the 0-5 volt range. I've used them before and for DC applications <BR>they are pretty darn easy to work with.<BR><BR>Best of
luck,<BR><BR>-James Jarvis KB0THN<BR><BR><BR><BR>------------------------------<BR><BR>Message: 9<BR>Date: Thu, 23 Feb 2006 19:17:58 -0500<BR>From: "Robert Bruninga" <BRUNINGA@USNA.EDU><BR>Subject: Re: [aprssig] Shunt for battery current / APRS Telemetry<BR>To: <AD6NH@ARRL.NET>, <APRSSIG@LISTS.TAPR.ORG><BR>Message-ID: <S3FE0A7F.064@FSGWHUB.USNA.EDU><BR>Content-Type: text/plain; charset=US-ASCII<BR><BR>Easiest way is to put a 0.02 ohm resistor in the negative<BR>lead of the battery. Then connect the battery terminal<BR>through a 10K resistor to an analog input and protective<BR>5v zener. When 12 amps is flowing through that<BR>resistor, the voltage will be 0.24 volts and the (0-5v) telemetry<BR>will be a count of 12 (out of 255). This is of course only<BR>1 amp resolution. 0-12 and is not taking full advantage of<BR>the 0-255 range of the A/D, but it is simple. de Wb4APR<BR><BR></div></BLOCKQUOTE><BR><BR><p>
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